ATMOSPHERIC CO2 ENRICHMENT - KINETICS OF CHLOROPHYLL A FLUORESCENCE AND PHOTOSYNTHETIC CO2 UPTAKE IN INDIVIDUAL, ATTACHED COTTON LEAVES

Chl fluorescence and gas exchange of attached cotton leaves (Gossypium hirsutum L.) were measured in ambient air and in a highly CO2-enriched atmosphere (4000 mu l l(-1) CO2; photosynthetic saturation). In the shore term (hours to one day), net CO2 uptake approximately doubled in all leaves examined. Photochemical (q(P)) and nonphotochemical (q(NP)) quenching of chlorophyll fluorescence, and calculated linear photosynthetic electron Row, did not change significantly when CO2 rose from 250 to 4000 mu l l(-1) CO2. These results show that high CO2 concentration did not inhibit photosynthesis in any leaf. In contrast, the long-term response of leaves to atmospheric CO2-enrichment was variable, Some leaves sustained the initial high level of photosynthetic stimulation for more than a week while in others photosynthetic CO2-uptake declined more or less. These leaves turned yellowish-green although chlorophyll content declined little. Variance in the degree of leaf yellowing was also encountered in experiments with clover when sets of plants were CO2-enriched. Gas exchange and chi fluorescence results suggest that yellowing of cotton leaves in high CO2 was not equivalent to 'natural' senescence although some chlorophyll fluorescence parameters changed similarly. During extended high CO2 treatment the level of q(NP) increased notably in the yellowing leaves. The high levels of q(NP) and relaxation kinetics of chi fluorescence quenching recorded upon darkening demonstrate that thylakoid energization increased during the decline of photosynthetic CO2 uptake in high CO2. This shows that the photosynthetic decline was not caused by decreasing thylakoid energization because of physical damage by oversized starch grains. Calculated photosynthetic electron flow declined little suggesting that CO2 at ribulosebisphosphate carboxylase-oxygenase fell and thus photorespiration rose. With regard to growth limitation in high CO2 concentration, these results support the concept that high CO2 concentration tends to induce low inorganic phosphate concentrations (Morin et al. Plant Physiol. 99, 89-95, 1992; Duchein et al. J. Exp. Bet. 44, 17-22, 1993) which can limit chloroplast ATP synthase and thus increase thylakoid energization. It is proposed that the different responses of individual leaves to atmospheric CO2 enrichment reflects variety among leaves in the phosphate status or in the capacity for Pi-recycling (assimilate utilization).