Modelling the canopy development of bambara groundnut

Canopy development of bambara groundnut (Vigna subterranea (L.) Verdc) is affected by temperature stress, drought stress and photoperiod. The quantification of these documented effects by means of a suitable crop model, BAMGRO is presented in this paper. Data on canopy development from five growth chamber, four glasshouse and three field experiments were analyzed to calibrate and validate the BAMGRO model to produce simulations for temperature stress, drought stress and photoperiodic effect on two contrasting landraces; Uniswa Red (Swaziland) and S19-3 (Namibia). The daily initiation rate of new leaves is calculated by means of a Gaussian function and is altered by temperature stress, drought stress, photoperiod and plant density. The rate in dead leaf number is dependent upon the maximum senescence fraction which can be explained by physiological maturity, mutual shading, temperature stress and drought stress. The leaf area expansion is calculated as a function of leaf number and individual leaf size. The canopy development of bambara groundnut is modelled (and tested) in BAMGRO model by means of leaf initiation and leaf area expansion and branching and stem production are not considered. Nash and Sutcliffe value (N-S) is used as the measure of the mean square error to the observed variance. The simulated leaf number per plant correlated well with the glasshouse measurements for Uniswa Red for the two considered years (2007 and 2008) with high N-S value (Nash and Sutcliffe; from 0.76 to 0.92), low mean absolute error (MAE; from ±2.91 to 3.98) and Botswana field grown in January 18 and February 1, 2007 (N-S, 0.88 and 0.74; MAE, ±0.59 and ±6.18). Overall, simulated LAI correlated well with measured values for the two tested landraces; S19-3 (N-S value, 0.84) and Uniswa Red (N-S value, 0.80) with maximum MAE less than ±0.50 thus explaining canopy development under temperature stress, drought stress and variable photoperiods.