Modelling genetic and environmental control of biomass partitioning at plant and phytomer level of sugarcane grown in controlled environments

Sucrose content has reached ceiling levels in several countries despite aggressive crossing and selection programmes aimed at improving this important trait for the sugarcane industry. Much of the recent research effort has been directed towards molecular means for improving sucrose content and while some breakthroughs have been made in the laboratory, no plants modified for this purpose have been grown successfully in the field. Sugarcane grown mainly for its sucrose in the past is now being considered for its fibre content as well because of increased interest in renewable energy. The paper offers an account of the variation in fibre, sucrose and hexoses in aboveground organs in relation to genotype, temperature and water regime with the aim of an improved understanding of biomass partitioning needed to effectively exploit sugarcane's potential for multiple production streams. Previous studies often focused on single genotypes and on partitioning within stalks and ignored the effects of whole-plant structural partitioning on sugar accumulation. A mathematical model was constructed of biomass partitioning (at whole-plant and phytomer levels) of two high and two low sucrose clones of sugarcane from data collected in two controlled environment experiments, with water and temperature as treatments. The model tested the hypothesis that genetic differences in sucrose accumulation and responses to water and temperature can be explained by differences in plant development and partitioning to structural components such as leaf and stalk fibre. Whole-plant biomass partitioning between leaf, stalk structure and stored sugars was adequately simulated using clone-specific partitioning fractions modified by water status and temperature. Leaf partitioning fractions varied significantly between clones (low sucrose clones had high leaf fractions) but not between treatments. Stalk fibre partitioning fractions did not vary between clones but increased with improved water status and increased temperature. These aspects were mostly represented successfully in the model mainly because partitioning parameters were derived from the same data. Sugar accumulation was simulated, reasonably successfully, as the remainder of the biomass pool after partitioning to structural pools. Phyllochron intervals determined the rate at which phytomers ceased structural growth and commenced sugar accumulation. Low sucrose clones had longer intervals and so started sucrose accumulation later than high sucrose clones. There were also clonal differences in the ratio of hexose to sucrose (low sucrose clones had high ratios) and this could largely be explained by the structural mass fraction present in biomass. Although the data did not allow independent tests of all model assumptions, modelling these experiments did assist in gaining improved understanding of the underlying mechanisms of genetic and environmental control of biomass partitioning at whole-plant and phytomer levels. Results suggest that a way to enhance sucrose yields could be to breed genotypes with appropriate phenological and structural partitioning traits such as rapid phytomer development and low leaf partitioning fractions. This needs to be confirmed by further studies on more genotypes and environments