PHOSPHORUS EFFICIENCY IN PASTURE SPECIES .8. ONTOGENY, GROWTH, P-ACQUISITION AND P-UTILIZATION OF ITALIAN RYEGRASS AND PHALARIS UNDER P-DEFICIENT AND P-SUFFICIENT CONDITIONS

The P efficiency of Italian ryegrass (Lolium multiflorum Lamk. cv Gras slands Tama) and phalaris (Phalaris aquatica L. cv Sirosa) was compare d on both a temporal and ontogenetic basis. As ontogeny and growth are interrelated, such a comparison allowed the growth and physiological responses to P level of the two species to be separated from responses due to the species being at different ontogenetic stages at the time of comparison. Plants were grown from seed through to anthesis under P deficient and P sufficient conditions in soil in a glasshouse. The on togenies of Italian ryegrass and phalaris were similar, but the rate o f development of Italian ryegrass was greater at both P rates. P defic iency resulted in arrested reproductive development in phalaris. At bo th P levels shoot, root and total biomass and net P uptake per plant b y Italian ryegrass were greater than by phalaris when the two species were compared on a temporal basis, but when compared on an ontogenetic basis the two species were similar. There were some differences in th e allocation of P between the acid-soluble P, lipid P, and residue P f ractions, but biomass production was not determined by the efficiency of P utilization. The superior biomass production of Italian ryegrass on a temporal basis was due to its greater seed size and rate of ontog eny rather than differences in photosynthetic rate, unit leaf rate, le af area ratio or shoot:root ratio. Similarly, the greater P uptake per plant of Italian ryegrass on a temporal basis was driven by its great er plant size and faster root extension rate rather than by P uptake p er unit root length. The level of vesicular arbuscular mycorrhizal (VA M) infection in the roots of the two species was similar under P defic iency but greater in phalaris under P sufficient conditions. Overall, the different temporal responses to P of Italian ryegrass and phalaris were largely related to their different rates of ontogeny and the int errelationships between ontogeny and growth rate rather than to differ ences in their physiology in relation to P acquisition and utilization .