Effects of drought during grain filling on PS II activity in rice.

The Rice varieties Araure 4 (A4) and Fonaiap 2000 (F2000) were grown in the glasshouse under natural sunlight and subjected to drought at heading. The drought induced changes in chlorophyll a fluorescence parameters, pigment composition, D1 contents and carbohydrate accumulation were investigated. Drought decreased phiPS II, FV'/FM' and qP, and increased qN in both varieties. F2000 had larger values of phiPS II and FV'/FM' at a lower RWC than A4. With the onset of drought only A4 increased the xanthophyll cycle pool, F2000 remaining constant throughout the drought cycle. Irrigated plants of A4 had a Larger de-epoxidation state (DEPS) of the xanthophyll cycle than F2000. A 40% increase in DEPS was induced by drought in both varieties but in A4 it was attained at a larger RWC than in F2000. Drought increased glucose and fructose contents of leaves 8-fold in A4 and 3-fold in F2000. Contrarily, sucrose contents decreased with drought but the effects were larger in A4 than in F2000. Sugars accumulation preceded and was proportional to the decrease in PS II activity elicited by drought in both varieties. In F2000 a decrease in D1 content smaller than 20% occurred at 70% of RWC, whereas droughted plants of A4 had lost 80% of D1 protein at 77% of RWC. Our data show that drought severely affected PS II activity and its main regulatory mechanisms in rice. There are genotypic differences in the response of PS II activity to drought that could be exploited as traits for selection to drought tolerance. There is a possible link between the drought-induced sugars accumulation in the flag leaf and the response of PS II to water deficit.