Invasion by Pittosporum undulatum of the forests of central Victoria. IV. Shade tolerance

Abstract

Seedlings of Pittosporum undulatum were grown under shade screens for 16 days at four flux densities: 97.8, 18.2, 3.4 and 0.4% full daylight, i.e. c. 420, 78, 14 and 2 W m^-2. Seedlings showed moderate tolerance of shade: the growth compensation point for light was 1 .6% daylight, the leaf area ratio in full daylight was low (48.96 cm² g^-1) and when plants grown at 0.4% were compared with those grown at 97.8% full daylight and there was a fourfold increase in chlorophyll concentration. Leaves were thinner at the lowest flux density because the cells were deflated and there appeared to be only one row of palisade mesophyll cells. On the other hand, there was no large increase in leaf area ratio with shading, no alteration in the chlorophyll a/b ratio and no depression of net assimilation rate in full daylight. P. undulatum can endure shade but has a higher growth rate when grown at higher photon flux densities. P. undulatum is found predominantly in shade owing to the uneven dispersal of seed and the site-dependent survival of seedlings and not because shade is required for maximum growth.