EFFECTS OF ENVIRONMENT DURING GROWTH ON THE SENSITIVITY OF LEAF CONDUCTANCE TO CHANGES IN HUMIDITY

Soybeans (Glycine max) and grain amaranth (Amaranthus hypochondriacus) were grown at a range of temperatures, carbon dioxide concentrations and light conditions in controlled environment chambers, and the respo nse of leaf conductance to water vapour to changes in humidity was the n measured under a standard set of conditions. The sensitivity of cond uctance was analysed in terms of (i) the absolute sensitivity of condu ctance to changes in leaf to air water vapour pressure difference (LAV PD), (ii) the sensitivity of conductance relative to the absolute valu e of conductance, and (iii) the slope of the relationship between cond uctance and an index incorporating assimilation rate, carbon dioxide c oncentration and relative humidity. The sensitivity of conductance var ied substantially with growth conditions for all three analyses in bot h species. The growth temperature of 25 degrees C increased the sensit ivity of conductance by all three measures compared with growth at 20 or 30 degrees C in amaranth, with little difference between 25 and 30 degrees C in soybean. Growth at elevated carbon dioxide decreased sens itivity in amaranth by all three measures, and decreased the absolute but not the relative sensitivity in soybean. Growth at reduced photon flux density and growth at high stand density reduced sensitivity in a maranth by all three measures. In soybean, growth at high stand densit y reduced sensitivity by all three measures, but growth at low photon flux density increased the relative sensitivity. The sensitivity of le af conductance to changes in humidity varied by a factor of two or mor e with growth environment by all measures of sensitivity in both the C 3 and the C4 species.