Photosynthetic responses of three citrus species to environmental factors

Gas exchange responses to irradiance, temperature, air vapor pressure deficit and intercellular CO2 concentration were evaluated in young plants of sweet orange 'Valencia', tangor 'Murcote' and acid lime 'Tahiti' plants, under controlled conditions. Maximum rates of CO2 assimilation were around 9.8, 12.8 and 13.0 mu mol m(-2) s(-1), respectively, for 'Valencia', 'Murcote' and 'Tahiti', and these differences were related to stomatal conductance and instantaneous carboxylation efficiency. Light saturation of photosynthesis was around 750 mu mol m(-2) s(-1) for 'Valencia' whereas 'Murcote' and 'Tahiti' did not show evident light saturation, exhibiting small increases of CO, assimilation above 1,000 mu mol m(-2) s(-1). The CO2 compensation point was 4.8, 5.8 and 5.4 Pa for 'Valencia', 'Murcote' and 'Tahiti', respectively, indicating differences in photorespiration of these citrus species. Leaf temperatures between 25 degrees C and 30 degrees C were optimum for photosynthesis of 'Valencia', whereas this optimum was around 30 degrees C for 'Murcote' and 'Tahiti'. At temperatures above or below the optimum range, CO2 assimilation was reduced by partial decrease of stomatal conductance and instantaneous carboxylation efficiency. Reduced CO2 assimilation rate was also caused by increasing vapor pressure deficit from 1.5 to 3.5 kPa, and this effect enhanced when temperature increased from 28 degrees C to 35 degrees C.