Environmental and physiological control of transpiration by groundnut crops

In central India, four populations of groundnut were grown to assess the interaction between population and water stress.Transpiration was calculated from measurements or estimates of stomatal resistance, r_s, boundary layer resistance, r_a, vapour concentration difference between leaf and air, δ_χ, and leaf area index, L.The frequency distributions of r_s, r_a, δ_χ and seasonal changes in L were plotted to analyse the dependence of transpiration rate on each variable, both per unit area of leaf surface, E_l, and per unit land surface, E_e. For estimates of E_l, both r_s and δ_χ were of similar importance, exerting a far greater influence than changes in r_a. However, in terms of E_e, changes in L were far more important than in any other variable, particularly late in the season when water was scarce.This study provides further experimental support for estimates of evaporation based on stomatal resistance and allied measurements and confirms similar estimates obtained earlier for crops of pearl millet maintained on stored water. The ability of the technique to describe temporal and spatial variations as well as the dominant environmental and physiological influences on transpiration may outweigh any small loss in accuracy of estimates thus obtained.