Differential inhibition of photosynthesis during pre‐flowering drought stress in Sorghum bicolor genotypes with different senescence traits

Young (16‐day‐old) Sorghum bicolor plants of a late‐ and slow‐senescing Texas A&M line (B 35) and of an early‐ and fast‐senescing descendant of an Ethiopian landrace (E 36‐1) were subjected to drought stress by decreasing the soil water content to 30% field capacity over 6 days. Plant water potentials decreased from − 2 bar (controls) to − 10 to − 18 bar, and this drought stress resulted in: (1) differential phenotypic reactions and (2) differential decreases in photosynthesis rates in the two cultivars. While E 36‐1 tended to lose viable leaf area from the leaf tips downwards, B 35 showed a gradual overall drying of the leaf. At the same time, photosynthesis rates decreased from 31.5 ± 1.6 to 12.3 ± 5.0 µmol CO2 m−2 s−1 (E 36‐1) and from 30.5 ± 1.6 to 3.3 ± 2.6 µmol CO2 m−2 s−1 (B 35), respectively. In vitro enzyme activities of phosphoenolpyruvate carboxylase (PEPCase), malate dehydrogenase (MDH) and malic enzyme (ME) on a leaf area basis exceeded the photosynthesis rates. Pyruvate phosphate dikinase (PPDK) activity was close to the photosynthesis rates in control plants and higher than the photosynthesis rates in drought‐stressed plants. Thus, none of the enzymes appeared to limit photosynthesis under drought stress, and likely bottleneck enzyme activities of the C3 pathway in the bundle‐sheath cells, i.e. ribulose‐1,5‐bisphosphate carboxylase (RubisCO) and stromal fructose‐1,5‐bisphosphatase (sFBPase), also showed sufficient activities to sustain higher photosynthesis rates than those observed in the stressed plants. However, under drought stress, total leaf malate concentrations were higher in B 35 (up to 33.1 µmol g−1 FW) than in E 36‐1 (up to 22.4 µmol g−1 FW). In particular, at the presumed cytosolic pH of 7.0–7.3, S. bicolor PEPCase was strongly inhibited by malate. In contrast with the in vitro PEPCase enzyme activities, the A/Ci curves suggested a stronger decrease in the in vivo activity of the enzyme in B 35 under drought stress than in E 36‐1. It is therefore suggested that photosynthesis under drought stress may be inhibited differentially through feedback malate inhibition of PEPCase in S. bicolor.