Diurnal decline in photosynthesis and stomatal conductance in several tropical species

Abstract

Photosynthesis ( A ) and stomatal conductance ( g s ) change diurnally due to internal signals, but the effects of diurnal rhythms on dynamic photosynthetic behavior are understudied. We examined diurnal changes in A and g s in ten tropical species: across species, there was a tendency for A and g s 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 3 and C 4 crop species each, namely fig ( Ficus carica ) and sugarcane ( Saccharum officinarum ). During the day, fig showed significantly slower photosynthetic induction and lower g s , as well as a slower g s increase, in the afternoon than in the morning and noon. Sugarcane showed a reduction in steady-state A reached under high irradiance and slower g s increase as well as lower g s 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 -2 s -1 , 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.

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