RADIATION INTERCEPTION, PARTITIONING AND USE IN GRASS CLOVER MIXTURES

Mixed swards of perennial ryegrass/white clover were grown in competit ion under controlled environmental conditions, at two temperatures and with different inorganic nitrogen supplies. The swards were studied a fter canopy closure, from 800 to 1200 degrees C d cumulative temperatu res. Clover contents did not vary significantly during the period. A s imulation model of light interception was used to calculate light part itioning coefficients and radiation use efficiencies for both componen ts of the mixture in this controlled environment experiment. Additiona lly, this same radiative transfer model was applied to the field data from Woledge (1988) (Annals of Applied Biology 112: 175-186) and from Woledge, Davidson and Dennis (1992) (Grass and Forage Science 47: 230- 238). The measured and simulated values of light transmission, at diff erent depths in the mixed canopy, were highly correlated (P < 0.001) w ith more than 80% of the total variance explained. The daily average o f photosynthetically active radiation (PAR) interception in a natural environment was estimated from simulations, for the field and controll ed environment data. Under these conditions, white clover captured sig nificantly more light per unit leaf area than perennial ryegrass at lo w, but not at high, nitrogen supply. In the controlled environment exp eriment, the radiation use efficiency of the legume was lower than tha t of its companion grass. For both species, radiation use efficiency w as negatively correlated with the mean irradiance of the leaf. The rol e of a compensation between light interception and light use for stabi lizing the botanical composition of dense grass-clover swards is discu ssed. (C) 1996 Annals of Botany Company