AMINO ACIDS AND THEIR ROLE IN DROUGHT RESISTANCE

WATER AVAILABILITY is a major limiting factor for plant growth and it is the main factor responsible for yield reductions in many crops. Water stress during drought limits the expansion of cells which results to restricted internode elongation and leaf expansion. It also causes losses in water content of plant tissues thus reducing cell rigidity. This inhibits enlargement and division of cells resulting to reduced plant growth and dry mass accumulation.[ The application of biostimulants such as amino acids was found to give positive effects on crops affected by environmental stresses such as drought. The importance of amino acids in plant development comes from their wide use in biosynthesis of different non-protein nitrogenous compounds such as vitamins, coenzymes, pigments, purine and pyrimidine bases.[ Moreover, studies have shown that amino acids directly and indirectly affects the plant yield and development

ROOT DEVELOPMENT The allocation of growth between the shoots and roots is a process that highly depends on environmental conditions. Nitrogen supply is a major determinant in this growth distribution process. A more general idea of nitrogen effects on plant biomass allocation is that at high rates of N supply, plants sense a decreased need for nitrogen uptake and results to decreased root growth and an increase need for carbon uptake and, hence, for shoot growth. Plants fed with high levels of nitrogen increases in shoot length but not the lateral length of its roots.

Amino acids serve as nitrogen sources for plants and has several advantages over inorganic sources of nitrogen with respect to the development of plant’s root system. Too much inorganic nitrogen absorbed by plants makes it vulnerable when subjected to water stress. High concentrations of nitrates (NO3-) result to inhibition of flow of auxins. Auxins are plant hormones that have significant role in coordination of many growth and behavioral processes in the plant’s life cycle and are essential for plant body development including lateral root development Thus, high concentrations of chemical fertilizers result in increased above-soil growth but poor root development. This problem is magnified during drought when the undeveloped root system cannot hold enough water to sustain plant growth. To overcome environmental stresses, plants need to modify root structure to enable them to forage heterogeneously distributed nutrients in the soil. Foraging response normally involves increased proliferation of lateral roots. Amino acids, specifically L-glutamate has been shown to induce foraging mechanism. This is characterized by slowing of primary root growth and increased root branching which increases the precision of root placement within the soil. Furthermore, there is strong evidence that both mycorrhizal and non-mycorrhizal plants in a variety of ecosystems directly absorb amino acids from the soil and through foliar feeding. Plants that have access to organic N directly would no longer have to rely on microbial mineralization to produce N from inorganic sources-which is generally considered to be bottleneck in the N cycle in soils.

OSMOTIC REGULATION DURING DROUGHT Osmotic pressure plays a fundamental role in water stress responses

Figure 1: Allocation of growth between roots and shoots. (At high nitrogen supply, plants sense a greater need for carbon uptake, from carbon dioxide, and thus increases its shoots while decreasing root lateral growth.)