INFLUENCE OF ELEVATED CO2 AND MILD WATER-STRESS ON NONSTRUCTURAL CARBOHYDRATES IN FIELD-GROWN COTTON TISSUES

Root, stem and leaf tissues, from cotton plants exposed to CO2 at ambi ent (370 mumol mol-1 (control)) or elevated (550 mumol mol-1 (FACE; fr ee-air carbon dioxide enrichment)) levels in the field during the 1990 and 1991 growing seasons, were analyzed for nonstructural carbohydrat es (glucose, fructose, sucrose and starch). Besides the FACE treatment , these plants were also exposed to two irrigation levels: 100% and 67 % replacement of evapotranspiration. FACE had a greater effect upon co tton plant nonstructural carbohydrates than did irrigation treatments. Leaf carbohydrate content was increased by FACE, but this increase wa s much more pronounced in the stems and roots. Starch and soluble suga rs in leaves in FACE plots tended to be consistently greater than in c ontrol leaves, without much change in carbohydrate content during the growing season. In contrast, root and stem, starch and soluble sugar p ools were strongly increased by FACE and fluctuated strongly during th e growing season. In both seasons, stem and taproot nonstructural carb ohydrate content passed through a minimum during periods of heavy boll set. The fluctuations in stem and root carbohydrate content were ther efore probably caused by the varying metabolic demands of the developi ng plant. These results suggest that a significant effect of CO2 enric hment on starch-accumulating plants is an increase of nonstructural ca rbohydrate, especially starch, in nonleaf storage pools. This buildup occurs somewhat independently of the water status of the plant, and th ese enlarged pools can be drawn upon by the growing plant to maintain growth during periods of high metabolic demand.