Invasion by Pittosporum undulatum of the Forests of Central Victoria. IV* Shade Tolerance Roslyn M. Gleadow, Kingsley S. Rowan and David H. Ashton School of Botany, University of Melbourne, Parkville, Vic. 3052. 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-?. 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-I) 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. Introduction Initially, invading Pittosporum undulatum Vent. is found predominantly at the butts of established trees and shrubs where the irradience is frequently between 4-8% full daylight and may be as low as 2% (Gleadow and Ashton 1981). This is due partly to the patchy distribution of seed by birds (Gleadow 1982) and partly to increased drought tolerance of seedlings in sheltered positions (Gleadow and Rowan 1982). However, the level of shade tolerance and whether or not seedlings have a higher growth rate in partial shade compared with full daylight are unknown. This experiment was to determine the shade tolerance of seedlings of P. undulatum by using the following criteria. Sun and shade plants are usually distinguished by the level of light radiation under which they normally grow (Went 1957). Many workers consider that a low compensation point for light at a given time ('instantaneous') or over a period of weeks ('growth' compensation point) indicates whether that species is a sun or a shade plant (Bieleski 1959; Boardman 1977; Huber 1978; Ashton and Turner 1979). Mahmoud and Grime (1974) criticized use of the light compensation point as an estimate of shade tolerance, preferring to measure the negative relative growth rate below the compensation point. Nevertheless, species accepted as shade-tolerant usually have a light compensation point of less than 2% full sunlight (Jarvis 1964). Other properties considered characteristic of shade plants are: high chlorophyll concentration per unit leaf area (Loach 1967; Nobel 1976; Bunce et al. 1977), expanded leaf area (Packham and Willis 1977), reduced development of the palisade mesophyll (Jackson 1967; Bjorkman et al. 1972; Nobel et al. 1975) and increased leaf area ratio in shade compared with full sunlight (Evans and Hughes 1961; Hiroi and *Part 111, Aust. J. Bot. 1982, 30, 347-57. 0067-1 924/83/02015 1$02.00