RESPONSE OF COTTON TO VARYING CO2, IRRIGATION, AND NITROGEN - YIELD AND GROWTH

The CO2 concentration of the atmosphere is increasing and is expected to double sometime near the middle of the next century. To determine t he effects of such a CO2 increase on cotton (Gossypium hirsutum L.) gr owth and productivity, a series of experiments from 1983 through 1987 were conducted with open-top CO2-enriched field chambers at ample as w ell as limiting levels of water and N at Phoenix, AZ. Comparisons with open-field plots showed that there was a significant chamber effect, amounting to a 30% average increase in growth inside, but under dry co nditions in 1985, the situation was reversed. No significant effects o f CO2 on harvest index, root-shoot ratio, or lint percentage were foun d, so the primary effect of elevated CO2 was to produce plants that we re larger. Comparing the results of 500 and 650 mumol mol-1 CO2 treatm ents, the increments of growth from ambient (about 350 mumol mol-1) to 500 mumol mol-1 were not significantly different from increments from 500 to 650 mumol mol-1. No statistically significant interactions wer e detected between CO2 level and either irrigation or nitrogen level, even when these variables were sufficiently low enough to limit growth . However, under well-maintained water stress conditions, the growth r esponse to CO2 tended to be somewhat larger than under normal irrigati on levels. Averaging over all the data available from these experiment s, seed cotton yield (lint plus seed) and above-ground biomass were in creased by 60 and 63%, respectively, by CO2 enrichment to 650 mumol mo l-1.