The seasonal growth characteristics of irrigated cotton in a dry monsoonal environment

Abstract

Cotton plantings were made at the Kimberley Research Station (128° 36´ E., 15° 42´ S.) from September to July to give overlapping growth curves extending from September 1961 to December 1962. Crop growth was examined in relation to radiation and temperature. Beginning at the third week from sowing, each planting was sampled 12 times at intervals of 15 days. At each sampling, dry matter, leaf area index, number and dry weight of bolls, and seed cotton yield were determined.

The development of total dry matter and leaf area index showed a strong seasonal trend. The number of bolls produced was less in the late plantings, and the seed cotton yield was correspondingly lower. The highest seed cotton yields and the highest ratios of seed cotton to total dry matter (30%) were obtained with January–February plantings.

The growth rates between 50 and 100 days were used to analyse seasonality; the highest seasonal growth rate was 164 mg dm^-2 day^-1 in December–January and the lowest value was zero in early September. By using fitted harmonics, a multiple correlation was established between maximum and minimum temperatures and growth rates 30 days later.

Potential rates of net photosynthesis of tops were calculated from observed radiation data with the use of the de Wit model and with allowance for light wastage, root growth, and respiration. There was good agreement between observed and potential growth rates from December to April. Midday stomatal apertures declined from November to May, and this appeared to be a seasonal effect. An annual pattern for the diffusive resistance of carbon dioxide through the leaf surface into the substomatal cavities is calculated.

Radiation did not appear to be the primary limiting factor, and the seasonal growth pattern was interpreted in terms of the composite effect of maximum and minimum temperatures on extension growth, stomatal opening, and the presence or absence of suitable sinks for carbohydrates. Growth rates are evaluated against rates observed elsewhere, and means of exploiting the Kimberley environment are discussed.