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

Abstract

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.

Differential inhibition of photosynthesis under drought stress in Flaveria species with different degrees of development of the C₄ syndrome

“…As in other C 3 species (Sharkey and Seemann 1989, Vassey and Sharkey 1989, Lal et al 1996, Escalona et al 1999, stomatal closure strongly reduced the intercellular CO 2 availability in F. pringlei (Table 1). In the other three species, the LiCor calculation revealed no significant decrease of C i , in accordance with previous studies on other C 4 plants (Premachandra et al 1994, Prakash and Rao 1996, Yu et al 2004, Beyel and Brüggemann 2005. However, the reliance of C i calculations from CO 2 and H 2 O exchange rates in DS plants is strongly under debate, not only because of the masked effects of stomatal patchiness (Terashima et al 1988, Mansfield et al 1990, Terashima 1992, but also because of the different diffusion resistances of the cuticle for CO 2 and H 2 O (Boyer et al 1997).…”

“…In Sorghum, where a similar finding was observed, Western blots gave no indications of a decrease of RuBPCO protein content in DS plants (Beyel and Brüggemann 2005). According to Rogers et al (2001), who studied pine needle tissue, insufficient extraction of RuBPCO before the analysis may often be the cause for low RuBPCO activities in vitro in leaves with strong cell walls.…”

“…According to Rogers et al (2001), who studied pine needle tissue, insufficient extraction of RuBPCO before the analysis may often be the cause for low RuBPCO activities in vitro in leaves with strong cell walls. However, any attempt to increase in vitro RuBPCO activities of C plants to or beyond their photosynthetic capacity by modifying the extraction procedure (liquid N 2 and excessive grinding) failed both in Sorghum (Beyel and Brüggemann 2005) and in Flaveria.…”

“…Electrophoresis: For electropheretic and Western blot analyses of leaf extracts according to standard procedures (Laemmli 1970, Beyel andBrüggemann 2005), preparations in HB without bovine serum albumin were performed and aliquots equivalent to defined leaf areas were run on 15 % PAA gels. Proteins were identified both by Coomassie Brilliant Blue staining and by electroblotting and incubation with anti-RuBPCO large subunit (Secale cereale) antiserum (Dr. Schmidt, University of Frankfurt, Germany, 1 : 5 000 dilution) or anti-PEPC (Sorghum bicolor) antiserum (Prof. Westhoff, University of Düsseldorf, Germany, 1 : 5 000 dilution).…”

Water-use efficiency in Flaveria species under drought-stress conditions

“…Threeto four-week-old plants were exposed to drought stress (DS) by receiving only so much water every evening to ensure a water content corresponding to 30% field capacity overnight, corresponding to water potential of approx. -1.6 MPa (Scheffer andSchachtschnabel 2002, Beyel andBrüggemann 2005). After 3-4 days, plants reached a water potential between -1.0 and -1.8 MPa during the DS treatment, which persisted for three days further.…”