The intensity of and recovery from photoinhibition under drought in a thermotolerant common bean compared to drought tolerant genotypes

The chlorophyll a fluorescence parameters of four Phaseolus vulgaris L. genotypes were evaluated under drought in two greenhouse experiments. Under severe water stress, the thermotolerant genotype 'Diplomata' maintained significantly higher values of predawn leaf water potential (Psi(w)), maximum F-v/F-m and effective (FPSII) quantum yield of photosystem II, and non-photochemical quenching than 'Ouro Negro', in the first experiment, and 'A 285' and 'A 222', in the second one. Among these parameters, F-v/F-m showed more differences that discriminated between the genotype responses even when measured at night. Next, a difference between F-v/F-m after sundown and F-v/F-m at dawn on the same day (day Delta F-v/F-m), i.e., the intensity of photoinhibition, and a difference between F-v/F-m at dawn and F-v/F-m after sundown on the day before (night Delta F-v/F-m), i.e. the photoinhibition recovery, were evaluated. Day Delta F-v/F-m and night Delta F-v/F-m were significantly higher for 'Diplomata' under severe water stress in both experiments. In addition, 'Ouro Negro' in the first experiment and all the genotypes in the second showed negative values of night Delta F-v/F-m on the last day of drought when their Psi(w) were also minimal indicating no recovery from photoinhibition and the need for rehydration. At maturation, stressed plants of 'Diplomata' showed a significantly higher yield than 'Ouro Negro' in the first experiment and the same as 'A 285' in the second. Therefore, the thermotolerant genotype 'Diplomata' also showed drought tolerance, and the use of day Delta F-v/F-m and night Delta F-v/F-m fluorescence analysis was able to discriminate between the tolerances of these genotypes and to indicate the need for rehydration.