Evaluation of assays for the analysis of thermo-tolerance and recovery potentials of seedlings of wheat (Triticum aestivum L.) cultivars

Response of eight cultivars of wheat (Triticum aestivum L.) to elevated temperature treatment during 60 h greening was examined from the analyses of leaf growth, pigmentation, membrane lipid stability, photosynthesis rates, and chlorophyll a fluorescence characteristics of primary leaves. The potential of 4 d etiolated seedlings for photomorphogenetic and photosynthetic development in response to greening at 40 °C (heat-stressed, HS), and for HS-seedlings to repair heat-injuries during 24 h post-stress growth at 25 °C was determined and compared with those of respective control seedlings greened for the same period at 25 °C. Heat-stress retarded leaf-growth and pigmentation but induced lipid peroxidation in HS-seedlings differentially among cultivars. Photosynthesis in primary leaves of HS-seedlings was markedly inhibited, 60 and 79 percnt; for HD2307 and HD2643, respectively. The quantum efficiency of photosystem (PS) 2 open centers in dark-adapted state, ΦPO (FV/FM, the ratio of variable to maximum chlorophyll a fluorescence), declined marginally, but that of PS2 open centers in light-saturated state, e′PS2 or ΦPS′ (FM′-FS/FM′), declined markedly in HS-seedlings of different cultivars. The e′PS2 values of HS-seedlings decreased marginally for HD2307, C306, and HD2329 but maximally for PBN51 cultivar. The excitation trapping efficiency of PS2, Φexc (ratio of variable to maximum fluorescence in light-saturated state, FV′/FM′) was reduced between 10-32 percnt; in HS-seedlings of different cultivars compared to respective control values. Compared to these chlorophyll fluorescence parameters, the relative fluorescence decrease (Rfd; ΔF/FS) index was more sensitive to heat-stress. Moreover, the pattern of thermal-sensitivity of this index among cultivars was comparable to that of heat-induced decline in photosynthesis rate determined from other in situ measurements. After 24 h post heat-stress, ΔF/FS index reversed markedly yet differentially, indicating varied responsiveness of cultivars tested. In summary, heat-stress inactivated photosynthesis primarily through a decline in e′PS2, the quantum efficiency of PS2 open centers, in addition to its effect on excitation trapping efficiency of PS2, Φexc. A comparative analysis of parameters revealed the potential of chlorophyll a fluorescence-derived e′PS2 and Rfd index as sensor of heat-stress; specifically the sensitivity of the latter index in screening wheat cultivars at an early developmental growth stage for seedling tolerance against heat-stress. Evaluation of photosynthetic stability in HS-seedlings and potential for post-stress reversal allowed screening of eight cultivars at four thermo-tolerance levels: thermotolerant (HD2307, HD2327), moderately thermotolerant (HD2329, C306, HD2402), thermotolerance-deficient (HD1553, HD2643), and thermosensitive (PBN51).