PHYSIOLOGICAL RESPONSES OF SORGHUM TO ITS ENVIRONMENT. I. LONG TERM EFFECTS OF SUBOPTIMAL TEMPERATURES ON DEVELOPMENT. II. MEASURING CONDUCTANCE AND WATER VAPOR AND CARBON DIOXIDE EXCHANGE IN CANOPIES

I. Eight sorghum genotypes, contrasting in temperature reaction and maturity characteristics, plus one corn hybrid were grown in two temperature regimes (22/35 and 8/27 C night/day) under a 12 h photoperiod. Low temperatures depressed growth, extended the periods from planting to panicle initiation and to anthesis, increased the total number of leaves, and increased the temperature requirement to reach both stages in most sorghum genotypes. 38 day Milo was the most insensitive sorghum genotype to these effects and actually decreased the temperature requirement to reach PI and anthesis in the 8/27 C temperature regime. Corn was also insensitive to the low temperature effects. There were no significant differences between genotypes for leaf apparent photosynthetic rate per unit leaf area (PS) within each environment. However, PS and transpiration rate (TR) were higher in the low temperature regime and were associated with higher conductance and leaf thickness. DK-28 suffered greater growth depression than Nyundo (at 8/22 C) and had a significantly higher TR and lower water use efficiency. II. A technique was developed for the transient measurement of night respiration and diurnal water vapor and CO(,2) exchange in sorghum canopies. Chamber bases were designed to avoid abnormal O(,2) depletion and CO(,2) buildup in the root zone. A Li-Cor 6000 Portable Photosynthesis System was attached to the chambers. Since the canopy was isolated from soil CO(,2) and water vapor fluxes during the transient time of measurement, it was possible to obtain measurements of conductance, transpiration and apparent photosynthesis simultaneously when the assimilation chamber was used. Canopy water use efficiency was calculated as the photosynthesis to transpiration ratio. This chamber should be useful for comparing genotypic responses under different water level and temperature treatment. The advantages this technique offers are: (1) the bases can be left in place for several days, (2) the chambers for both photosynthesis and respiration can be used alternatively during the day and night on the same base, and (3) the problems associated with an open bottom of the chambers are avoided