Age-related abscission patterns of cotton fruiting forms: timing of the end of abscission susceptibility in relation to water content and growth of the boll

Abstract

The susceptibility of cotton fruiting forms (squares or bolls) to abscission is age dependent. After flower opening, abscission probability peaks and then decreases rapidly, suggesting the existence of a stage beyond which boll abscission does not occur. However, the timing and characterization of this stage need further refinement. This paper compares the susceptibility to abscission of fruiting forms with age according to sympodial position, and relates abscission probability of bolls to the time-course patterns of boll diameter, seed-cotton weight and water content. Square appearance, flower opening and abscission events were monitored in three experiments conducted with high inputs and in one experiment receiving no fertilizer. In the latter, the duration of fruiting and the appearance rates of fruiting forms were lower than in the high-input experiments. For a given age-class of fruiting form, the abscission rate increased from the innermost to the outermost position on the sympodium but the relative pattern of susceptibility with age did not change. Abscission followed a bimodal pattern with squares and bolls abscising most during early stages of development (≤80 Degree Days from appearance for squares, and ≤160 DD from flower opening for bolls). Applying a 60% light reduction for four or eight days during the late flowering period increased the abscission rate in proportion to the duration of shading. However, the frequency distribution of abscission with age was not affected by shading. In general, there was a consistent period at 200–240 DD after flower opening beyond which abscission ceased to occur. This period corresponded to a change in boll water content, which fell below 0.80 g g −1, and in seed-cotton dry weight which started to increase linearly and rapidly. Boll diameter was found to be a convenient measure to delimit this transition period. In our experimental conditions a critical diameter of 30 mm marked the end of abscission sensitivity of bolls.