LEAF WATER RELATIONS AND GAS-EXCHANGE IN 2 GRAIN-SORGHUM GENOTYPES DIFFERING IN THEIR PRE-FLOWERING AND POST-FLOWERING DROUGHT TOLERANCE

Two grain sorghum [Sorghum bicolor (L.) Moench.] lines (TX7078 and B35) of similar maturity but known to exhibit differences in sensitivity to drought in pre- and post-flowering growth stages were evaluated in order to quantify the physiological bases for differences in drought tolerance between lines and growth stages. The relationships among leaf water status, osmotic adjustment, leaf gas-exchange characteristics, and epicuticular wax load were characterized for irrigated and non-irrigated plants during pre- and post-flowering growth stages. Osmotic adjustment during pre-flowering stress did not differ between lines, and average adjustment was approximately 0.42MPa. In contrast, osmotic adjustment occurred only in TX7078 during post-flowering stress, and the extent of adjustment relative to irrigated plants was only 0.16 MPa. Neither osmotic adjustment nor epicuticular wax load appeared to be related to the relative drought tolerance of these genotypes during pre- or post-flowering growth stages. A reversal in the ranking of water-use efficiency (WUE) of TX7078 and B35 occurred between pre- and post-flowering growth stages. Non-irrigated plants of TX7078 exhibited significantly higher WUE than B35 at the pre-flowering stage, but the opposite was true in the post-flowering stage. The reason for the reversal in WUE is not known, but there was no significant relation between osmotic adjustment and WUE. TX7078 and B35 showed a contrasting response for intercellular CO2 concentration (C-i) in both pre- and post-flowering experiments; C-i was strongly reduced by water deficit in TX7078, but it was not significantly affected in B35, suggesting that these genotypes may differ fundamentally in their photosynthetic response to low leaf water potential.