Strongly increased stomatal conductance in tomato does not speed up photosynthetic induction in ambient nor elevated CO2 concentration

Irradiance-dependent opening and closure of stomata is comparably slow. Therefore, stomatal conductance (gs) often limits photosynthesis in fluctuating irradiance, particularly after large stepwise increases in irradiance in leaves adapted to low irradiance or darkness. This limitation to photosynthesis may reduce crop productivity in natural environments and greenhouses, where irradiance incident on a leaf can fluctuate rapidly. To test whether this limitation applies to typical greenhouse crops such as tomato (Solanum lycopersicum L.), photosynthetic gas exchange of flacca, a mutant with very high gs (2-4 times wildtype level) was compared with its wildtype (WT), cv. Rheinlands Ruhm. The steady-state response to leaf internal CO2 concentration (Ci) was similar between genotypes, indicating similar properties of the leaf photosynthetic apparatus. Surprisingly, when exposing dark-adapted leaves in ambient CO2 concentration (400 ppm) to a stepwise increase in irradiance, photosynthetic induction was not faster in flacca than in WT, despite flacca having 3.5 times higher gs in darkness (0.26 ± 0.03 mol·m-2·s-1 in WT; 0.90 ± 0.01 m-2·s-1 in flacca). The same was true for leaves in 800 ppm, despite dark-adapted gs being 4.6 times higher in flacca. During photosynthetic induction of leaves in 200 ppm CO2 concentration, however, photosynthesis in flacca had a significantly higher induction state 60 seconds after illumination (19% in WT; 35% in flacca; P=0.04), and took less time to reach 90% of full photosynthetic induction (19.5 minutes in WT; 8.5 minutes in flacca; P=0.04). Hence, the rate of photosynthetic induction was only increased in reduced CO2 concentration in flacca, but not in ambient nor elevated CO2, indicating that wildtype gs did not pose a limitation to photosynthetic induction in ambient nor elevated CO2 concentration.