GUMCAS: a model describing the growth of cassava (Manihot esculenta L. Crantz)

Abstract A process-oriented model, GUMCAS, describing the growth of cassava ( Manihot esculenta L. Crantz) was developed for inclusion in the IBSNAT decision support system. Potential dry matter production is calculated from existing leaf area, and is modified by effects of light, temperature, water stress, and vapour pressure deficit. Leaf and stem growth are assumed to be the dominant sinks for assimilate, with fibrous roots receiving a fraction of that allocated to the shoot, decreasing as the crop ages. The storage roots receive any remaining assimilate. The Ritchie water balanced model is used to estimate water status. Leaf size is calculated empirically as a function of time. However, there was a strong correlation between leaf size and the assimilate supply/demand ratio represented by δ W/W (daily change in weight/total plant weight). This approach is included in the model as an option for simulating environmental influences on leaf size. This relationship functioned well, particularly on release of drought stress. Phenology is described by assuming two independent “clocks” controlling vegetative and reproductive development respectively. Both clocks are influenced by temperature and water status, while reproductive development is also influenced by photoperiod (φ). Fitting the standard model for photoperiod response to observed reproductive branching data gave a minimum optimum photoperiod ( φ 0 ) of 15.5 h and a sensitivity ( S φ ) of 0.25 h −1 , assuming the crop was sensitive to photoperiod from emergence. However, an improved fit was obtained by also assuming branching is inhibited when the rate of change of photoperiod (d φ /d t ) was above 0.01 h day −1 . Sensitivity analysis confirmed previous reports that leaf longevity is an important character determining storage root yield; other characters are the age at which first branching occurs, and specific leaf area. However, the importance of some characters changes under drought, such as the date at which the maximum leaf size occurs. The model was validated with the limited number of datasets available; good agreement between simulated and measured values was obtained for a range of cultivars in a range of environments. Further validation is necessary, however, before widespread application. The factors controlling leaf size and times of branching were identified as areas in which there is a scarcity of knowledge, and to which future cassava physiology research should be directed.