- Retaining Soil Moisture: What To Do When Soil Dries Out Too Fast In The Garden
- Retaining Soil Moisture
- What to Do When Soil Dries Out too Fast
- 1. Mulch, mulch, and mulch some more!
- 2. Water deeply.
- 3. Use drip irrigation and an automatic timer.
- 4. Mix water-absorbing materials into your soil.
- 5. Check your weekly watering number!
- The Dirt on Dirt – Sand
- Compost increases the water holding capacity of droughty soils
- How Do Plants Deal with Dry Days?
- Some Plants Have Special Structures That Help Them to Survive in Drought Conditions
- Some Plants also Have Internal Defenses Against Drought
- Plants Still Need to Perform Photosynthesis During Drought
- Plants Need to Protect Themselves from Dangerous Free Radicals
- Plants Need to Control the Amount of Water within Their Cells
- A Plant’s Genes Control its Responses to Drought
- Conflict of Interest Statement
- Scientists Unveil New Chemical to Help Plants Retain Water
Retaining Soil Moisture: What To Do When Soil Dries Out Too Fast In The Garden
Is your garden soil drying out too fast? Many of us with dry, sandy soil know the frustration of watering thoroughly in the morning, only to find our plants wilting by the afternoon. In areas where city water is costly or limited, this is especially a problem. Soil amendments can help if you’re soil dries out too quickly. Continue reading to learn about retaining moisture in the soil.
Retaining Soil Moisture
Keeping garden beds weeded helps in retaining moisture in the soil. Excessive weeds can rob soil and desirable plants of the water and nutrients they need. Unfortunately, many weeds can thrive and flourish in dry, sandy soils where other plants struggle.
If your soil dries out too quickly, mulch can also help with retaining soil moisture. Mulch helps prevent water evaporation. When mulching for moisture
retention, use a thick layer of mulch 2-4 inches deep. While it is not recommended to heap thick mulch around the crown or base of plants, it is a good idea to mound mulch in a donut-like fashion a few inches away from the plant crown or tree base. This little raised ring around the plants encourages water to flow down toward the plant roots.
Soaker hoses can be buried under mulch when soil still dries out too quickly.
What to Do When Soil Dries Out too Fast
The best method of retaining moisture in the soil is by amending the top 6-12 inches of the soil. To do this, till or mix in organic materials that have high water holding capacity. For instance, sphagnum peat moss can hold 20 times its weight in water. Humus rich compost also has high moisture retention.
Other organic materials you can use are:
- Worm castings
- Leaf mold
- Shredded bark
- Mushroom compost
- Grass clippings
Many of these amendments have added nutrients that your plants will benefit from too.
Some outside-the-box ideas for retaining soil moisture include:
- Creating moat-like basins around planting beds or cross-cross irrigation ditches.
- Burying unglazed terra cotta pots in the soil with the lip sticking just out of the soil surface.
- Poking holes in plastic water bottles and burying them in the soil near plants with the bottle top sticking out of the soil surface – fill the bottles with water and place the lid on the bottle to slow the seepage of the water from the holes.
As summer heats up you don’t have choose between conserving water or letting your garden cook in the summer sun! Use these five tips to maximize your watering potential and keep your home garden hydrated.
1. Mulch, mulch, and mulch some more!
Cover your soil with a blanket of organic material such as straw, leaves, shredded paper or cardboard, or bark. This will moderate soil temperature, prevent runoff and evaporation, and hold moisture in the for longer periods between waterings.
2. Water deeply.
Less frequent, deeper waterings are more effective for most plants than frequent, shallow waterings. Plant roots will grow stronger and healthier, and you will not need to water as often.
To check whether it’s time to water, push your finger down into the soil. If it is still moist a knuckle or two deep, then it doesn’t need water yet. If it’s dry, then give the soil a nice long, deep soak so that the water reaches the root zone.
3. Use drip irrigation and an automatic timer.
Large amounts of water tend to run off the soil surface rather than being absorbed into the lower layers. For this reason, it’s best to water slowly, allowing the moisture to soak into the soil and permeate down to the root level of the plants.
Drip lines, which are available at nurseries and home centers, provide very slow and effective irrigation. If your plants suffer from various leaf diseases, drip watering may help to prevent these diseases by keeping the leaves dry.
An automatic timer can be used for watering your garden, as well. Whether you use a drip system or a sprinkler, both can be attached to timers, which you can set for automatic, daily or regular watering cycles.
4. Mix water-absorbing materials into your soil.
Organic material, such as coconut coir, peat moss, or even compost, will absorb water, retaining moisture that plants can use during dry spells. Organic material also improves the structure, aeration and overall health of the soil, resulting in better long-term success for your garden.
5. Check your weekly watering number!
The Weekly Watering Number is the amount of water in inches that your lawn will need that week. You can also use the Weekly Watering Number (WWN) for watering other types of plants, by using these general guidelines.
- Shrubs: 50% of the WWN
- Perennials: 50% of the WWN
- Vegetables: 75% of the WWN (new starts may require more water)
- Trees: Newly planted trees need regular watering for up to the first couple of years, while established trees may need a deep soak or two in summer.
Be sure to check with your local garden center or landscape professional for more information that is specific on how much water your plant needs.
Where Does The Weekly Watering Number Come From?
The Regional Water Providers Consortium (www.conserveh2o.org) contracts with a weather forecasting service to provide a free weather forecast and Weekly Watering Number each Thursday (April ‐ September). The WWN is based on historical data (evapotranspiration, rainfall, and other data points) from the previous week, but it is used to determine how much to water lawns and landscapes during the current week.
You can find your weekly watering number at www.conserveh2o.org or right here on CRW’s website at www.crwater.com/conservation.
The Dirt on Dirt – Sand
Sandy soil is often cursed by gardeners but sand can be a wonderful thing. The Dirt on Dirt – Sand will teach you about sandy soils, why you should love them, and how to make them even better.
Soil comes in a whole array of types. The basic categories are clay, silt, loam and sand with constant variation within each of these classes. If you have silt or loam soils you are sitting pretty, gardening will be easy and you will love your soil. If you have clay or sandy soils it will take a bit more input from you before you love your soil. Trust me, you can love your clay or sandy soil, it just takes a bit of knowledge and a bit of elbow grease. So the question is: how, exactly do you learn to love sandy soil? Read on to learn more about what sandy soil is, why you should be glad you have it and how you can make it even better.
First, things first, how do you know you have sandy soil? Does water quickly drain through your soil with puddles a rarity even after hard rains? Is it difficult to squeeze the soil into a ball? It these things are true then you probably have sandy soil. Sandy soils offer both benefits and disadvantages when compared to clay soils. They may require more water, more fertilizer and more amending, but they are much easier to work with and many plants prefer this type of soil. If you have clay soils, click here to read about working with clay soils.
What is Sandy Soil?
What does it mean that you have sandy soil? A sandy soil is composed of many irregular to rounded tiny grains of sand, as opposed to the many tiny plate-like soil particles that make up a clay soil. If you imagine a glass jar filled with ping pong balls, this is what a truly sandy soil looks like under magnification. If you imagine a jar filled with poker chips, this is more how a clay soil would appear when magnified. As you can imagine there is a lot more air space between the rounded sandy soil particles and this larger amount of air under the soil surface is what gives your soil the characteristic of being well-drained. This simply means that water moves quickly through the soil and air replaces it quickly.
Before we go into too much detail, a sandy soil will replace water with air more quickly, and this is why sandy soils dry out faster than clay soils. Is this bad? Well it all depends on your soil and what you are trying to grow, sandy soils are best for plants that like to have their roots dry out quickly, but it can also be adjusted to support plants that do not. It is always hard to know which kind of soil you have without doing a soil test, but your local county extension service will help you in doing a basic soil test to let you know what particular type of clay soil you have.
What’s good about sandy soil & what’s bad about it?
Let’s take a look at what a sandy soil offers to gardeners, both the good and the bad.
The good parts: A sandy soil is so much easier to work with than clay soils, it is lighter weight, doesn’t compact, and in general is easy to dig in or amend with compost, and most flowering plants benefit from the fact that it is well drained. You will rarely have to worry about over-watering and root rot problems are less likely. Transplanted plants seem to establish a little bit faster in sandy soils as well, since it is easier for their roots to get a foothold in this looser type of soil. Sandy soils also tend to warm up a little faster in the spring when compared to clay soils, so if you are an impatient gardener having a sandy soil gives you a little bit of head start in spring.
A few bad things: Since sandy soils are made up of well…sand you will find that it doesn’t hold water or nutrients very well. Sand is composed of silica, usually quartz crystals, and these have relatively no ability to hold onto nutrients and little ability to hold on to water. Hopefully you are not gardening in pure sand, but even then there is hope. You just have to plan to use water more efficiently, and to water deeply, slow release types of fertilizer are better than liquid fertilizers, and you’ll want to spend a bit more time adding compost or other organic matter into your soil to beef it up. In these days of drought warnings and water restrictions sandy soils are getting a bad reputation, but like most bad reputations this is largely a misconception. A sandy soil has a lot of great qualities including that it is much more difficulty to compact a sandy soil, clay soils can be compacted by driving over them with lawn mowers, cars etc, and sandy soils are more resilient. .
How fertilize sandy soils most effectively – We all need to learn is how to avoid wasting fertilizers as they eventually run off into our lakes, streams and groundwater if we use them improperly. Nowhere is this more important than with sandy soils. Since sandy soils cannot hold either nutrients or water as well as clay type soils, they allow more water and nutrients to run through the soil, which means they end up somewhere else other than your garden.
Fertilizer manufacturers have come up with a type of fertilizer that mimics the way a clay soil adheres to and then releases fertilizer. The name for these fertilizers that hold and slowly release fertilizer is “slow-release fertilizers”. There are two types and you may want to experiment with both to see what works best for you. Plastic coated or resin coated fertilizers (such as Osmocote®, Dynamite®, and Nutricote®) are marvels of technology with multiple layers of plastic surrounding the fertilizer, each layer of plastic has minute holes which allow fertilizer to leak out slowly where plants can grab it up before it moves through the soil.
Sulphur coated slow release fertilizer act in a similar way only using sulphur (itself a fertilizer) layers to restrict how quickly the fertilizer breaks down. In both cases you’ll get better results with the fertilizer mixed into the soil at planting rather than placed on top your mulch after you have finished planting. Mixing the fertilizer into the soil allows soil bacteria and underground moisture levels to help with uniform delivery of your fertilizer. Also a small percentage of fertilizer gets atomized back into the atmosphere unless it is covered with soil.
Regardless of which type of fertilizer you choose a slow release fertilizer will usually allow you to fertilize about ¼ as often as regular granulated fertilizers or water soluble fertilizers. This can really make your life easier during the spring and summer. Most landscapes and gardens need a liquid fertilizer about every 2 weeks, OR a granular fertilizer about every month, OR a slow release fertilizer 2-3 x per season. Over fertilizing is simply wasting money and potentially harming the environment. You can actually cause more problems by fertilizing too much; over-fertilized plants tend to be more susceptible to insect and disease problems because they have been pushed so hard to make them grow that they are weakened and more likely to have problems. Click here for more information on fertilizer.
How to water sandy soils most effectively – Watering is the biggest challenge most gardeners’ face and most people over-water their plants, it is the single biggest cause of plants dying. Luckily if you have a sandy soil, you are not likely to be an over-watering statistic. The key to watering sandy soils is to water less frequently but for longer each time, this encourages deeper root systems on plants and also allows them to penetrate deeper into the soil where there is more water available than there is at the surface. Less frequent deeper watering will help develop deep root systems and frequent light watering encourage shallow roots which make plants less drought tolerant. Check with your local county extension service to see what recommended watering rates are in your town.
The best way to water is deeply and infrequently (except for recently planted flowers and landscapes, these need water frequently to get established). If you have a sprinkler system, make sure to check and see that is not overwatering on a regular basis, plants get used to whatever watering cycle you give them, so plants that are regularly overwatered are more likely to collapse when the water isn’t there, on the contrary plants that have to work just a little bit in between watering are tougher and more likely to handle short dry periods. For more on watering landscapes click here.
How to make sandy soils better:
Incorporating compost – For gardeners with sandy soils adding organic matter to the garden soil is simply a matter of survival. Luckily this type of soil is easy to dig in and a breeze for a rototiller. You want to add the same types of organic matter regardless of what soil type you have: compost, straw, shredded wood bark, etc) by adding these things to your soil you can help it to retain more water and fertilizer as well as providing additional nutrients as these organic bits decompose. For most sandy soils it may be better to use a slightly coarser material for your amendments because they break down so quickly in well drained soils, especially if local rains are heavy.
Sometimes adding large amounts of organic matter all at once can temporarily reduce the nutrient nitrogen, so when adding uncomposted materials, you may want to bump up your fertilizer levels until plants appear to be growing actively with no problems. The first sign of a nitrogen shortage is plants turning a yellowish green. The compost you add each year will also act as a slow release fertilizer and a as an additional way to hold water for your growing plants! Click here for more information on compost.
Mulching – For sandy soils mulching is essential to get plants established. Because sandy soils have so much more air space than other types of soil, water evaporates from the surface of the soil at a much faster rate than clay soils. Applying a 2-3” layer of mulch composed of compost or other organic matter will stop water evaporation almost entirely. This helps keep the water where the plants need it, underground. A layer of mulch will also act to cool the soil during summer heat and extend the life of flowers and vegetables in the garden as well as reducing temperatures overall in the garden.
Having a sandy soil is actually a lot less work than clay IF you know how to handle it. They are easier to work with, less effort to dig in and easier to adjust if problems occur. The key to success in sandy soil is less frequent deeper watering, using slow release fertilizers to reduce the amount of fertilizer run off and environmental pollution, and adding as much organic matter as possible to the soil to help hold water, nutrients, and keep plant roots in place. Another key to success is selecting plants that do well in well drained soils, ask your local garden center or county cooperative extension service about what plants work best in your area.
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|A sample of vermiculite. Photo credit: Brian Pettinger Flickr CC BY-NC-SA 2.0|
Improving water retention in soils begins with understanding the soil type found in the garden. Soils are generally made up of varying mixtures of three sizes of soil particles; sand, silt and clay, known as soil texture.
Generally, water retention is inversely related to permeability. Sandy soils have the lowest water retention, followed by silt, and then soils high in clay.
Various soil amendments are available that can improve water retention, particularly soil high in sand.
Organic Soil Amendments
- Sphagnum peat moss (please note that Sphagnum peat may not be a sustainably harvested amendment)
- Coir dust (a byproduct of coconut harvest)
- Aged manure (may have high salt content)
- Other organic materials such as grass clippings and straw
- Vermiculite (expensive and breaks down quickly, better for container gardening)
- Perlite (excellent for container gardening)
These are some of the common and more readily available soil amendments for improving water retention. Each has its own unique benefits as well as limitations.
Additional Resources by Region:
Colorado: Choosing a Soil Amendment
Maryland: Soil Amendments for the Garden
Florida: Coir Dust, A Viable Alternative to Peat Moss
Compost increases the water holding capacity of droughty soils
Compost is an earthy-smelling, humus-like material that is a product of the controlled aerobic decay of organic nitrogen (such as manure) and carbon (such as sawdust, straw or leaves). One advantage of compost is its ability to hold moisture. The focus of this article is to understand how to choose composts that increase the soil’s water holding capacity.
It is important to understand at the outset that not all composts are alike. For example, composts made from manure are not the same as composts made from leaves. The nutrient content, microorganism diversity and population, cation exchange capacity and water holding capacity of compost can be different based on the feedstocks used to make the compost, the process used to make the compost and the maturity of the compost at the time of application. Therefore, it is important to understand the quality of a compost before using it to ensure you get the intended benefit you are seeking. Further information on compost quality can be found in the following publications: Field Guide to Compost Use, AAPFCO Soil Amendment/Compost Uniform Product Claims and Compost: Matching Performance Needs with Product Characteristics.
Water holding capacity of soil organic matter
Soil scientists report that for every 1 percent of organic matter content, the soil can hold 16,500 gallons of plant-available water per acre of soil down to one foot deep. That is roughly 1.5 quarts of water per cubic foot of soil for each percent of organic matter, according to Sullivan in “Drought Resistant Soil. Agronomy Technical Note. Appropriate Technology Transfer for Rural Areas” at the National Center for Appropriate Technologies in 2002. Increasing the organic matter content from 1 to 2 percent would increase the volume of water to 3 quarts per cubic foot of soil. Rodale Institute presenters, on the other hand, assume that 1 pound of carbon can hold up to 40 pounds of water. That calculates out to be approximately 38,445 gallons of total water per acre six inches deep. The point here is that organic matter holds a lot of water, thus, the amount of organic matter in a soil directly influences the availability of water to a crop over time. However, organic matter in droughty soils breaks down so rapidly that getting above 2 or 3 percent is difficult to do, but getting to 2 to 3 percent can have major positive impacts.
How effective is compost at holding water?
A 1994 study by A. Maynard found that a 3 inch layer of leaf compost rototilled to a 6 inch depth increased water holding capacity 2.5 times that of a native sandy soil and provided almost a 7 day supply of plant available water. In a 2000 study, Maynard found that increasing the water holding capacity of the soil by adding compost helped all crops during summer droughts by reducing periods of water stress. The amount of water in a plow layer (8 inches) of the compost amended soil increased to 1.9 inches compared with 1.3 inches in unamended soil. Since vegetables require 1 inch of water a week, at field capacity, the compost amended soil held a 2-week supply of water.
Reduce water application
The U.S. Compost Council (2008) has stated that the frequency and intensity of irrigation may be reduced because of the drought resistance and efficient water use characteristics of compost. Compost reduces soil crusting, which helps with water absorption and penetration into the soil. Recent research suggests that the addition of compost in sandy soils can facilitate moisture dispersion by allowing water to more readily move laterally from its point of application.
How much compost must be added to soil to increase organic matter content?
The limiting factor for compost application in Michigan is soil phosphorus levels. In the Generally Accepted Agricultural and Management Practices for Nutrient Utilization (Nutrient GAAMPs) it states when soil phosphorus levels exceed 300 pounds per acre, no source of phosphorus can be applied. That means no compost can be applied to soils that exceed 300 pounds per acre. When soil phosphorus levels are between 150 and 299 pounds per acre, compost is to be applied based on the phosphorus removal rate of the crop. When soil phosphorus levels are less than 150 pounds per acre, compost is to be applied based on the nitrogen requirements of the crop.
For most cropland in Michigan, this means that low amounts of compost will be applied, so choosing composts high in organic matter is critical if increasing soil water holding capacity is your goal.
According to the U.S. Compost Council’s Field Guide to Compost Use, farmers should choose composts that have an organic matter content between 50-60 percent and a water holding capacity of 100 percent or higher.
When purchasing compost, ask to see an analysis to verify organic matter content and water holding capacity. Commercial sources of compost in Michigan can be found at the FindAComposter.com website.
Soil organic matter is built up over time with continuous applications of compost. Some farmers in Michigan’s Thumb area have found that applying 1 to 2 tons of compost/acre/year on field crops makes a difference in the soil’s ability to grow a crop. It is estimated that applying a ton of compost to the acre on a soil with 1 percent organic matter can increase that soil’s organic matter content by 10 percent. Compost spread evenly over one acre at a depth of one inch equals about 135 cubic yards or 54 tons, assuming the compost has 60 percent organic matter and a bulk density of 800 lbs./cubic yard at 30 percent moisture.
Michigan State University Extension educators are available to assist farmers with compost use and application. To locate an educator, go to Michigan State University Extension Bioeconomy page and click on the “MSUE Find an Expert” button on the left side of the page. When the “Find an Expert” page is displayed, type the word “compost” in the line next to “Educator Area of Specialty” and names of individuals you can contact will be displayed.
- MSU Extension’s Drought Resources
How Do Plants Deal with Dry Days?
Plants regularly face dry conditions. Not having enough water poses a serious threat to a plant’s ability to grow and develop or even just survive! If plants die, we will not have enough food to eat! How do plants manage to survive during water shortages? They must somehow be able to sense, respond, and adapt to changes in water availability. They do this through a range of techniques that allow for a plant to combat water shortages. A plant’s structural “armor” helps it to decrease the amount of water it loses to the environment and increase water storage. Plants respond to water shortages in very complex ways. These responses can include changes in the plants’ growth and in their ability to protect themselves against toxic chemicals that accumulate in the plant during dry periods. All of a plant’s responses are directly controlled by the plant’s genes. If we can understand the genes that are involved in protecting plants against drought, in the future we might be able to make genetically modified crops that can tolerate global warming and climate changes.
Have you heard people speaking about global warming and climate change? Do you know what these terms mean? These terms basically imply that the earth is getting hotter every year. These higher temperatures lead to unexpected and unusual weather patterns. One of these extreme weather patterns is frequent and severe droughts. Droughts are very long dry periods without any rain. What do severe droughts mean for plants? Well, plants are sessile, which means they stay in one place and can’t move around like we can. They can’t pull up their roots and relocate to a shady or damp spot. Therefore, plants somehow need to deal with these ever-increasing drought conditions, or they will simply die. Remember, plants are our food. We eat plants raw or cooked (those vegetables your mom insists you eat!) or processed, like your favorite box of breakfast cereal . So, if plants die because of droughts, we will not have enough food to eat!
If there is no water around, what can plants do to survive? Amazingly, all plants seem to have a number of genes for drought-defense strategies encoded in their DNA. Genes are small sections of DNA, like chapters in a book. How they use these genes determines their ability to survive drought.
Some plants are drought-resistant. When we talk about drought-resistant plants, we mean plants that can withstand dry conditions without dying. A drought-resistant plant can survive drought by using three defense strategies: escaping, avoiding or tolerating the loss of water . Drought tolerant plants are quite rare in nature and can endure long periods with no water at all. Some of the most spectacular drought tolerant plants are called resurrection plants. Resurrection plants are able to survive long periods (up to 3 years!) without any water. However, give them a little water and they will spring back to life in a day or two. Other drought-resistant plants may not be as spectacular, but they too can survive short periods of drought using special techniques and defense strategies.
Some Plants Have Special Structures That Help Them to Survive in Drought Conditions
Some plants are able to survive droughts because of their unique structures. These structural features include the external armor of plants that protects them against water loss, as well as tools to help the plants absorb and store water. Drought-resistant plants can be specially adapted to live and survive in very dry environments. These plants often look quite different from plants living in areas where water is easily available. The drought-resistant plants normally have special “avoidance” (one of the defense adaptations!) features to make sure less water is lost to the environment or that more water gets absorbed and stored in the plant. Plants called desert succulents are a good example of plants that have drought avoidance strategies . Desert succulents have thick fleshy leaves, which often don’t resemble leaves at all, and they have a thick waxy layer to prevent water loss. Desert succulents also have extensive root systems that search for water under the dry desert soil (Figure 1). Some succulents have specialized roots that form large bulb structures, which are actually underground water reservoirs for the plant. These plants can survive years of drought using the water stored in their bulbs.
- Figure 1 – Extreme structural adaptations found in plants to combat water loss and store more water.
Most of the water a plant loses is lost due to a natural process called transpiration. Plants have little pores (holes or openings) on the underside of their leaves, called stomata. Plants will absorb water through their roots and release water as vapor into the air through these stomata. To survive in drought conditions, plants need to decrease transpiration to limit their water loss. Some plants that live in dry conditions have evolved to have smaller leaves and therefore fewer stomata. Extreme examples are plants with leaves that resemble spiky thorns. Some plants may also completely shed their leaves in a drought, to prevent water loss. The basic rule is that fewer leaves mean less water loss through transpiration. These extreme leaf adaptations can also protect the plants from hungry and thirsty birds and animals (Figure 1). You certainly would not like to have a prickly meal!
Some adaptations are quite clever and involve plants “escaping” drought as seeds (remember, escape is another defense strategy). The seeds survive during the dry spells and very quickly germinate (sprout), grow and produce more seeds when rains fall. These seeds are then scattered and can also survive extreme harsh conditions for long periods of time. Looking closely at desert soils, you will find a lot of seeds lying around, just waiting for rain before germinating again.
Some Plants also Have Internal Defenses Against Drought
In addition to special structures, plants have internal defenses to protect them against water shortage, as well. When a plant experiences drought conditions, some reactions will quickly happen inside the plant to help the plant with the stress of the drought. These reactions that occur in the plant are often quite complex and sophisticated. We will give you some examples.
Plants Still Need to Perform Photosynthesis During Drought
Plants are green because they contain a green chemical called chlorophyll. Chlorophyll is packed into special structures called chloroplasts, which are the energy factories of plants. Together with water and carbon dioxide (CO2), chlorophyll uses sunlight to create sugars. These sugars allow the plant to grow and flourish. This is the process of photosynthesis and it is linked to the availability of water.
When there is not a lot of water in the plant’s soil, the process of photosynthesis will happen a little differently and will result in the build-up of damaging chemicals called free radicals. This means that plants need to carefully control how they use the energy of the sun. During photosynthesis, CO2 must enter the plant through its stomata (the little pores mentioned earlier). But remember, open stomata mean that water will be lost through transpiration! So, the plant is faced with the difficult problem of making sure it has enough water and also enough CO2 for photosynthesis to occur. To do this, plants use a “manager” called abscisic acid (ABA).
When a plant experiences a shortage of water, ABA is rapidly produced and transported to the stomata. In the stomata, ABA controls how the stomata open and close by manipulating something called turgor pressure (Figure 2) . Turgor pressure is the pressure applied on the wall of the plant cell by the fluids inside the cell. The more water is in the cell (the fuller the cell) and the bigger the pressure. Management of turgor pressure provides a balance between CO2 intake and water loss, so that photosynthesis can occur. But, if water remains limited in drought conditions, eventually the plant will be unable to cope with the stress of the drought and the entire photosynthetic process can stop working properly. However, drought-resistant plants have figured out a clever way to avoid the problem of losing water during photosynthesis. They only open their stomata during the cool of the night to take up CO2. They then store this CO2 and use it in the daytime for photosynthesis. This way, they lose less water during the day because they can keep the stomata closed, but they can continue to grow—although a little slower than normal.
- Figure 2 – Internal defenses of plants under water stress.
- (A). When plenty of water is available in the soil, plants will absorb water through its roots. This water will be used by the plant or released through transpiration by open stomata in the leaves. Photosynthesis will also occur normally with CO2 and oxygen being absorbed and released through the open stomata. (B). But when limited water is available in the soil, plants try to prevent water loss. Water loss through transpiration can be reduced by closing the stomata in the leaves using a substance called ABA. When the stomata is closed photosynthesis will decrease because no CO2 can enter through the closed stomata. Less photosynthesis means less energy is produced by the plant and the plant stops growing.
Plants Need to Protect Themselves from Dangerous Free Radicals
In drought conditions when a plant cannot seem to balance photosynthesis and water loss properly, the plant will have to deal with nasty little molecules called free radicals. Free radicals occur naturally during photosynthesis, but when there isn’t a lot of water available more free radicals form. Free radicals can be very dangerous for the cell, because they can cause damage to DNA, cell membranes, proteins, and sugars (all of these substances are essential for a cell’s survival)!
Plants are used to dealing with low amounts of free radicals. However, drought tolerant plants are really good at dealing with free radicals, because they accumulate protective substances. These protective substances are called free radical scavengers. The presence of free radical scavengers often causes a change in the color of the plant. Plants often turn red or purple when these scavengers accumulate (do you see the purple leaves of the dry plant in Figure 3B?). The free radical scavengers occur widely in nature and are very good at mopping up free radicals to protect plants from their harmful effects.
- Figure 3 – The resurrection plant, Craterostigma pumilum.
- (A). This is how the plant looks when it is growing in conditions where enough water is available. (B). The two middle pictures show the plant when no water is available, after 3 weeks without water. Doesn’t it look dead to you? (C). If the same dry, dead-looking plant is watered, within 2 weeks the plant will recover from the drought and start producing seeds.
Plants Need to Control the Amount of Water within Their Cells
Osmosis is an important concept in biology. Basically, osmosis is the movement of water across a membrane (like a cell membrane) to an area where certain molecules (like salts, sugars, and free radicals) occur in higher concentrations. By doing so, the water will dilute the concentration of these molecules so that the concentration is equal on both sides of the membrane. Now think about what happens to a plant that is suffering from the loss of water. There is not enough water to allow osmosis to occur, so molecules become super concentrated inside the plant cells. This is generally not a good thing, especially if these molecules are free radicals.
Once again, drought tolerant plants have some very cool strategies to fight this problem. At the first signs of drought, the cells of these plants will accumulate a bunch of molecules involved in what is called osmotic adjustment (OA) . OA is the change is solute concentration in a cell. This is like when you dissolve sugar in water, where sugar is the solute. These molecules (solutes) can be sugars, amino acids or small proteins. The purpose of these molecules is to limit the movement of water out of the cell. What makes these OA molecules unique in drought tolerance is that they serve many functions. The OA molecules can physically bind to DNA and proteins to protect them from free radicals. They can also bind water itself, preventing it from moving out of the plant cells. These OA molecules also bind to membranes, stabilizing the structure of the plant when water is restricted.
Resurrection plants are perfect examples of how drought tolerant plants bring together the concepts we’ve discussed so far. Resurrection plants are able to survive complete loss of water. They accumulate vast quantities of OAs, release free radical scavengers and produce special protective proteins to survive long and severe droughts. They do all of this while they also fold their leaves away and wait until rain falls (Figure 3). The process can be compared to bears going into hibernation.
A Plant’s Genes Control its Responses to Drought
Keep in mind that we have discussed these processes used to protect plants from drought in a very simplified manner. Looking closely at these processes is actually very complicated. At the very basic level, these processes are regulated by the plant’s use of its genetic code—its genes. Substances necessary to survive drought will be produced by accessing this code at the right time. This accessing of the genetic code to help a plant survive a drought is called the genetic response of the plant.
The genetic responses of a plant experiencing the stress of a drought are very complex—lots of genes are switched on or off. Using advanced computer technologies, scientists are now able to identify most of the genes that play a role in protecting a plant from drought. This technology has found that literally hundreds of genes are switched on and off, depending on where and when they are needed! We can’t list all of these genes, because you will be completely bored at the end of the first page! What we will say is that these genes fall mainly into three groups: (1) genes that control other genes important for switching genes on and off; (2) genes that produce substances that help with drought protection in the plant; and (3) genes involved in water uptake and transport.
Why do you think it might be important to know which genes play a role in helping plants avoid or tolerate drought? Most of our crops are actually not able to survive droughts. How are we going to protect our crops or make them more resistant to these droughts? We need to use the knowledge of the genes that are turned on or off during drought conditions to produce plants that are more resistant to drought.
Over the years, plant scientists have had some success in producing drought-resistant crops. These drought-resistant crops were produced mainly by selecting and breeding individual plants that survived well under drought conditions. Over the past few decades, scientists working on genetically modified (GM) plants also started to focus on producing drought-resistant crops .
To produce a GM plant, a new gene (from any source!) is inserted into the DNA of the plant. By inserting this new gene/s, the scientist hopes to introduce a new, useful trait into the GM plant. Imagine being able to choose from hundreds of helpful genes in a resurrection plant and introduce some of them into wheat! Unfortunately, only a handful of GM drought-resistant plants (such as maize/corn and sugarcane) have been successfully produced. Much more work needs to be done, including convincing the general public that GM plants are not dangerous!
Plants are really vulnerable when it comes to water scarcity. Drought will influence a plant’s growth, development, productivity and ultimately its survival. However, plants do have some built-in protection against drought. They can have some structural adaptations to avoid or tolerate dehydration. They also have some internal defenses that are activated to try to limit water loss when they realize water is becoming scarce. All of these defense systems are regulated by the plant’s genes. Knowledge of these genes and how they are involved in protecting the plant against drought provide humankind with a hope to make drought-resistant GM crops.
Sessile: An organism that can’t move and stays in one place, like a plant.
Succulents: Plants that have thickened and fleshy leaves and stems, in which water can be stored.
Transpiration: The process where plant roots will take up water and then release water vapor through pores (stomata) in the leaves.
Stomata: Little holes in the lower surface of a leaf through which water and gas can move in and out of a plant.
Photosynthesis: The process where plants use water, light and CO2 to produce their own food (in the form of sugars) and release oxygen into the air.
Free radicals: Molecules that will react with, and damage, anything they come in contact with.
ABA: A plant hormone called abscisic acid that helps take care of the water balance in plants.
Turgor pressure: The tension exerted on a plant cell wall by the fluids inside the cell. Imagine filling a balloon you’ve placed inside a glass jar. As you fill the balloon more, it presses up against the rigid glass jar just like the fluids against the rigid plant cell wall.
Osmosis: Moving water through a cell membrane from one cell to the next cell. Why? To ensure equal concentrations of solutes on both sides of the membrane.
Osmotic adjustment: Changing the concentration of solutes in a plant cell.
Solute: The substance (like sugar) you are dissolving in a solution (like water).
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Figures were created in the Mind the Graph platform (www.mindthegraph.com).
Blum, A. 2014. Genomics for drought resistance – getting down to earth. Funct. Plant Biol. 41:1191–8. doi:10.1071/FP14018
Scientists Unveil New Chemical to Help Plants Retain Water
A study by researchers at the University of East Anglia in the US, reported that 25 percent of the world’s lands are threatened by drought, as water resources are declining due to the effects of climate change.
A solution by the University of California could address this problem. It involves a chemical that helps plants hold onto and preserve water, which could stem the wave of massive annual crop losses from drought.
Senior author Sean Cutler, a plant biologist at University of California, said: “Drought is the No. 1 cause of annual crop failures worldwide.”
“This chemical is an exciting new tool that could help farmers better manage crop performance when water levels are low.”
Details of the team’s work on the newer, more effective anti-water-loss chemical are described in a paper published in the Science journal. They named the new chemical, Opabactin, or “OP.”
The drug mimics abscisic acid, or ABA, the natural hormone produced by plants in response to drought stress. ABA slows a plant’s growth, so it doesn’t consume more water than is available and doesn’t wilt.
Scientists have known for a long time that spraying plants with ABA can improve their drought tolerance. However, it is too unstable and expensive to be useful to most farmers. This led farmers to seek an affordable replacement, the Opabactin.
When ABA binds to a hormone receptor molecule in a plant cell, it forms two tight bonds, like hands grabbing onto handles. Scientists searched millions of different hormone-mimicking molecules and resulted in OP.
“OP is 10-times stronger than ABA, which makes it a ‘super hormone.’ And it works fast. Within hours it could give growers more flexibility around how they deal with drought,” Cutler told Asharq Al-Awsat via email.
“One thing we can do that plants can’t is predict the near future with reasonable accuracy. Two weeks out, if we think there’s a reasonable chance of drought, we have enough time to make decisions like applying OP that can improve crop yields,” he explained.