Far-red light enhances mesophyll conductance and photosynthesis in tomato on short timescale below sunlight far-red fraction only

Abstract

Far-red (FR) light can enhance photosynthesis over short timescales, even at irradiances above the light-limited range. However, it remains unclear whether increasing FR beyond the fraction naturally present in direct sunlight further stimulates photosynthesis, and whether changes in mesophyll conductance to CO2 (gm) contribute to this response. Tomato leaves developed under artificial sunlight-mimicking PAR (16 h; 300 μmol PAR m−2 s−1) with a far-red fraction representative of direct sunlight (R:FR ratio 1.1) were exposed to two actinic light regimes (constant and sinusoidal intensity patterns) combined with three R:FR ratios (10.5, 1.1 and 0.48), representing FR fractions below, similar to, and above those occurring in direct sunlight. High-temporal-resolution gas exchange and carbon isotope (δ13C) measurements (0.2 Hz) were used to quantify gm, stomatal conductance (gs), net CO2 assimilation (Anet) and chloroplastic CO2 concentration. Increasing FR from a low level to the fraction occurring in direct sunlight enhanced both gm and Anet throughout the photoperiod in both actinic light regimes. However, a further increase in FR to levels characteristic of foliar shade did not result in additional increases in either gm or Anet. Estimates of chloroplastic CO2 concentration suggest that the FR-induced increase in apparent gm may help sustain chloroplastic CO2 supply as assimilation increases, but is not sufficient to explain the stimulation of Anet. These results indicate that the short-term photosynthetic response to FR is already established at the FR fraction occurring in direct sunlight and does not increase further with additional FR.