AbstractsBiology & Animal Science

The interception and reflection of solar radiant energy by maize crops was measured at Cedara using tube solarimeters. From the measured data the amount of PHAR that was intercepted was related to the L value of the crop. K(v), the extinction coefficient of PHAR thus obtained was found to vary with row spacing, plant population density and cultivar, whereas growth stage, time of day and weather conditions did not appear to effect the K(v) value markedly. On clear days the albedo (R(v)) of a ma1ze crop was of the order of 0,05 to 0,08 during the middle of the day. However, at low sun angles in the early and late hours of the day, R(v) increased to values of the order of 0,2. On overcast days, the diurnal variation was considerably lower. The rate of photosynthesis in a ma1ze crop was measured using the plastic enclosure technique. The rate of CO2 uptake was related to the amount of PHAR absorbed by the crop. A rectangular hyperbola (Eqn. 2. I) was found to describe this relationship well. The photosynthetic response of a crop to population density, plant spacing and temperature at different developmental stages is discussed. The seasonal distribution of dry matter in the maize crop was studied using normal growth analysis techniques. From these data functions describing the partitioning of DM to the various plant organs (e.g. roots, leaves, grain) at different growth stages were developed. The functions describing the absorption of light by the crop canopy, its photosynthetic rate and the partitioning of photosynthate were incorporated in a computer simulation model, which was used to estimate the growth of a maize crop from meteorological radiant flux density and temperature data. The functioning of the model and its possible use are discussed.