EFFECT OF CANOPY STRUCTURE ON EFFICIENCY OF RADIATION INTERCEPTION AND USE IN SPRING WHEAT CULTIVARS DURING THE PREANTHESIS PERIOD IN A MEDITERRANEAN-TYPE ENVIRONMENT

Previous studies in Western Australia showed that the extinction coeff icient (K) is greater for modem cultivars of spring wheat than for old er cultivars. To identify the factor(s) responsible for the difference s in K, measurements of green area index (GAI) and light interception for the whole canopy and strata within the canopy were made on three s pring wheat cultivars of contrasting canopy characteristics released b etween 1929 and 1986: Bencubbin (released in 1929), Gamenya (1960) and Kulin (1986). Values of K calculated from the measurements of photosy nthetically active radiation (PAR) and GAI data for the whole preanthe sis period averaged 0.76 for Kulin, 0.72 for Gamenya and 0.63 for Benc ubbin, but these differences were not significant; the high K for Kuli n was associated with its large awned ears that were more effective th an the leaves in intercepting radiation. However, before ear emergence , K was always significantly lower for Kulin (average 0.38) with few a nd erect leaves, than for Bencubbin (0.59) and Gamenya (0.60) both of which had more and lax leaves. Values of K based on total solar radiat ion (K-t) measurements followed the same trends as those based on phot osynthetically active radiation (K-p), but the former was about 30% lo wer than the latter. Radiation-use efficiency (RUE) based on above-gro und dry matter (DM) and the interception of photosynthetically active radiation for the pre-anthesis period was significantly greater for th e modem cultivars than the older ones; RUE was in the order Kulin (2.4 0 g DM MJ(-1)) > Gamenya ( 1.63) > Bencubbin (1.52). Reasons for highe r K values and improved RUE for the recent cultivars are discussed.