Fluorescence techniques as suitable methods to discriminate wheat genotypes under drought and high-temperature conditions

The chlorophyll fluorescence parameters Fv/Fo and Fd/Fs (equals Rfd690), related to the quantum conversion capacity at dark-adapted and light-adapted state of the photosynthetic apparatus respectively, have been evaluated as possible indicators of drought and heat tolerance in winter wheat. The measurements were carried out on primary leaves of 8-day old seedlings. Rfd values decreased in 8 days by 20% (p less than or equal to 0.01) only under severe water limitation and for the drought susceptible genotype. The photosynthetic apparatus was more sensitive to high temperature with both ratios, Fv/Fo and Rfd690, showing mean decrease (p less than or equal to 0.001) of 27% and 43%, respectively, in 5 days at 35 degrees Celsius. The susceptible cultivars decreased of up to 42% and 65% and the drought and heat tolerant genotypes only 7% and 12% for Fv/Fo and Rfd690, respectively. The Fv/Fo ratio correlated well (p less than or equal to 0.05 and p less than or equal to 0.01) with seedling responses to oxidative and osmotic stresses. The Rfd690-values correlated better with all physiological parameters considered and with the deviations from linear regression of drought susceptibility index DSI (r equals -0.84, p less than or equal to 0.01) on yield potential showing the highest potential to predict drought and heat tolerance. In addition the blue, green, red and far-red fluorescence have been determined using a laser-induced-fluorescence imaging system in entire seedlings of wheat and triticale grown under optimal laboratory conditions. The ratios F690/F740 and F440/F520 correlated well (p less than or equal to 0.05) with the total chlorophyll content (detected by the SPAD-chlorophyll-meter) and the specific leaf dry weight (SLDW) showing the potential of the both fluorescence ratios to discriminate genetic differences between cultivars for these leaf structural sources of water use efficiency (WUE) improvement.