RESUMO
For oil palm, yield variation is in large part due to variation in the number of harvested bunches. Each successively-produced phytomer carries a female (productive), male or aborted inflorescence. Since phytomer development takes 3-4 years and nearly two phytomers are produced per month, many inflorescences develop in parallel but have different phenological stages. Environment-dependent developmental rate, sex and abortion probability determine bunch productivity, which, in turn, affects other phytomers via source-sink relationships. Water deficit, solar radiation, temperature and day length are considered key external factors driving variation. Their impact is difficult to predict because of system complexity. To address this question we built a simple model (ECOPALM) to simulate the variation in number of harvested bunches. In this model, trophic competition among organs, expressed through a plant-scale index (Ic), drives sex determination and inflorescence abortion during specific sensitive phases at phytomer level. As a supplemental hypothesis, we propose that flowering is affected by photoperiod at phytomer level during a sensitive phase, thus, contributing to seasonal production peaks. The model was used to determine by parameter optimisation the influence of Ic and day length on inflorescence development and the stages at which inflorescences are sensitive to these signals. Parameters were estimated against observation of number of harvested bunches in Ivory Coast using a genetic algorithm. The model was then validated with field observations in Benin and Indonesia. The sensitive phases determined by parameter optimisation agreed with independent experimental evidence, and variation of Ic explained both sex and abortion patterns. Sex determination seemed to coincide with floret meristem individualisation and occurred 29-32 months before bunch harvest. The main abortion stage occurred 10 months before harvest - at the beginning of rapid growth of the inflorescence. Simulation results suggest involvement of photoperiod in the determination of bunch growth dynamics. This study demonstrates that simple modelling approaches can help extracting ecophysiological information from simple field observations on complex systems.
RESUMO
Selection for early vigour can improve rice (Oryza sativa L.) seedlings' access to resources, weed competitiveness and yield. Little is known about the relationships between early vigour and drought tolerance. This study explored a panel of 176 rice genotypes in a controlled environment regarding a diversity of traits and trait combinations related to early vigour and water use under drought. The design excluded genotypic differences for root depth. We hypothesised that early vigour (as determined by biomass accumulation under well-watered conditions) was not independent from drought tolerance (determined by biomass accumulation maintenance under drought). Leaf size, developmental rate (DR) and tiller number contributed positively to shoot DW and leaf area, and thus vigour. Early vigour was negatively correlated with growth maintenance and water use efficiency under drought, suggesting tradeoffs. Three clusters of genotypes were identified based on the constitutive traits DR, specific leaf area, tiller number and leaf size. The less drought-tolerant cluster, mainly with lowland O. sativa indica rices, showed a sensitive transpiration response to the fraction of transpirable soil water; however, under well-watered conditions these genotypes were vigorous, with small leaves, high DR and high tillering. This experiment showed that the tradeoff between early vigour and drought tolerance was physiological and not a matter of access to water. The results are discussed with a view to identify drought adaptation strategies for crop improvement. Further improvement of multitrait phenotyping approaches is proposed.
