AUTHOR=Suwannarut Wachira , Vialet-Chabrand Silvere , Kaiser Elias 

TITLE=Diurnal decline in photosynthesis and stomatal conductance in several tropical species

JOURNAL=Frontiers in Plant Science

VOLUME=14

YEAR=2023

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1273802

DOI=10.3389/fpls.2023.1273802

ISSN=1664-462X

ABSTRACT=<p>Photosynthesis (<italic>A</italic>) and stomatal conductance (<italic>g<sub>s</sub></italic>) change diurnally due to internal signals, but the effects of diurnal rhythms on dynamic photosynthetic behavior are understudied. We examined diurnal changes in <italic>A</italic> and <italic>g<sub>s</sub></italic> in ten tropical species: across species, there was a tendency for <italic>A</italic> and <italic>g<sub>s</sub></italic> to decline diurnally when these were repeatedly measured under either steady-state or fluctuating irradiance conditions. We then examined in more detail the irradiance-induced kinetics of gas exchange in a C<sub>3</sub> and C<sub>4</sub> crop species each, namely fig (<italic>Ficus carica</italic>) and sugarcane (<italic>Saccharum officinarum</italic>). During the day, fig showed significantly slower photosynthetic induction and lower <italic>g<sub>s</sub></italic>, as well as a slower <italic>g<sub>s</sub></italic> increase, in the afternoon than in the morning and noon. Sugarcane showed a reduction in steady-state <italic>A</italic> reached under high irradiance and slower <italic>g<sub>s</sub></italic> increase as well as lower <italic>g<sub>s</sub></italic> reached under high irradiance, but no changes in the rate of photosynthetic induction, in the afternoon, compared to morning and noon. These reductions in the afternoon were not reverted by a dark treatment in the middle of the day, suggesting that the decrease was not proportional to diurnal time-integrated carbon fixation. Repeated exposure to light- and shadeflecks (1000 and 50 μmol m<sup>-2</sup> s<sup>-1</sup>, lasting 20 min each) revealed fundamental differences in stomatal regulation between species: in fig, stomata opened and closed slowly, and their opening became progressively slower under a series of lightflecks, whereas sugarcane showed much faster stomatal opening than closure that was unchanged during the course of the day. Our results highlight that steady-state rates and irradiance-induced kinetics of photosynthesis and stomatal movement change diurnally in most species studied, and that they do so differently in fig and sugarcane.</p>