ONTOGENIC CHANGES IN RADIATION USE EFFICIENCY OF SUNFLOWER (HELIANTHUS-ANNUUS L) CROPS

Radiation use efficiency (epsilon) values for sunflower were derived using data from periodic determinations of crop aerial or total (aerial + root) biomass and radiation interception in two experiments conducted at Junin and Buenos Aires, Argentina. Analysis showed that three distinct phases, each characterized by a different value of epsilon, could be distinguished. The phases were establishment (0-47 days after emergence (DAE)), rapid growth (47 DAE-anthesis) and postanthesis (anthesis-physiological maturity). The crops had reached the bud-visible stage and intercepted at least 85% of incident radiation by 47 DAE. Efficiency values based on aerial biomass were highest during the rapid growth (epsilon(rg) = 2.4 +/- 0.22 g/MJ PAR) phase and lower during the establishment (epsilon(e) = 1.01 +/- 0.25 g/MJ PAR) and postanthesis (epsilon(pa) = 1.3 +/- 0.15 g/MJ PAR) phases. The differences between epsilon(pa) and epsilon(rg) are attributed to the increased respiration load, high synthesis costs of the oil-rich grains and leaf senescence during grain filling. The value of epsilon(e) was less than that of epsilon(rg) even when efficiencies were calculated on the basis of total crop biomass (i.e. including root biomass). Hence, ontogenetic changes in partitioning of biomass between the root and aerial organs do not seem to be the cause of the changes in the value of epsilon. No variations in photosynthetic capacity with leaf position could be found, and it is concluded that the most likely reason for the lower values of epsilon(e) lies in saturation of canopy photosynthesis at less than full sunlight while crop cover is incomplete.