Temperature effects on cotton yield and fibre properties of three cotton cultivars were determined. Plants were grown in pots maintained in growth rooms at varying day and night temperatures representing seasonally constant or varying (C) or daily varying (V) regimes.Yield and fibre characters responded to variation of daily mean and amplitude of temperature. Mean temperature reduction improved yield components, but fibre length, uniformity, strength and micronaire were increased by high, particularly high day, temperatures. A large daily temperature amplitude produced an intermediate number of flowers and the lowest retention percentage.Fruiting and yield were increased by reduction in temperature down to the threshold mean temperature of 22°C. However, V-regimes with a low minimum temperature acted as a further drop (below 22°C) of temperature and adversely affected these characters. An adverse effect of low minimum temperature combined with a moderate day temperature was observed also on lint percentage and fibre properties.Varietal differences were more pronounced for highly heritable characters such as fibre properties, for which significant interactions between varieties and temperature also occurred. Differences in reproductive development were not sufficient to be of much practical importance.

The growth and development of cotton was studied on cotton plants grown in pots in growth rooms under constant day\night temperature (C) and varying temperature regimes throughout the day and\or night (V) The V-treatments had a common mean temperature of 22 mC. The objectives were to determine the thermal requirements of three cultivars and to observe the extent of genotypeithermal environment interactions throughout the entire growth period.Vegetative growth was found to be almost exclusively time and temperature dependent, varietal differences being largely insignificant. Plant material was found to accumulate four times faster under the warmest C-regime, which was 7 mC warmer than the coolest. The C-treatments caused variation in the number and size of lateral shoots and leaves, causing leaf area to be larger at the squaring stage in cool environments but at maturity in warm regimes. Relatively cool nights lowered the position of the first floral or fruiting branch, whereas warm days shortened flowering intervals and thus promoted earliness. However, the effect of temperature in altering the position of the first floral branch seems to be less important than its effect on the shedding of early squares.The effects of fluctuating temperature (V) on vegetative growth and earliness were similar to those from constant temperature environments. Growth and development rates were low in the low minimum V-regimes, especially when the maximum temperature was also low. Temperature variation affected vegetative growth to a greater extent in the early than in the later stages of development. At squaring, leaf area and dry weight were lowest under the regime with the highest minimum and maximum temperatures. Later on, only leaf area at flowering and total dry weight at maturity significantly differed between treatments. High maximum or minimum temperatures produced effects similar to a higher or lower mean temperature, respectively. The low minimum raised the node number of the first floral or fruiting branch, whereas in the case of boll dry weight it acted in the same way as a further drop in temperature, decreasing the weight, regardless of the maximum temperature. Boll period was affected mainly by the temperature variation itself rather by than the type of variation.The inverse of time to a certain stage and the corresponding mean temperature were linearly related and allowed threshold temperatures (T ! ) and thermal time requirements to be estimated. T ! l 12 mC, except for the initial stage, when it was lower. A cotton growing season cooler on average by only 1 mC will considerably delay maturity.

A series of experiments investigating the interactive effects of light and temperature on vegetative growth, earliness, fruiting, yield and fibre properties in three cultivars of cotton, was undertaken in growth rooms. Two constant day/night temperature regimes with a difference of 4 °C (30/20 and 26/16·5 °C) were used throughout the growing season in combination with two light intensities (75 and 52·5 W m−2).The results showed that significant interactions occurred for most of the characters studied. Although the development of leaf area was mainly temperature-dependent, plants at harvest had a larger leaf area when high temperature was combined with low rather than with high light intensity. Leaf area was least in the low temperature–low light regime. However, the plants grown under the high temperature–low light combination weighed the least.Variations in the number of nodes and internode length were largely dependent on temperature rather than light. Light did, however, affect the numbers of branches, sympodia and monopodia. The first two of these were highest in the high light–high temperature regime and the third in the low light–low temperature regime.All other characters, except time to certain developmental stages and fibre length, were reduced at the lower light intensity. Variation in temperature modified the light effect and vice versa, in a character-dependent manner. More specifically, square and boll dry weights, as well as seed cotton yield per plant, were highest in high light combined with low temperature, where the most and heaviest bolls were produced. But flower production was favoured by high light and high temperature, suggesting increased boll retention at low temperature, especially when combined with low light. Low temperature and high light also maximized lint percentage.Fibres were shortest in the high temperature–high light regime, where fibre strength, micronaire index and maturity ratio were at a maximum. However, the finest and the most uniform fibres were produced when high light was combined with low temperature.Cultivar differences were significant mainly in leaf area and dry matter production at flowering.