The Effects of Leaf Pruning and other Stresses on Sex Determination in the Oil Palm and their Representation by a Computer Simulation

Oil palms produce an inflorescence in each leaf axil. These show irregular cycles of gender with occasional abortions. Physiological stress is known to induce a higher frequency of males and abortions, but it is not known how such factors interact with endogenous cycles nor is/are the developmental state(s) when sex is determined known with accuracy. A computer simulation model, based on published results of field experiments to define critical timings of sex determination and stress generation, with feedback loops generating “stress” one-half wavelength ahead of the sex determination and abortion stages, produced oscillations of male, female and abortion cycles. The model assumes that female inflorescences are formed unless the stress exceeds a specified threshold level, when males are formed. At higher stress levels abortions occur. The model predicts that in the absence of external stimuli there is a natural sex cycle the duration of which depends on the timing of sex determination. A shorter abortion cycle 24 nodes in duration interacts to modify the sex cycle. Annual stress cycles (e.g. drought) can over-ride the endogenous cycles and synchronize them. The response obtained to a transient stress simulating leaf pruning resulted in extra males and abortion but depended on when the stress was applied in the natural flowering cycle. Simulations of irrigation experiments are presented and disbudding experiments discussed. The model allows wide flexibility of inputs. The examples shown are based on sex determining stages at a variety of times between leaf axil −5 (spikelet initiation) and leaf axil −27 (first bract initiation) with the signal generated during bunch development. Abortions occurring at leaf axial +10 are also induced by a signal from developing bunches. The timing of these stages can be changed at will. By changing the stress threshold levels at which females, males and abortions occur, it was found that this double loop feedback model gives good simulations of the endogenous cycles. The loop timings used are consistent with published data on the critical stages of oil palm floral development. The effects of pruning stress on two oil palm clones with different flower-cycle characteristics were only partially simulated, and deeper understanding of the sex determination processes is needed. Several predictions are made which would allow the model to be tested experimentally.