WATER-DEFICIT, LEAF ROLLING AND SUSCEPTIBILITY TO PHOTOINHIBITION IN-FIELD GROWN SORGHUM

Chlorophyll fluorescence and gas-exchange techniques were used to inve stigate chang es in photosynthetic performance in response to high fig ht and mild water deficit, in two cultivars of the C-4 plant sorghum ( Sorghum bicolor [L.] Moench), grown under field conditions. For all le aves fully exposed to the sun, the efficiency of photosystem II (PSII) showed a mid-day decline, but with substantial over-night recovery; t he magnitude of the mid-day decline was enhanced by water deficit. The re was no corresponding decline in leaves not exposed to full sunlight , either because they were shaded by other leaves or else because of l eaf-rolling. Net assimilation rates appeared more sensitive to water-d eficit than was PSII efficiency. Shade-adapted leaves had lower rates of photosynthesis in full sun (and lower stomatal conductances) than w ell-exposed leaves. When these shade-adapted leaves were suddenly expo sed to full sunlight, fluorescence quenching was slow, especially when plants were well-watered. For the latter, photochemical quenching (q( P)) was small even after several minutes, indicating a continuing imba lance between energy funnelled to PSII and subsequent electron transpo rt. Shade-adapted leaves that were water stressed were better able to withstand a sudden increase in irradiance than those that were well wa tered. It is suggested that the shade-adapted leaves from unirrigated plants, having a lower stomatal conductance than the irrigated leaves, had been acclimated by receiving energy in excess of that required to fix CO2 thus leading to the operation of dissipative mechanisms. A sh ortened protocol for quenching analysis is proposed that enables non-p hotochemical quenching to be partitioned into rapidly and slowly relax ing components (the latter including photoinhibition) by relating resu lts to a theoretical maximum yield of variable fluorescence. This is p articularly suitable for screening field material.