Pulses and Effective Drought Response

Pulses are an important source of protein and vital for global nutritional security. Irregular and reduced rainfall, along with temperature rises due to climate change, are making food production more challenging. Since 80% of crops are rainfed, it is imperative that we understand how crops react to drought. Research into the internal physiological processes that help plants cope with water deficit is rapidly expanding and requires precise and portable scientific field instruments.

Drought and Pulses

Being the cheapest form of proteins, pulses make up approximately 75% of the diet in developing countries, as opposed to 25% in developed countries, according to FAO. Pulses, which include beans and peas, can be dried and stored for long periods at room temperature. Hence, this food group is crucial in preventing malnutrition for some of the poorest people around the globe.

Pulses are water-efficient, and their nitrogen-fixing ability reduces reliance on chemicals. This group has a huge amount of diversity, so it is easy to choose new varieties that can cope with longer droughts.

During drought, plants do not get the water necessary for their growth and survival. When faced with a water deficit, plants react at various levels. As a whole plant, at the tissue level, and at the cellular level, alterations are made in their physiology and biochemical processes.

Differences exist in plant response depending on the species and genotypes within that species. Moreover, drought can vary in its effects on the different life-stages of a plant.

The usual research method in studying drought effects has been to monitor the reduction in crop yeilds. However, this involves a long experimental time and does not tell us why and what processes affect plant productivity. To speed up breeding programs and make them more effective, researchers are now focusing on physiological changes due to drought.

The following plant strategies have been identified:

• Developing more and deeper roots
• Increasing water use efficiency
• Faster transport of nutrients and carbohydrates
• Maintaining higher relative water content
• Phenotypic plasticity

Here again, the pattern of plant or environmental conditions can influence expression. So, unfortunately, it is not possible to generalize drought-coping mechanisms in plants. They must instead be identified and studied for each crop and its varieties.

More Roots and Osmotic Adjustment Make Beans Drought Tolerant

About 60% of beans (P.vulgaris) are grown in areas that are water deficient, and are therefore at risk of losing up to 80% in yield.

In Brazil, two of the three growing seasons experience water stress. So, to see how beans coped with drought in tropical regions, Brazilian scientists tested two varieties: the BAT 477, which is resistant to water stress, and Perola, which is sensitive to water deficiency.

The scientists grew the plants in a greenhouse and assessed drought by its effect on leaf area, root development, and shoot dry matter produced after 30 days of soil water deficit in the early vegetative stage.

The two varieties were subjected to two levels of irrigation. The control/no stress plants were watered to 80% field capacity throughout their growing period. After ten days of emergence, the other batch was given no water for 30 days. On the last date of the experiment, gas exchanges and relative water content (RWC) were recorded. RWC studies included osmotic potential and adjustment.

Among the instruments used was the CI-600 In-Situ Root Imager, manufactured by CID Bio-Science Inc.

CI-600 is a minirhizotron that can make non-destructive, digital images of living roots. A meter long transparent root tube was inserted into the soil at the start of the experiment, allowing the roots to grow around it. The camera in the CI-600 can be rotated 360 degrees to take scans of all the roots. The accompanying RootSnap! software was used to measure root length, surface area, and volume of the roots. Readings were taken once a week without damaging or disturbing the roots.

Both varieties had greater root mass in shallower soils, up to 25 cm deep. In drought conditions, BAT 477 had 50% more roots than Perola, in terms of length, surface area, and volume at 25-45 cm soil depth. Hence, BAT 477 could use more of the available water in the soil.

Physiologically, BAT 477 was also quick to react to the water stress, reducing photosynthetic and transpiration rate; Perola was not.

In drought, BAT 477 achieved a reduction in stomatal openings of 50%, compared to only 44% reduction in Perola. Since BAT 477 produced greater yield, even with increased closure of stomata and intake of carbon dioxide gas, it could also mean that the variety is better at retaining and fixing carbon.

BAT 477, regulates its stomata by maintaining cell turgor. It does this by keeping its osmotic potential low, increasing intercellular solute concentration and thereby attracting more water. Perola maintained similar osmotic potential, showing it could not adjust its rate of osmosis.

By having better control over the stomata and adjusting osmotic potential, BAT 477 managed to be more water efficient. Unlike Perola, BAT 477 retained its water efficiency at 80% even at times of drought.

Consequently, drought-sensitive Perola suffered a 71% reduction in shoot biomass production and a 70% reduction in leaf area, while BAT 477 suffered only 50% and 41%, respectively.

Both varieties have similar yields when they are irrigated. However, due to water stress, the yields were reduced by 33% in BAT 477 and 53% in Perola.

The scientists found it was varying root development and adjustment of osmosis that was responsible for the difference in drought response of the two bean varieties.

Drought Response at Different Stages of Growth

Figure 1: “Effect of drought on leaf area of (a) black gram and (b) green gram; (C-control, D-drought),” Boroowa and Gogoi, 2016. (Image credits: Research Journal of Recent Sciences, Vol. 5(2), 43-50)

An experiment in India explored drought’s effect at different life stages, in the case of black gram (Vigna mungo) and green gram (Vigna radiata).

Two cultivars of black gram, T9 and KU-301, and green gram, Pratap and SG 21-5, were tested under four irrigation conditions. The control plants were irrigated throughout the growing period. In other treatments irrigation was not given for fifteen days,

• at the vegetative stage,
• early reproductive stage,
• and pod formation stage.

Water-stress levels were maintained at 30% of plant-available water.

Plant height, number of leaves, and leaf area were measured. The dry weight of the shoot and roots were recorded at the end of the experiment to calculate shoot-to-root ratios. The relative water content of leaves was the physiology monitored in this study.

Leaf area was measured by the CI-203 Handheld Laser Leaf Area Meter, a portable instrument, which makes rapid, non-destructive measurements of leaves, manufactured by CID Bio-Science Inc. The CI-203 measured leaf length, width, perimeter, and area. Data was stored and transferred via SD card for analysis.

Across all the varieties in both pulses, scientists found that plants that maintained better leaf water content had better grain yield. All measured attributes were positively correlated with yield.

All varieties in both pulses were affected by drought in all the attributes that were examined. However, the effect of drought at various stages affected different attributes.

Plant height was the attribute most affected by drought at the vegetative stage.

• Leaf numbers also showed the most reduction at the vegetative stage due to drought. This occurred due to a decrease in new leaf formation and the loss of those that grew due to ethylene production.
• Leaf area decreased due to drought at all stages; see Figure 1. However, the difference to irrigated plants was greatest at the vegetative stage, due to reduced cell division as a result of the loss of cell turgor. Leaf area was able to recover from drought at the vegetative stage and give good yield later.
• The shoot-to-root ratio was most affected by drought at the early reproductive stage. This is due to more root development and to increase water uptake in all drought-affected plants.
• Drought at the early reproductive stage had the greatest negative impact on the pulses’ ability to produce yield. The yield reduction was lowest in plants subjected to vegetative drought and moderate for plants that suffered drought at pod filling.

The most drought-tolerant varieties were T9 in black gram and Pratap for green gram. Not only did these varieties have the best final yield, but both also recorded higher values for all the tested attributes. Additionally, T9 and Pratap had the deepest root development and relative water content.

The scientists recommend using relative water content as a stress marker in pulses.

Criteria for Crop Breeding Programs

Both studies found that plants respond physiologically, at the whole plant and cellular level, to reduce yield due to water stress, though effectiveness differed considerably across varieties. Scientists hope to use the specific insights they gained about the processes behind drought tolerance as criteria for future breeding programs. The results could be used in variety selection or trait development to make pulses more drought tolerant and ensure nutritional security around the world.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Feature image courtesy of Vegan Photo

Sources

Baroowa, B., Gogoi, N., & Farooq, M. (2016). Changes in physiological, biochemical and antioxidant enzyme activities of green gram (Vigna radiata L.) genotypes under drought. Acta Physiologiae Plantarum, 38(9). doi:10.1007/s11738-016-2230-7

FAO. (2016). Pulses for food security and nutrition – Food and Agriculture- How can their full potential be tapped? Retrieved from http://www.fao.org/3/a-i6690e.pdf

FAO. (2016, Jan 6). Let the countdown to the International Year of Pulses begin! Retrieved from http://www.fao.org/zhc/detail-events/en/c/358100/

Lanna, A.C., Mitsuzono, S.T., Terra, T.G.R., Vianello, R. P., & Carvalho, M.A. F. (2016). Physiological characterization of common bean (Phaseolus vulgaris L.) genotypes, waterstress induced with contrasting response towards drought. Australian Journal of Crop Sciences, 10(1):1-6.

UNESCO. Fact 24: Irrigated land: United Nations Educational, Scientific and Cultural Organization. (n.d.). Retrieved from http://www.unesco.org/new/en/natural-sciences/environment/water/wwap/facts-and-figures/all-facts-wwdr3/fact-24-irrigated-land/