Seasonal variation in yield and developmental processes in an oil palm density trial on a peat soil: 1. Yield and bunch number components.

Abstract

Bunch production in an oil palm density trial on a peat soil in Perak, West Malaysia, displayed a regular annual cycle that was highly synchronized across densities. The phase of the cycle differed from that displayed for West Malaysia as a whole but resembled those at some other sites. Annual cycles were found in both bunch number and mean bunch weight and in the proportion of palms that yielded bunches in any one month. While the long-term trends in bunch numbers and single bunch weights were negatively correlated, in the shortterm there was a highly significant positive correlation between the two. The variation in mean bunch weight was also reflected in the variation in the main bunch components. There were also seasonal variations in the ratios of bunch components. In addition to yield, regular annual cycles were also apparent in the rates of frond emission, male and female inflorescence production and sex ratio, and in inflorescence abortion. While the phases of bunch and female inflorescence cycles could be matched using physiologically meaningful lag periods, the cycles of frond emission and total inflorescence production, and of frond emission and female inflorescence production could not, giving rise to non-significant or negative correlations. Abortion rates were low throughout the trial and while a regular sex ratio cycle became apparent from the eighth year, this was not the main determinant of bunch number cycling. Rather, variation in the rates of inflorescence development may be the crucial factor in causing the yield cycles. Other external and internal factors that might contribute to the yield cycles are discussed. the more uniform climates of Southeast Asia, the causes of yield cycles are less obvious although rainfall and other climatic factors do exhibit seasonality and could still play a role. However, even in the absence of obviously varying external factors there may be internal, i.e. endogenous, processes regulating development which give rise to cyclic behaviour. These may operate such that the cycles persist even when the external factors appear uniform or non-limiting. Several developmental processes may contribute to cycles in fruit bunch production in oil palm. These include variation in the rate of frond emission, timing of inflorescence development stages, inflorescence