Effects of Irradiance and Solar Radiation on Dry Matter Growth and Net CO₂ Exchange of Trifolium subterraneum L. Swards at a Constant Temperature

Abstract

Small swards of subterranean clover were grown at 20°C at different times of the year and at the same time under shades of different light transmission. Dry matter production and net CO₂ exchange rates at different levels of irradiance were measured during growth. Crop growth rates and net CO₂ exchange rates are examined in relation to plant attributes and to the daily solar radiation. Dry matter growth curves varied according to the daily solar radiation. Increase in crop growth rate with increase in daily solar radiation was taken to be linear for swards with similar dry weight and was more rapid for closed canopies than for those not showing full light interception. Once a closed canopy was attained, further increase in total dry matter resulted in decreased crop growth rate due, apparently, to increased loss of dry weight through increased respiration rate. The shape of the CO₂ exchange/irradiance curve was linear up to 100 W m^-2 (PAR) and curvilinear above that value. The rate of net CO₂ exchange at 50 W m^-2 (PAR) decreased with increase in shoot dry matter above 100 g m^-2 due, apparently, to an increased rate of dark CO₂ efflux: the rate was not affected by daily solar radiation during growth. The net CO₂ exchange rate at 250 W m^-2 (PAR) increased with increase in LAI up to 3 after which it became almost constant, varying only with the variation in daily solar radiation during growth. The rate of dark CO₂ efflux was strongly affected by the amount of dry matter present and to a smaller extent by the crop growth rate or the daily solar radiation.