AUTHOR=Kaiser Elias , Morales Alejandro , Harbinson Jeremy , Heuvelink Ep , Marcelis Leo F. M. 

TITLE=High Stomatal Conductance in the Tomato Flacca Mutant Allows for Faster Photosynthetic Induction

JOURNAL=Frontiers in Plant Science

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.01317

DOI=10.3389/fpls.2020.01317

ISSN=1664-462X

ABSTRACT=<p>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 <italic>flacca</italic> mutant in tomato (<italic>Solanum lycopersicum</italic>) displays very high stomatal conductance (<italic>g<sub>s</sub></italic>). We aimed to determine to what extent this substantially increased <italic>g<sub>s</sub></italic> affects the rate of photosynthetic induction. Steady-state and dynamic photosynthesis characteristics were measured in <italic>flacca</italic> and wildtype leaves, by the use of simultaneous gas exchange and chlorophyll fluorometry. The steady-state response of photosynthesis to CO<sub>2</sub>, maximum quantum efficiency of photosystem II photochemistry (<italic>F<sub>v</sub>/F<sub>m</sub></italic>), as well as mesophyll conductance to CO<sub>2</sub> diffusion were not significantly different between genotypes, suggesting similar photosynthetic biochemistry, photoprotective capacity, and internal CO<sub>2</sub> permeability. When leaves adapted to shade (50 µmol m<sup>−2</sup> s<sup>−1</sup>) at 400 µbar CO<sub>2</sub> partial pressure and high humidity (7 mbar leaf-to-air vapour pressure deficit, VPD) were exposed to high irradiance (1500 µmol m<sup>−2</sup> s<sup>−1</sup>), photosynthetic induction was faster in <italic>flacca</italic> compared to wildtype leaves, and this was attributable to high initial <italic>g<sub>s</sub></italic> in <italic>flacca</italic> (~0.6 mol m<sup>−2</sup> s<sup>−1</sup>): in <italic>flacca</italic>, the times to reach 50 (<italic>t<sub>50</sub></italic>) and 90% (<italic>t<sub>90</sub></italic>) of full photosynthetic induction were 91 and 46% of wildtype values, respectively. Low humidity (15 mbar VPD) reduced <italic>g<sub>s</sub></italic> and slowed down photosynthetic induction in the wildtype, while no change was observed in <italic>flacca</italic>; under low humidity, <italic>t<sub>50</sub></italic> was 63% and <italic>t<sub>90</sub></italic> was 36% of wildtype levels in <italic>flacca</italic>. Photosynthetic induction in low CO<sub>2</sub> partial pressure (200 µbar) increased <italic>g<sub>s</sub></italic> in the wildtype (but not in <italic>flacca</italic>), and revealed no differences in the rate of photosynthetic induction between genotypes. Effects of higher <italic>g<sub>s</sub></italic> in <italic>flacca</italic> were also visible in transients of photosystem II operating efficiency and non-photochemical quenching. Our results show that at ambient CO<sub>2</sub> partial pressure, wildtype <italic>g<sub>s</sub></italic> is a substantial limitation to the rate of photosynthetic induction, which <italic>flacca</italic> overcomes by keeping its stomata open at all times, and it does so at the cost of reduced water use efficiency.</p>