Pima cotton is grown mostly in Arizona, New Mexico, California and high plains of Texas where extreme temperatures often prevail. Current literature on Pima cotton growth and development is limited. Quantitative information on Pima cotton responses to temperature would be useful for accurate yield predictions by crop models.

Cotton, Gossypium barbadense L., cv. S-6, plants were grown from seed in a 3:1 sand, vermiculite growing medium using Soil-Plant-Atmospheric-Research (SPAR) units at the Crop Simulation Research Unit, Mississippi State during the summer of 1990. Seedlings emerged in four-days and were grown in the units until 64 days after emergence (DAE). Plant growth and development were measured in 20/12, 25/17, 30/22, 35/27 and 40/32 C day/night temperature regimes. Carbon dioxide was maintained at 350 µL L^-1. The plants were irrigated with a drip system and were grown under luxuriant water and nutrient conditions.

Plant height, main-stem nodes and leaf lengths were measured weekly. Early season estimates of plant component dry matter accumulation was measured by harvesting six rows, five plants each, at DAE 16 and two rows at DAE 26 leaving three rows to 64 DAE with a final a population of 15 plants m-2. Leaf area was measured at 16, 26 DAE and at the final harvest. Expansion and development of individual leaves on the main-stem were measured daily. At the final harvest all plant component dry weights, including roots, were dried to constant weight at 75 C to determine biomass partitioning of various plant organs.

Stem lengths of cotton seedlings were low and insensitive to temperature for about 15 days after emergence, after which, they became very temperature sensitive. Plants at 35/27 C were taller than those of grown at lower temperatures. Growth of cotton seedlings at 40/32 C was similar to those grown at 35/27 C up to 43 DAE, but after that,growth at 40/32 C was significantly less. Average stem elongation rate of plants over the 64 DAE period was markedly affected by temperature, rising from 6.12 mm d^-1 at 20/12 C to 20.22 mm d^-1 at 35/27 C, but falling again to 17.5 mm d^-1 at 40/32 C.

Number of nodes on plants grown at different temperatures increased linearly with the increase in temperature. Nodes were added at the rate of 0.16, 0.23, 0.31, 0.39, and 0.43 d^-1 at 20/12, 25/17, 30/22, 35/27 and 40/32 C, respectively.

The number of fruiting branches per plant increased rapidly as temperature increased to 30/22 C, but declined over 15% at 35/27 C compared with the 30/22 C treatment. Plants grown at 40/32 C did not produce fruiting branches. In contrast, the number of vegetative branches per plant was higher at 20/12 C, decreased almost to zero at 30/22 C and again increased at the two higher temperatures. Lengths and nodes per branch of both vegetative and fruiting branches reacted similarly to branch numbers at different temperatures.

Leaf area and above-ground biomass per plant were very low at all temperatures during the 0 to 16 DAE and 0 to 26 DAE periods, but increased rapidly between 27 and 64 DAE periods across all temperatures. Leaf area was added at the rate of 36, 133, 146, 236, and 134 cm² d^-1 per plant at 20/12, 25/17, 30/22, 35/ 27 and 40/32 C, respectively, during the 27 and 64 DAE period. Aboveground biomass accumulated between 27 and 64 DAE period was significantly affected by temperature, rising from 0.42 g d^-1 at 20/12 C to 2.29 g d^-1 at 35/27 C and again declined to 1.54 g d^-1 at 40/32 C.

Days to squaring and flowering were strongly influenced by temperature. The time required to produce first square was 26 days at 30/22 C, while it took nearly 43 days at 20/12 C and 30 and 29 days at 25/17 and 35/27 C. The number of fruiting sites p@r plant increased linearly as temperature increased to 30/22 C, but declined over 50% at 35/27 C compared with the 30/22 C treatment. The number of squares and bolls retained was about 65 per plant at 30/22 C, but decreased to 23 per plant at 35/27 C. Plants grown at 40/32 C did not produce reproductive structures during the entire 64 DAE period.

Optimum temperature for reproductive growth in Pima cotton in terms of number of fruiting branches, length and nodes per branch was 30/22 C. This was also the optimum temperature for square and boll production and retention. There was increased vegetative growth at temperatures above 30/22 C, resulting in greater main stem lengths and higher leaf area at nodes initiated higher on the main stem. More squares and bolls were aborted at 35/27 C than at the optimum or lower temperatures. Plants grown at 40/32 C remained vegetative during the 64 DAE period. These results emphasize the sensitivity of pima cotton plants to temperatures above 30/22 C, and cause one to appreciate more fully the need for heat-tolerant cultivars in today's cotton production environments. Heat-tolerant cultivars will be even more essential in the future, if global warming is imminent.