An experimentally-calibrated model of photosynthesis, assimilate partitioning and tuber yield in cassava in response to water supply to assist crop management and improvement

Abstract

A model (DYNCAS 1.0) is presented of photosynthesis and partitioning of carbon-based (C) assimilate to respiration and growth of leaves, stems, tubers and roots in cassava in response to environmental atmospheric conditions and water supply. It comprises 9 state variables and 40 parameters (including 7 functional relationships), all of direct physiological significance. The important characteristic of the model is the simultaneous consideration of assimilate supply and crop water status by hourly calculations that integrate diurnal physiological responses of photosynthesis, respiration and transpiration to seasonal crop growth and water use under existing environmental conditions.Crop photosynthesis is the integral of a single leaf-response through the canopy according to intercepted radiation determined by an extinction coefficient and by temperature together with canopy conductance determined from crop water potential and vapour pressure deficit. Respiration of organs is calculated individually according to chemical composition and growth processes, including mobilization of reserves. Transpiration proceeds with photosynthesis according to canopy conductance, but in response to evaporative demand, extracting water from the root zone depending upon crop and soil water potentials and the root length that explores it. Partitioning of carbon assimilate from a single crop pool to leaves, stems, tubers and roots depends upon diurnal oscillations in supply according to organ-specific Michaelis-Menten kinetics. Phenological development, here the branching of stems, rate of node and leaf production, and maturity and senescence of leaves and of roots, is determined by temperature and crop water status.The model is shown to capture the response reported in a published study of the growth and yield of two cultivars of cassava of distinct branching habit to water supply. The principles of assimilate partition and the simulation of diurnal responses of growth processes employed here are presented as a guide for further research in cassava productivity and response to environment and management.