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EcoPalm: A new model simulating seasonal variation of palm yield : I. Day lenght sensitivity of flowering and competition among sinks explain bunch number

Combres J.C., Mialet-Serra I., Bonnal V., Soulie J.C., Rouan L., Braconnier S., Dingkuhn M.. 2009. In : 16th International Oil Palm Conference and Expopalma. Challenges in sustainable oil palm development, 22 to 25 September 2009, Cartegena de Indias, Colombia. Bogota : FEDEPALMA, 14 p.. International Oil Palm Conference and Expopalma. 16, 2009-09-22/2009-09-25, Carthagène des Indes (Colombie).

Seasonal variation of fresh bunch production is difficult to explain and predict. A population scale model, EcoPalm, was developed to explain those using new physiological hypotheses. A central hypothesis was that day length influences time of flowering during a photosensitive period (PSP) at the scale of individual fronds (phytomers). The variable PSP duration between successive phytomers result in a queue of phytomers in "waiting state", which partly explains seasonal variations. The monthly, potential number of harvested bunches, based on a 100% sex ratio, and the absence of abortions and failures of fruit set, was simulated. The second hypothesis was that sex ratio, intensity of bunch abortion and bunch failure, , depend on the trophic status of a given phytomer during sensitive phases of its development, expressed through a plant-scale Index of Competition for assimilates (Ic), which is the ratio between instantaneous assimilate supply and aggregate demand. Annual demand for vegetative organ growth was assumed to be constant (stabilized growth). Bunch dry matter production was assumed to be variable. If Ic < 1 during sensitive stages of bunch development, (high competition among sinks), sex ratio was considered to be low and abortion and bunch failure high. If Ic > 1, sex ratio was high. The 3rd hypothesis was that resulting sink-source imbalances are buffered by stem carbohydrate reserves. If Ic > 1, demand was not down-sized and excess assimilates were stored (overflow model). If Ic < 1 reserves were mobilized. The model was tested on a multi-annual period in La Mé (Ivory Coast) on a control progeny, LM2T X D10D. Sensitive periods to Ic were adjusted by optimization of parameters using evolutionary algorithm method. The PSP occurs at the early stage of development of phytomer, begin 36 months before harvest (mbh) and ends 28 mbh. The sex determination is very complex and occurs during PSP. The initial sex ratio of palm population was determined by mean Ic from nearly 34 to 28 mbh and the last month a second phase increases sensitivity to low Ic. This sexualization phase ends with PSP. A first stage of abortion is founded 17 mbh (frond emergency). A second stage of abortion occurs from 10 to 8 mbh. The model shows good monthly and annual simulations of bunches number harvested by ha at La Mé station. Validations at sites in Indonesia are in progress. Other model features including simulation of oil production and impacts of drought are presented in subsequent papers.

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