Barley [photo] 


Improve water use through plant physiology

By Willem Jonker: Specialist - OmniSap®

Why plant physiology?

Understanding the underlying mechanisms involved during drought stress, gives insight into possible strategies to alleviate drought stress and to improve plant water use efficiency (WUE), nutrient use efficiency (NUE) and optimal plant productivity. The global impact of drought and plant stress symptoms related to drought is well documented. In this summary the focus will be on the main areas where nutrient management can be used to enhance plant production under drought stress.

Economic yield reduction though drought stress in some representative field crops
Crop Growth stage Yield reduction
Barley Seed filling 49-57%
Maize Grain filling 79-81%
Maize Reproductive 63-87%
Maize Reproductive 70-47%
Maize Vegetative 25-60%
Maize Reproductive 32-92%
Rice Reproductive (mild stress) 53-92%
Rice Reproductive (servere stress) 48-94%
Rice Grain filling (mild stress) 30-55%
Rice Grain filling (servere stress) 60%
Rice Reproductive 24-84%
Chickpea Reproductive 45-69%
Pigeonpea Reproductive 40-55%
Common beans Reproductive 58-87%
Soybean Reproductive 46-71%
Cowpea Reproductive 60-11%
Sunflower Reproductive 60%
Canola Reproductive 30%
Potato Flowering 13%

Managing plant drought stress with mineral nutrition

Under low nutrient concentrations in soil, plants have to absorb more water to be able to take up the same amount of mineral nutrients than they would from soil with satisfactory fertility. On the other hand, in conditions of lacking soil moisture, plants are unable to get optimal amounts of nutrients, which has negative effects on the overall condition of plants, especially their growth and fruit quality. As nutrient and water requirements are closely related, fertilizer application is likely to increase the efficiency of crops in using available water. This indicates a significant interaction between soil moisture deficits and nutrient acquisition. Studies show a positive response of crops to improved soil fertility under arid and semi-arid conditions. Currently, it is evident that crop yields can be substantially improved by enhancing the plant nutrient efficiency under limited moisture supply.

Nitrogen (N): Many studies have indicated changes in behaviour of NO3 assimilatory enzymes in plants under water stress conditions. Nitrate reductase (NR), the first enzyme in the pathway of nitrogen assimilation,  has been shown to decrease in water-stressed leaves of sunflower. Increased nitrogen application to water-stressed plants improves nitrate uptake and increases NR activity. Possible mechanisms to minimise the detrimental effects of drought by improving water use efficiency with N nutrition were described by Waraich et al. (2011).

Phosphorous (P): The application of P fertilizer can improve plant growth considerably under drought conditions. The positive effects of P on plant growth under drought have been attributed to an increase in stomatal conductance, photosynthesis, higher cell-membrane stability, water relations and drought tolerance. P improves the root growth and maintains high leaf water potential. This results in improved water and nutrient uptake and increases the activity of nitrate reductase which improves the assimilation of nitrate under drought conditions. 

Potassium (K): In drought-treated sunflower, the degree of stomatal opening of K-applied plants declined faster initially. However, at an equally low soil water potential, diffusive resistance in the leaves of K-applied plants remained lower than those receiving no K. Water stress causes grain yield reductions and K application could enhance yield to a great extent. Production of above ground biomass, grain yield and relative water content were highly correlated with the tissue K concentration, showing that concentration of K in leaves played a vital role in increasing water stress resistance and stabilising yield.

Calcium (Ca): Calcium has been established as a ubiquitous intracellular second messenger in plants. Calcium also improved water stress tolerance in Catharanthus roseus by increasing ã-glutamyl kinase and reducing the proline oxidase activities.

Zinc (Zn): Increases in auxin levels due to Zn application enhances the root growth which in turn improves the drought tolerance in plants. Normal auxin functions are likely to be disrupted in drought conditions. Maintaining adequate hormone levels, gives the plant a competitive advantage to withstand adverse conditions of all kinds.

Boron (B): By improving B nutrition, the detrimental effects of drought can be corrected. Boron improves the drought tolerance in plants by improving sugar transport, flower retention, pollen formation and seed germination. Seed and grain production are also increased with proper B supply. Boron nutrition under drought condition decreased the incidence of growth (rosetting), barren ears due to poor pollination, hollow stems and fruit (hollow heart) and brittle, discoloured leaves and loss of fruiting bodies.

Copper (Cu): Copper is an important micronutrient essential for carbohydrate and nitrogen metabolism. Copper is also required for lignin synthesis which is needed for cell wall strength and prevention of wilting. Proper Cu nutrition alleviates the adverse effects of drought by reducing dieback of stems and twigs, yellowing of leaves, stunted growth, pale green leaves that wither easily.

Manganese (Mn) and Iron (Fe): Studies showed anti-oxidant enzymes concentrations were increased in the range of 48-89% when treated with Fe+Zn+ Cu+Mn. The results showed that under drought stress micronutrient applications increase drought resistance in sunflower.

Silicon: It has been proven that the addition of silicon increased water use efficiency by reducing leaf transpiration and the water flow rate in the xylem vessel in maize. Silicon could also facilitate water uptake and transport in sorghum in drought conditions. Silicon nutrition increases the antioxidant production and reduces the generation of reactive oxygen species, which in turn reduces the photo-oxidative damage and maintains the integrity of chloroplast membrane and enhances the drought tolerance in plants. Ample evidence is available indicating that when silicon is readily available to plants, it plays a significant role in their growth, mineral nutrition, mechanical strength and resistance to several stresses.

Growth Regulators: Under drought, endogenous concentrations of auxins, gibberellins and cytokinin usually decrease, while those of abscisic acid and ethylene increase. Nevertheless, phyto hormones play vital roles in drought tolerance of plants. Auxins induce new root formation by breaking root apical dominance induced by cytokinins. As a prolific root system is vital for drought tolerance, auxins have an indirect but key role in this regard. Drought stress limits the production of endogenous auxins, usually when concentrations of abscisic acid and ethylene increase (abscisic acid is a growth inhibitor and produced under a wide variety of environmental stresses, including drought). All plants respond to drought and many other stresses by accumulating abscisic acid. Increases in abscisic acid and a decline in cytokinins levels favor stomatal closure and limit water loss through transpiration under water stress. When plants wilt, abscisic acid levels typically rise as a result of increased synthesis. Increased abscisic acid concentration leads to many changes in development, physiology and growth. Abscisic acid alters the relative growth rates of various plant parts such as increase in the root-to-shoot dry weight ratio, inhibition of leaf area development and production of prolific and deeper roots. It triggers the occurrence of a complex series of events leading to stomatal closure, which is an important water-conservation response. By its effect in closing stomata, abscisic acid can control the rate of transpiration and, to some extent, may be involved in the mechanism conferring drought tolerance in plants.