PHOTOSYNTHESIS IN TOWNSVILLE LUCERNE (STYLOSANTHES HUMILIS HBK)

Photosynthetic rates of attached leaves of Townsville lucerne (Stylosanthes humilis H.B.K.) were measured under controlled conditions in a leaf chamber. The effect of radiation, ambient carbon dioxide concentration, and leaf temperature were measured, and the leaf resistances to water vapour and carbon dioxide transfer determined. The maximum rate of photosynthesis recorded, 25 mg CO2/dm2 h, is comparable with rates reported for other members of the Leguminosae. Respiration was measured as carbon dioxide evolution in the dark, and at light saturation in CO2-free air. The respiration rate in the light was at least 1.8 times that in the dark, thus any estimate of gross photosynthesis must take photorespiration into account. A detailed field study of crop structure and microenvironment clearly showed that while less than 10% of the total crop surface is located in the top 10 cm, this layer intercepts 80-85% of the radiation, due to active phototropism by the uppermost leaves. The field and laboratory data were built into a model describing crop photosynthesis. Net photosynthesis was computed for leaf and stem material in each 10 cm layer of the crop for each hour of the day. The model predicts a maximum growth rate of ca. 55 g/m2 day for a sward about 20 cm high, and that lower growth rates would be attained by taller swards. The maximum growth rate so far recorded for Townsville lucerne at Katherine is ca. 28 g/m2 day, excluding root growth. © 1968.