Evidence for light-dependent recycling of respired carbon dioxide by the cotton fruit

Conservation of respired CO₂ by an efficient recycling mechanism in fruit could provide a significant source of C for yield productivity. However, the extent to which such a mechanism operates in cotton (Gossypium hirsutum L.) is unknown. Therefore, a combination of CO₂ exchange, stable C isotope, and chlorophyll (Chl) fluorescence techniques were used to examine the recycling of respired CO₂ in cotton fruit. Respiratory CO₂ losses of illuminated fruit were reduced 15 to 20% compared with losses for dark-incubated fruit. This light-dependent reduction in CO₂ efflux occurred almost exclusively via the fruit's outer capsule wall. Compared with the photosynthetic activity of leaves, CO₂ recycling by the outer capsule wall was 35 to 40% as efficient. Calculation of ¹⁴CO₂ fixation on a per Chl basis revealed that the rate of CO₂ recycling for the capsule wall was 62.2 micromoles ¹⁴CO₂ per millimole Chl per second compared with an assimilation rate of 64.6 micromoles ¹⁴CO₂ per millimole Chl per second for leaves. During fruit development, CO₂ recycling contributed more than 10% of that C necessary for fruit dry weight growth. Carbon isotope analyses (δ¹³C) showed significant differences among the organs examined, but the observed isotopic compositions were consistent with a C₃ pathway of photosynthesis. Pulse-modulated Chl fluorescence indicated that leaves and fruit were equally efficient in photochemical and nonphotochemical dissipation of light energy. These studies demonstrated that the cotton fruit possesses a highly efficient, light-dependent CO₂ recovery mechanism that aids in the net retention of plant C and, therein, contributes to yield productivity.