Stomatal factor in the water use and dry matter production by sorghum

Abstract

Direct measurement of stomatal resistance using a porometer in a sorghum crop over a range of time scales is described, and its response to changes in solar radiation, S, and soil water potential, ΨS, is analysed. Working in conductances, radiation response is given by: (k - kc) = (km - kc)(1 - e −αS) where kc is the cuticular conductance, and k and kM are the epidermal conductances (stomatal + cuticular) in normal and in maximum irradiance. With k in cm sec−1, and S in W M−2 (0.3–3.0 μm region), the best value of α for sorghum is 2 · 10−3. From a smaller set of observations, the response of the stomatal component, (kM - kc) to mean soil water potential measured on the field in the top 30 cm of the soil is: (kM - kc) = (kM∗ - kc)(1 − 0.75 exp ]-exp[0.25(-Ψs - 6.9)][) where kM∗ is the conductance at full turgor. When allowance is made for leaf area index, the porometer measurements of stomatal resistance agree very well, both on daily and hourly basis, with canopy resistances calculated from the simultaneous micrometeorological records, further demonstrating the validity of the canopy resistance concept. Sample calculations of water use in sorghum, incorporating realistic values for leaf area index and stomatal resistance in the modified Penman formula gave 315 mm of evaporation for the growing season of May, June and July; in good agreement with measured crop water use of 325 mm in Central Texas. Repeating the calculations suggests that doubling crop density of the present practice and irrigating to maintain stomatal resistances near minimum may increase yields by about 100%, whereas evaporation by only 33%, significantly improving the efficiency of water use.