Abstract
Due to their slow movement and closure upon shade, partially closed stomata can be a substantial limitation to photosynthesis in variable light intensities. The abscisic acid deficient flacca mutant in tomato (Solanum lycopersicum) displays very high stomatal conductance (gs). We aimed to determine to what extent this substantially increased gs affects the rate of photosynthetic induction. Steady-state and dynamic photosynthesis characteristics were measured in flacca and wildtype leaves, by the use of simultaneous gas exchange and chlorophyll fluorometry. The steady-state response of photosynthesis to CO2, maximum quantum efficiency of photosystem II photochemistry (Fv/Fm), as well as mesophyll conductance to CO2 diffusion were not significantly different between genotypes, suggesting similar photosynthetic biochemistry, photoprotective capacity, and internal CO2 permeability. When leaves adapted to shade (50 µmol m−2 s−1) at 400 µbar CO2 partial pressure and high humidity (7 mbar leaf-to-air vapour pressure deficit, VPD) were exposed to high irradiance (1500 µmol m−2 s−1), photosynthetic induction was faster in flacca compared to wildtype leaves, and this was attributable to high initial gs in flacca (~0.6 mol m−2 s−1): in flacca, the times to reach 50 (t50) and 90% (t90) of full photosynthetic induction were 91 and 46% of wildtype values, respectively. Low humidity (15 mbar VPD) reduced gs and slowed down photosynthetic induction in the wildtype, while no change was observed in flacca; under low humidity, t50 was 63% and t90 was 36% of wildtype levels in flacca. Photosynthetic induction in low CO2 partial pressure (200 µbar) increased gs in the wildtype (but not in flacca), and revealed no differences in the rate of photosynthetic induction between genotypes. Effects of higher gs in flacca were also visible in transients of photosystem II operating efficiency and non-photochemical quenching. Our results show that at ambient CO2 partial pressure, wildtype gs is a substantial limitation to the rate of photosynthetic induction, which flacca overcomes by keeping its stomata open at all times, and it does so at the cost of reduced water use efficiency.
Highlights
In the leaves of higher plants, stomata balance carbon uptake against water loss
Plants were grown in a climate chamber under a day/night cycle of 16/8 h, 20/18°C temperature, ambient CO2 partial pressure, 70% relative air humidity, and 154 μmol m−2 s−1 photosynthetically active radiation (PAR; measured 10 cm above table height), which was provided by fluorescent tubes (Master TL-D 58W/840 Reflex Eco; Philips, Eindhoven, the Netherlands)
Mesophyll conductance and its components were not significantly different between genotypes, flacca tended to show greater values for A, J, mitochondrial respiration (Rd), and chloroplast CO2 partial pressure (Cc) (Figure 2)
Summary
In the leaves of higher plants, stomata balance carbon uptake against water loss. They achieve this balance by dynamically regulating stomatal aperture in response to intrinsic and extrinsic factors. The other two main limitations during photosynthetic induction arise from slow rates of change in the activity of enzymes involved in ribulose-1,5bisphosphate (RuBP) regeneration, and from slow activation of Rubisco (Pearcy et al, 1996; Way and Pearcy, 2012; Kaiser et al, 2015; Kaiser et al, 2018). These limitations occur to the limitations at steady state due to, e.g., the rate of electron transport, Calvin cycle metabolism, sucrose metabolism, or mesophyll conductance (gm, mol m−2 s−1)
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