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-enriche d 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 photosy nthetic electron Row, did not change significantly when CO2 rose from 250 to 4000 mu l l(-1) CO2. These results show that high CO2 concentra tion did not inhibit photosynthesis in any leaf. In contrast, the long -term response of leaves to atmospheric CO2-enrichment was variable, S ome leaves sustained the initial high level of photosynthetic stimulat ion for more than a week while in others photosynthetic CO2-uptake dec lined more or less. These leaves turned yellowish-green although chlor ophyll content declined little. Variance in the degree of leaf yellowi ng 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 'nat ural' senescence although some chlorophyll fluorescence parameters cha nged similarly. During extended high CO2 treatment the level of q(NP) increased notably in the yellowing leaves. The high levels of q(NP) an d relaxation kinetics of chi fluorescence quenching recorded upon dark ening demonstrate that thylakoid energization increased during the dec line of photosynthetic CO2 uptake in high CO2. This shows that the pho tosynthetic decline was not caused by decreasing thylakoid energizatio n because of physical damage by oversized starch grains. Calculated ph otosynthetic electron flow declined little suggesting that CO2 at ribu losebisphosphate carboxylase-oxygenase fell and thus photorespiration rose. With regard to growth limitation in high CO2 concentration, thes e results support the concept that high CO2 concentration tends to ind uce 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 energiz ation. It is proposed that the different responses of individual leave s to atmospheric CO2 enrichment reflects variety among leaves in the p hosphate status or in the capacity for Pi-recycling (assimilate utiliz ation).