The effect of drought stress (DS) on photosynthesis and photosynthesis-related enzyme activities was investigated in F. pringlei (C3), F. floridana (C3-C4), F. brownii (C4-like), and F. trinervia (C4) species. Stomatal closure was observed in all species, probably being the main cause for the decline in photosynthesis in the C3 species under ambient conditions. In vitro ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and stromal fructose 1,6-bisphosphatase (sFBP) activities were sufficient to interpret the net photosynthetic rates (PN), but, from the decreases in PN values under high CO2 (C a = 700 µmol mol- 1) it is concluded that a decrease in the in vivo rate of the RuBPCO reaction may be an additional limiting factor under DS in the C3 species. The observed decline in the photosynthesis capacity of the C3-C4 species is suggested to be associated both to in vivo decreases of RuBPCO activity and of the RuBP regeneration rate. The decline of the maximum PN observed in the C4-like species under DS was probably attributed to a decrease in maximum RuBPCO activity and/or to decrease of enzyme substrate (RuBP or PEP) regeneration rates. In the C4 species, the decline of both in vivo photosynthesis and photosynthetic capacity could be due to in vivo inhibition of the phosphoenolpyruvate carboxylase (PEPC) by a twofold increase of the malate concentration observed in mesophyll cell extracts from DS plants. and M. C. Dias, W. Brüggemann.
A low irradiance mediated regulation of C4 metabolism during acclimation is reported for first time in Amaranthus hypochondriacus L., a NAD-ME dicot, Eleusine coracana (L.) Gaertn., an NAD-ME monocot, and Gomphrena globosa, a NADP-ME dicot. Significant decline in activities of key C4 enzymes were observed under limited irradiances in each of the species studied. When the plants were transferred to full natural irradiance, the enzyme activities were restored to originál State in 3-5 d, a similar time frame needed for acclimation to limiting irradiance. This identifies the decarboxylation reaction involving NAD-ME in Amaranthus and Eleusine or NADP-ME in Gomphrena as crucial sites of regulation under limiting irradiance.
Understanding distribution and transport of carbon assimilates and photosynthesis contribution to grain yield in wheat spike is important in assessing the photosynthetic process under stress conditions. In this study, photosynthetic characteristics were evaluated in a pot experiment. Transport of spike photosynthates to grain was demonstrated using 14C isotope tracer technique. Yield and key enzyme activities of C3 and C4 pathways were examined after anthesis in wheat cultivars of different drought resistance. The ear net photosynthetic rate, chlorophyll content of the spike bracts (glume, lemma, and palea), and relative water content slightly decreased under water deficit in drought resistant variety Pubing 143 (Pub) during the grain filling stage, whereas all parameters decreased significantly in drought sensitive variety Zhengyin 1 (Zhe). Grain 14C-photosynthate distribution rate fell by 3.8% in Pub and increased by 3.9% in Zhe. After harvest, the water-use efficiency of Zhe dropped by 18.7% under water deficit. Rubisco activity in ear organs declined significantly under water deficit, whereas activity of C4 pathway enzymes was significantly enhanced, especially that of phosphoenolpyruvate carboxylase and NADP-malate dehydrogenase. Water deficit exerted lesser influence on spike photosynthesis in Pub. Ear organs exhibited delayed senescence. Accumulation of photosynthetic carbon assimilates in ear bracts occurred mainly during the early grain filling and photosynthates were transported in the middle of grain filling. C4 pathway enzymes seem to play an important function in ear photosynthesis. We speculate that the high enzyme activity of the C4 pathway and the increased capacity of photosynthetic carbon assimilate transport were the reasons for the drought tolerance characteristics of ears., S. Jia, J. Lv, S. Jiang, T. Liang, C. Liu, Z. Jing., and Obsahuje bibliografii