GROWTH-RESPONSE OF COTTON TO CO2 ENRICHMENT IN DIFFERING LIGHT ENVIRONMENTS

Experiments were conducted to examine the growth responses of cotton ( Gossypium hirsutum L. cv. Coker 315) to CO2 enrichment under different light regimes. Plants were exposed to 350 or 700 mu l(-1) CO2 and six light treatments differing in photosynthetic period length (8 or 16 h ) and in photosynthetic photon flux density (PPFD) for 32 days of vege tative growth. Higher PPFD (1100 mu mol m(-2) s(-1)) was provided by a combination of high intensity discharge and incandescent lamps (HID), and lower PPFD (550 mu mol m(-2) s(-1)) was provided by fluorescent a nd incandescent lamps (F) or HID and incandescent lamps with shade clo th (HIDs). Growth was generally much slower with the 8-h photosyntheti c periods, but the growth stimulation by CO2 enrichment was larger tha n with 16-h photosynthetic periods. After 28 to 32 days of treatment, the growth enhancement with CO2 enrichment was 152 and 78% for 8- and 16-h photosynthetic periods, respectively, under HID; 100 and 77% in F , and 77 and 56% in HIDs. The higher PPFD of HID positively influenced the CO2 effect only at the slower growth rate in the 8-h light period . The stimulation of leaf area expansion by CO2 enrichment was also gr eater with the 8-h photosynthetic period for all light sources. These results, and others on net assimilation rate, shoot to root dry weight ratios and specific leaf weights, suggest that the growth response to CO2 enrichment with the longer photosynthetic period was depressed by limiting factors, perhaps nutritional, in the growth environment. The results also show that extensive variability in CO2 response can occu r under light intensities which are often used in growth chamber exper iments.