Photosynthetic responses of three citrus species to environmental factors

Abstract

Gas exchange responses to irradiance, temperature, air vapor pressure deficit and intercellular CO₂ concentration were evaluated in young plants of sweet orange 'Valência', tangor 'Murcote' and acid lime 'Tahiti' plants, under controlled conditions. Maximum rates of CO₂ assimilation were around 9.8, 12.8 and 13.0 mmol m^-2 s^-1, respectively, for 'Valência', 'Murcote' and 'Tahiti', and these differences were related to stomatal conductance and instantaneous carboxylation efficiency. Light saturation of photosynthesis was around 750 mmol m^-2 s^-1 for 'Valência', whereas 'Murcote' and 'Tahiti' did not show evident light saturation, exhibiting small increases of CO₂ assimilation above 1,000 mmol m^-2 s^-1. The CO₂ compensation point was 4.8, 5.8 and 5.4 Pa for 'Valência', 'Murcote' and 'Tahiti', respectively, indicating differences in photorespiration of these citrus species. Leaf temperatures between 25^oC and 30^oC were optimum for photosynthesis of 'Valência', whereas this optimum was around 30^oC 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^oC to 35^oC.