Cotton production during the tropical monsoon season. I – The influence of variable radiation on boll loss, compensation and yield

Abstract

Growth and reproductive structure development in cotton (Gossypium hirsutum) is dependent on light intensity, which is often irregular in tropical environments due to shading from cloudiness associated with the monsoon. The objective of this study was to evaluate the effects of cloudiness on reproductive organ abscission and how subsequent compensatory organ replacement, with the provision of irrigation, might recover yield in the semi-arid monsoon tropics. Cotton cultivars representing different morphology were sown in field experiments at closely timed intervals during the early monsoon over five seasons to expose crops to varying amounts of cloud-related shading, creating a matrix of crop responses. A wide range for lint yield (1099 to 2900 kg ha−1) was symptomatic of the treatments’ exposure to highly varied environmental conditions. Lint yield was moderately correlated (P < 0.01; Adj R2 = 0.409) with monsoon cloudiness, reducing 235 kg ha−1 per MJ m−2 decrease in the average daily incident radiation during flowering. Final boll number rather than boll size was the key yield component (P < 0.001), with the number of bolls retained and their location within the canopy correlated with the solar radiation pattern spanning flowering. Boll number m−2 decreased linearly in proportion to mean daily incident solar radiation, by 5.02 and 4.39 bolls MJ m−2 during early (P = 0.007) and late flowering (P = 0.02) respectively. Cloudiness-induced boll loss from the early flowering canopy produced a negative correlation with late boll numbers, increasing by 7.2 bolls per MJ m-2 d−1 decrease in mean incident radiation during early flowering (P < 0.001) as lower canopy boll loss enabled surplus assimilates for later set bolls. Fibre quality parameters were not negatively affected by reduced incident solar radiation during flowering and boll filling. The clustering of sowing times into groups according to similarities in the number and final distribution of bolls within the crop canopy provided a mechanism to identify sowing time treatments that shared similar compensation responses. The highest group yields occurred when minimal cloudiness during flowering allowed uninterrupted boll accumulation or alternatively when moderate boll losses from the lower canopy were compensated via additional later set bolls when radiation improved. Yield recovery was restricted for groups where cloudiness was ongoing or increased during late flowering or when canopy factors detracted from efficient use of available radiation. Extrapolation of the linear relationship between boll number and incident radiation showed boll retention was maximised at a mean daily value of 24.2 MJ m−2 (Rmax). Quantification of Rmax was a key finding as it could be applied to calculate a cumulative radiation deficit (RD) = ∑ (if daily radiation is < Rmax then daily RD = Rmax-daily radiation) and provided an alternative independent variable for quantifying the impact of cloudiness during flowering on boll survival. Wider application of this concept requires research in larger solar radiation and temperature regimes than observed here. Yield compensation from cloud induced fruit abscission in the semi-arid tropics will be assisted by management that can extend flowering to create new fruiting sites whilst balancing the partitioning of canopy growth. © 2020