Analyzing and modelling biomass accumulation in sorghum stem and its drought regulation at tissue and organ level – genotypic variability and implications for ideotype conception

Sorghum can significantly contribute to growing needs in ligno-cellulosic biomass for bio-sourced product diversification, particularly in water-limited conditions. However, the genotypic and environmental variability of stem biomass production and quality is still poorly understood, limiting its genetic improvement. This study aimed to identify the morphogenetic, biochemical and histological traits underlying at internode level the genotypic and environmental variability of stem biomass accumulation by sorghum. Three field experiments were organized to compare 8 genotypes under irrigated and drought (applied during stem elongation). The eco-physiological model Ecomeristem was used to explore trait impact for different cropping situations and production targets. Both stem biomass production and quality were affected by water deficit due to the reduction of the number, length and ligno-cellulosic content of expanded internodes, whereas their soluble sugar content was increased and diameter unaffected. Internodes developed after re-watering observed a remarkable recovery whereas those developed under stress did not recover. Genotypic variability for drought sensitivity and recovery was highlighted but no correlation was found between them. The drought response of growth, biochemical and histological traits was slightly correlated, suggesting only partial trade-offs between stem biomass production and quality response to drought, obviously under complex physiological and genetic controls. Once validated on available data, the crop growth model Ecomeristem was used to in silico explore trait impact on biomass production depending on the variation in key cropping criteria for biomass sorghum worldwide: targeted production (structural, nonstructural carbohydrates), planting density and water availability. Different ideotypes were suggested for each simulated situation, mainly defined by the trade-off between tillering propensity and internode sink capacity (related either to size or biomass density). It is suggested that not only internode biomass accumulation but also tillering capacity should be further considered for phenotyping and ideotyping biomass sorghum in its targeted cropping environments