Adaptive Strategies – Leaf Area and Physiological Plasticity in Drought

It is becoming increasingly important for ecologists and agronomists to understand how plants adapt to drought. Natural adaptations, which help a plant withstand changing climate, are now not just of theoretical interest but are also relevant for maintaining and improving agricultural production and ecosystem conservation. Leaf area can be an indicator or strategy for drought coping, making the leaf area meter one of the most commonly used bio-science devices by scientists. 

Drought Affects Plant Distribution and Crop Yield

Drought is the most common stress affecting plants. Consequently, plants adapt to drought. How well a plant adapts to drought can depend on species and varieties.

Drought adaptation is crucial for both natural populations and cultivated species, especially due to changing temperature and rainfall ranges caused by global warming, altering water availability temporally and spatially.

For natural populations, drought adaptation can determine survival, propagation success and, in the long run, distribution. Sessile plants cannot choose their environment and, therefore, have to be plastic enough to grow in a wide range of micro-climates. The plasticity can result in plants showing different traits under varying circumstances.

Drought-resistant or tolerant features in crops are relevant, as scientists search for varieties that can produce more under warmer and drier conditions.

Hence, scientists are studying the morphological and physiological features that adapt plants to water stress. Species that occur in a wide range of climatic conditions make ideal subjects for this research.

• Morphologically, leaf area modifications, number of leaves, size of plants, floral size and number, and size of roots are important characteristics.
• Physiologically important characteristics can be stomatal conductance, water use efficiency, and plant development rate. These help the plant reproduce and senesce quickly before being affected by drought.

Drought Coping Traits Can Be Plastic

While scientists know plasticity exists, there have been few studies that show what adaptive advantage they give plants. A species subjected to environmental heterogeneity is expected to have more plasticity than those growing in uniform conditions. Similarly, more plasticity is seen in species that experience extreme drought in some years. Thus, spatial and/or temporal heterogeneity can make a species more plastic.

Biologists Lambrecht, Morrow, and Hussey decided to see if both the plasticity in vegetative and floral size and the physiological traits allowed four populations of the annual Leptosiphon androsaceus, found in different rainfall regions, to cope with drought. They also wanted to know if the variations in the populations were due to plasticity/phenology or due to selection/genotype.

Leptosiphon androsaceus, or false babystars as it is commonly known, is a kind of phlox (Polemoniaceae). It is an herb that is native to the chaparrals and woodlands in the Mediterranean climate found in California, during the uneven winter rains.
The Field and Growth Chamber Studies
The field studies to track changes in morphology due to seasonal/temporal variations, besides geographical differences, lasted five years.

Each year, 15 to 30 plants were selected randomly for data collection.

One to three flowers from each plant were used for measuring floral traits. The traits calculated were length and width of corolla lobes, diameter of the flower, length of floral tube, anther-to-stigma distance recorded by digital calipers, and averages for each flower.

The biologists measured most of the vegetative traits, such as calyx length, leaf length, and plant height, using a simple ruler.

Five to ten plants were collected from each population and the leaf area of the uppermost pair of leaves was recorded. The false babystars’ leaves are typically about 3 cm long and are linear to needle-like. The leaves’ minute size makes it difficult to record small differences that occur, so the scientists needed an instrument that was precise but also easy to use. A digital caliper would be difficult to use for needle-like leaves, and a simple ruler was not accurate enough.

Lambrecht, Morrow, and Hussey solved the problem by using a leaf area meter. The CI-202 Portable Laser Area Meter is light and can be carried to the field and is ideal for intricate leaves like those of false babystars. Using laser technology, the leaf area meter takes fast and precise, non-destructive measurements.

The leaves are placed on the palette and covered with the transparent sheath, which protects the tender false babystars leaves while flattening and smoothing them for accurate readings. Then, the laser scanner is slid over the sheath, and for every 1 mm of movement, the dimensions of the leaves are recorded. These dimensions include leaf length, width, perimeter, and area. The instrument has a high resolution of 0.01 cm2, so it could easily capture differences in leaf area between different populations. A data logger stores the information and scientists can transfer data using a USB port. The tool is rapid and needs no calibration.

Stable carbon isotope ratios were measured to estimate water use efficiency (WUE).

To check within and between population variations, Lambrecht, Morrow, and Hussey also conducted an experiment in a growth chamber. They took seeds from three populations—Bobcat, Domino, and Woodpecker—and grew them in two treatments: well watered and drought. After ten weeks, the biologists collected all the data, as in field trials. They also measured photosynthesis (A) and stomatal conductance (gs).

At each site, Lambrecht, Morrow, and Hussey also measured the volumetric water content of the soil in 3 to 5 locations for the rooting depth of the herb (10 cm).

Genotype and Phenotype Shape Drought Coping Strategies

The scientists found variation in the traits within the four areas over five years. Wetter areas and years produced similar results. The plants and their leaves and flowers were larger, and the WUE was lower than in drier years or locations.

The growth chamber study showed that the vegetative and physiological traits were plastic but the floral traits were not. Regardless of the area of origin, all plants had lower specific leaf area (leaf area per unit leaf mass), photosynthesis, and stomatal conductance, but higher WUE due to water stress. However, there was a difference in the adaptiveness. Bobcat and Domino showed the strongest difference when grown in wet versus dry conditions, while Woodpecker populations remained similar regardless of water availability.

Floral sizes differed between populations and did not change with watering, indicating they were influenced by selection and not plasticity, with smaller flowers in drier regions to reduce water loss through their surfaces.

The scientists found that specific leaf area was the only trait that showed plasticity due to water availability and population.

The false babystars did show drought coping responses, and they were both plastic and genotypic. These strategies are vital for their survival, as the species is a short-lived annual herb. The herb increases WUE while reducing stomatal conductance and specific leaf area to cut water losses and make the most of the water it can get.

Drought Adapted Varieties

In Chile, agricultural scientist—Acuña, Inostroza, Sánchez, and Tapia—focused on genotypic differences to develop drought-resistant varieties of forage legumes

As better soils are used for food, forage crops are being grown on marginal soils. Water stress is common in these areas. Finding varieties that are naturally adapted to drought can help maintain higher yields in the absence of irrigation.

It is particularly challenging to grow crops in the Mediterranean climate in Chile, with hot and dry summers, 80% of rainfall occurs in winter. Therefore, the scientists tested twelve populations of the perennial forage legume Lotus tenuis in areas with varying water availability for growth and plant water status.

The agricultural scientists used a greenhouse experiment and sowed seeds in pots to establish ten plants in each pot. The pots were subjected to four treatments that received 100, 70, 40, and 10% SWA (soil water availability).

The relative rate of stem elongation, the dry matter (DM) of shoots and roots, and the relative water content (RWC) were measured. The Chilean scientists also used the CI-202 Portable Laser Area Meter, manufactured by CID Bio-Science Inc., to estimate SLA.

Acuña, Inostroza, Sánchez, and Tapia found RWC, stem elongation, shoot and root DM, and the root:shoot ratio of DM were affected by drought. Water stress, or 10% SWA, decreased all these traits.

However, RWC and SLA did not differ greatly in the twelve populations, even though the RWC did decrease by over 15% under water stress. SLA in fully watered treatments (100% SWA) was 20% higher than in water-stressed (10% SWA) plants.

The DSI, derived from DM growth of a population, did differ significantly among the 12 populations of legumes, from 0.49 to 1.34 due to severe stress.

An increase in the root:shoot ratio of DM is considered to make plants more adaptive to drought, and among the 12 populations/genotypes, there was a difference in this ratio.

Population Lt12 had the highest root:shoot DM. It was also the population with the least DM growth because genotypes that invest heavily in root production have more respiration so they build up less DM.

Population Lt14 was drought-sensitive, being the most responsive to the environment, and was most affected by water stress. As conditions became more favorable, it allocated more resources to shoots and was able to optimize photosynthesis and water absorption; this genotype had the best combination of values for DM growth, root:shoot ratio of DM, and stem elongation. It is generally a good performer when the conditions are ideal.

Population Lt4 was drought tolerant and the most adaptive to stress, with the lowest DSI-value but an intermediary DM growth. This shows that the genotype has low productivity but performs well under water stress.

The agricultural scientists identified varieties Lt4 and Lt14 as genotypes with the greatest variation between each other. They recommend using these two genotypes for chromosomal selection for different traits, which could be combined to develop varieties suitable for drought.

While further research is, indeed, necessary, reccomendations regarding varieties for existing agroclimatic regions, depending on the traits studied, could have also been greatly helpful. With this information, growers could also capitalize on the vigor of natural varieties against pests and other stressors through existing adaptations not present in artificially bred varieties.

Leaf Area Variations

Though leaf area decreases as a coping mechanism due to drought, it is difficult to generalize whether the control is genotype or the environment. From both the studies considered, leaf area seems to be plastic and varies in response to the environment. This decreased its use as an indicator to breed new varieties of legumes. However, in the case of false babystars, both genotype and plasticity were important. Since studies into adaptive mechanisms have just begun, it may take time before more information is available. The relevance of leaf area may be species-specific. Studies should at least take this approach for current research on the importance of leaf area in drought response.

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

Feature Photo courtesy of Don Loarie.

Sources

Acuña, H., Inostroza, L., Sánchez, M. P., & Tapia, G. (2010). Drought-tolerant naturalized populations of Lotus tenuis for constrained environments. Acta Agriculturae Scandinavica, Section B – Plant Soil Science, 60(2), 174-181. doi:10.1080/09064710902800224

Lambrecht, S. C., Morrow, A., & Hussey, R. (2017). Variation in and adaptive plasticity of flower size and drought-coping traits. Plant Ecology, 218(6), 647-660. doi:10.1007/s11258-017-0718-x