Photosynthesis in the water‐stressed C4grassSetaria sphacelatais mainly limited by stomata with both rapidly and slowly imposed water deficits

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Abstract
A comparison of the effects of a rapid and a slowly imposed water deficit on photosynthesis was performed inSetaria sphacelatavar.splendida(Stapf) Clayton, a C4NADP‐ME grass. Gas exchange was measured in rapidly and slowly dehydrated adult leaves either under atmospheric CO2partial pressure with an infrared gas analyser or under saturating CO2partial pressure with a leaf disc oxygen electrode. These measurements were used to calculate stomatal and non‐stomatal limitations to photosynthesis. These were further investigated using modulated chlorophyllafluorescence measurements and photosynthetic pigment quantification. The decrease of net photosynthesis, leaf conductance and water use efficiency was more pronounced under rapid stress than in slow stress. However, photosynthesis is always mainly limited by stomata in both types of stress, albeit the contribution of non‐stomatal limitations increases at severe water deficits in slow stress experiments. The substomatal CO2partial pressure significantly increased in both types of stress, suggesting an increased resistance due to an internal barrier to CO2diffusion. Physical alterations in the structure of the intercellular spaces due to leaf shrinkage may account for these results. The maximal photochemical efficiency of photosystem II (PSII) was remarkably resistant to stress, as theFv/Fmratio decreased only at severe water deficit. On the contrary, the effective photochemical efficiency of PSII (ΔF/F′m) measured under high actinic light decreased linearly in both types of stress, although in a more pronounced way under rapid stress. A similar variation in photochemical quenching suggests that the decrease of ΔF/F′mis mainly due to the closure of PSII reaction centres. The non‐photochemical quenching did not change significantly except under severe dehydration indicating that the energization state of thylakoids remained stable under stress. The decrease observed in photosynthetic pigments may be an adaptation to stress rather than a limiting factor to photosynthesis. Results suggests that, although intrinsic mesophyll metabolic inhibitions occur, stomatal limitation to CO2diffusion is the main reason for the decrease in photosynthesis.
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