Mechanisms of Drought Resistance in Grain Legumes I: Osmotic Adjustment

When crops are exposed to drought there is commonly an increase in solute-pool of the plant tissue. A series of experiments were conducted in 1994 and 1995 in the University of Hohenheim, Germany, to evaluate whether drought-induced solute accumulation per se will improve drought resistance, to determine the species of active osmotica involved, and to identify alternative mechanisms in grain legumes, namely Common bean (Phaseolus vulgaris) faba bean (Vicia faba), pea (Pisum sativum), and chickpea (Cicer arietinum). Drought, equivalent to soil water potential of -0.64 MPa, decreased the leaf water potential to -1.67, -1.83, -0.34 and -1.32 MPa and resulted in dry matter yield reduction of 36.4%, 23.9%, 17.6% and 14.5% in faba bean, pea, common bean, and chickpea, respectively. Higher yield of chickpea and common bean under drought was augumented by turgor maintenance. The highest degree of turgor (1.0 MPa) was in common bean, though it had the lowest solute accumulation. Thus, a decrease in osmotic potential was not the only strategy of turgor maintenance in legumes. Increase in solute pool in all was due to concentration effect as a result of water loss and growth inhibition, except in chickpea. The major osmotica in chickpea were of organic origin, namely sugars and sugar alcohols contributing more than 50%, and amino compounds 20% of the osmotic pool. Calcium was the only inorganic osmoticum contributing 19% of the pool. Concentration of solutes in the cell-sap did not necessarily indicate the level of contribution to the osmotic pool, as the most abundant ions (e.g., potassium) did not contribute to osmotic adjustment under drought. It was concluded that it is vital to differentiate solutes accumulated as a concentration effect from active osmotica using cell water volume of control plants before considering solute concentration as a selection criterion for breeding drought resistant varieties/crops. Key words/phrases: Active osmotica, concentration effect, drought, grain legumes