Effect of Different Photon Flux Densities (PAR) on Seedling Growth and Morphology of Metrosideros collina

Seedlings of Metrosideros were capable of net accumulation of dry matter at photon flux densities as low as 13 /lmol m2 S-I PAR (about 0.6 percent sunlight), when grown under a 12-hour day1ength at 20° or 25°C for over 4 months. Seedlings became adapted to shade at low levels of PAR by an increased leafiness of the plant (expressed as the leaf area ratio). This increased leafiness was brought about by a marked reduction in leaf thickness rather than by an increase in the proportion of assimilates distributed to leaves. 'OHI'A (Metrosideros sp., Myrtaceae) AND KOA (Acacia Koa Gray) are the two major endemic tall trees found in the Hawaiian rain forests. Metrosideros collina (Forst.) Gray is a highly variable species, with seedlings capable of colonizing the high-radiation environment of a lava flow exposed to full sunlight or the highly shaded environment of a treefern stem (Cibotium sp.) deep within the rain forest (Corn 1972, Smathers and MuellerDombois 1974). As part of studies on the importance of radiation in controlling the regeneration of Metrosideros in the forest environment, the following experiments were carried out to determine the lower limit of photosynthetically active radiation (PAR) over the wavelength range 400 to 700 nm needed to support early growth of seedlings. The methods of growth analysis (reviewed by Kvet et al. 1971) were used to further examine the adaptive response of seedlings to the level of PAR at which they were grown. MATERIALS AND METHODS Mature seed ofM etrosideros collina (Forst.) Gray subsp. polymorpha (Gaud.) Rock was 1 Contribution no. 74, Island Ecosystems IRP/IBP Hawaii, NSF grant GB-23230. Manuscript accepted 13 July 1979. 2 University of Hawaii, Botany Department, 3190 Maile Way, Honolulu, Hawaii 96822. obtained from the Kilauea rain forest on the island of Hawaii. All seed was from one seed cluster, and was sown on the surface of pieces of shredded hiipu'u (tree-fern root) in 4-inch plastic pots. The pots were watered from below and the tops of the pots were covered with plastic Petri dish lids. Germination took place in about one week in a growth room maintained at a constant temperature of 25°C and a daylength of 12 hours provided by white fluorescent lamps. The photon flux density of wavelengths in the photosynthetically active part of the spectrum from 400 to 700 nm (PAR level) was measured with a Lambda Instruments quantum sensor LI-185. For white fluorescent lamps, 1 /lmol ms1 PAR was equivalent to 48 lux. Seeds were germinated at PAR values of 40 /lmol ms-1 and then transferred to a series of PAR levels ranging from 8 to 235 /lmol ms1 in growth rooms maintained at a 12-hour daylength at either 20° or 25°C. A second series of plants was transferred to a similar series of PAR levels but under a 24-hour daylength at 20°C. The different levels of PAR were obtained by varying the distance of the plants from the light. Plants were watered with one-quarter strength Hoaglands solution (Hoagland and Arnon 1939), at 2to 3-day intervals, and harvested after 126 days growth at the 12hour daylength and after 262 days at the 24hour daylength. The number of leaves and