RESPONSES OF FABA BEAN (VICIA-FABA L.) TO SOWING RATE IN SOUTH-WESTERN AUSTRALIA II - CANOPY DEVELOPMENT, RADIATION ABSORPTION AND DRY-MATTER PARTITIONING

Sowing rate influences plant density, canopy development, radiation ab sorption, dry matter production and its partitioning, and seed yield. The canopy development, radiation interception, and dry matter partiti oning of faba bean (cv. Fiord) were examined using 6 sowing rate treat ments from 70 to 270 kg/ha in field experiments conducted over 3 years at Northam as part of a larger investigation of sowing rate responses in faba bean in south-western Australia. High sowing rates resulted i n significantly earlier canopy closure, larger green area indexes, mor e radiation absorption, more dry matter accumulation particularly duri ng the early vegetative stages, and greater seed yield than treatments where a low plant density was established. The results suggest that f urther increases in canopy development, radiation absorption, dry matt er accumulation, and seed yield are possible by using sowing rates in excess of 270 kg/ha. The rate of node appearance was relatively consta nt within and across seasons (1 every 65.9 degree-days), whereas the n umber of branches per plant declined with increasing plant density, an d less branches survived through to maturity at high density. The peak photosynthetically active radiation absorption (75-85%) measured at g reen area index of 2.9-3.8 in the highest sowing rate treatment in thi s study is similar to previous reports for other crops. The estimated radiation use efficiency (1.30 g/MJ) was constant across sowing rate t reatments and seasons. High sowing rates produced tall crops with the lowest pods further from the soil surface than those at low plant dens ity, and hence, mechanical harvesting was easier. The growth of indivi dual plants may have been limited by the low growing season rainfall ( 266-441 mm) and/or low soil pH (50 in CaCl2) at the site, and competit ion between plants for radiation was probably small even at the highes t sowing rate. Early canopy closure and greater dry matter production with high sowing rates may also cause greater suppression of weeds and aphids.