No abstract

Temperature is a major environmental factor that affects cotton (Gossypium hirsutum L.) production. Temperatures of 35 to 40°C are frequently observed in cotton‐producing areas. High‐temperature environments are sometimes associated with cotton sterility and boll retention problems; however, there is little specific information available on cotton sensitivity to temperature. We conducted a series of experiments in naturally lit growth chambers where the temperature was accurately controlled so we could measure its effects on cotton fruiting site production, abscission of flower buds (squares), flowers, and bolls. Cotton plants grown from seedlings at 40°C for 12 h d−1 shed all their squares. Plants grown from seedlings in the natural environment, then exposed to daytime temperatures of 30, 35, or 40°C during the fruiting period accumulated 47, 5.7, and <1%, respectively, of their mass as bolls. The time of day when plants were exposed to high temperature did not influence percent boll retention. Number of bolls produced, bolls retained, and percent retention were progressively reduced as time per day at 40°C was increased. Three week exposure to 40°C for 2 or 12 h d−1 resulted in 64 or 0% bolls, respectively, retained on the plants. Cultivars appeared to be similar in sensitivity to high temperature. Cultivars tolerant to short periods of high temperature would probably be more productive in today's environment and increased tolerance to high temperature will be even more essential in a warmer environment.

The ecological range of temperature conditions where cotton (Gossypium hirsutum L.) is commercially grown frequently exceeds the temperature range of available data. To predict growth, development, and responses of the primary physiological processes to those conditions, data at both lower and higher temperatures are needed. Cotton plants were grown in naturally lit, temperature‐ and CO2− controlled chambers. Plants grown at near‐optimum temperatures (30 and 35 °C day temperature) fixed twice as much CO2 at high photosynthetically active radiation during the fruiting period as did plants growing at 20 °C. Growth exhibited similar differences due to temperature during the boll‐filling period. Plants with heavy fruiting load partitioned less to vegetative structures. Most plants grown at 35/25 °C day/night temperatures and all those grown at 40/30 °C had serious flower survival problems. No fruits were harvested from the plants grown in the 40/30 °C chamber and only a few from those grown in the 35/25 °C chamber. Gross photosynthesis, canopy light utilization efficiency, and canopy conductance were found to be highly variable from day to day. This variability should be considered by investigators measuring short‐term CO2 exchange rates. Respiration rates of cotton plants were 45% higher during the first hour of dark than during the rest of the night. These data suggest that carbohydrate is more readily available for respiration during and immediately after the light period than later in the dark period.

Temperature is a primary environmental factor controlling growth and developmental rates of plants, yet little specific information is available regarding cotton (Gossypium hirsutum L.) responses to temperature. Information covering a wide range of temperatures would be useful for predicting both developmental and growth rates in cotton. Therefore, an experiment was conducted in naturally lit, temperature-and CO,-controUed cabinets from soon after emergence until 56 d after emergence (DAE). The cabinets were maintained at 20/12, 25/17, 30/22, 35/27, and 40/3rc day/night cycles. Plant heights, number of nodes, and leaf areas were determined weekly throughout the experiment, and dry weight measurements were obtained at three intervals. Mainstem elongation, leaf area growth, and biomass accu· mutation rates were very sensitive to temperature about 3 wk after emergence. Prior to that time, they were relatively insensitive to temperature. The temperature optimum for stem elongation, leaf area expansion, and biomass accumulation was 30/2rC. Developmental rates, as depicted by number of mainstem nodes produced, number of fruiting branches, and fruiting branch nodes, were not as sensitive to temperatures above 30/2rC as were growth rates. Four times as many fruiting branches were produced at 30/2rC as at 20/1rC; whereas more vegetative branches were produced at low temperatures. All flower buds (squares) abscised from plants grown at 40/3rC. Essentially, all bolls and squares were retained at 30/22 •c while a 10o/c boll and square loss was observed at 35/27 •c during the early reproductive period. Less time was required for this cultivar to produce squares at any temperature, suitable for growing cotton, than was suggested by previous experiments.

Cotton, (Gossypium hirsutum L.) when grown in a fertile, well‐watered, and suitable environment, produces excessive vegetative growth. Such luxuriant growth causes several production problems. A plant growth regulator, mepiquat chloride1 (1,1‐dimethylpiperidinium chloride; BASF Wyandotte Corp., Parsippany, NJ) has been found to reduce vegetative growth, but the crop responses to it appears to be temperature sensitive. An experiment was conducted to determine the crop responses to temperature in naturally lighted, temperature‐ and CO2‐controlled plant growth chambers. The experiment was conducted at five temperatures and measurements were made of several plant parameters. A preliminary experiment showed that mepiquat chloride (MC) caused a dramatic reduction in plant height and number of main‐stem nodes. A more detailed study showed that the impact of MC on main‐stem elongation was temperature dependent, with the greatest effect occurring at 35/25 °C day/night temperatures. The application of MC also caused a reduction in mainstem, vegetative‐branch and fruiting‐branch node formation. Leaf area was reduced on plants treated with MC at each of the temperatures. Specific leaf area (m2 kg−1) was also reduced on leaves from various positions on the plants. It appears that MC affects more than one physiological process. The primary effects were reduction in stem elongation, node formation, and leaf expansion. The reduced stem growth and leaf area development also caused greater leaf densities—probably a secondary or tertiary effect.