Changes in light and nutrient availabilities do not alter the duration of use of seed reserves in Araucaria angustifolia seedlings

Abstract

Initial plant growth is entirely dependent on seed reserves, until the photosynthetic organs become a significant net carbon source to the growing plant. The time during which these reserves are used may vary within species, depending on the availability of external resources. We have investigated the duration of use of the seed reserves in the large-seeded tree species Araucaria angustifolia, when growing under different light and nutrient availabilities. We hypothesised that this duration would not vary with growing conditions, because of the high pressure exerted by predators on these seeds and because of the existing possibility of transferring to and restoring part of their starchy reserves in the underground hypocotyl. Plant growth took place for 139 days under shade cloth in outside gardens, under each of the four combinations of high and low light or nutrient availabilities. Plants within each treatment were separated into six different groups, each including seedlings which were separated from their seeds by cotyledon excision at five different time periods after seed sowing (43, 63, 83, 103 and 124 days), plus a Control group of seedlings which were not deprived from their seeds. Final mass accumulation by seedlings that had full access to the seed reserves was compared with that of seedlings which were separated from their seeds at different time periods after seed sowing. Seed mass evaluations at each excision period allowed us to follow the temporal pattern of seed consumption. We found that seedlings growing under low light used seed reserves for growth longer than those under high light, regardless of their nutrient condition, pointing to the importance of a large seed for seedling establishment under deep shade. However, seed consumption followed a very similar pattern in both light treatments. Nutrient availability did not affect the duration of growth dependency on seed reserves or the pace of seed mass depletion. Our initial hypothesis was then accepted, and the possible implications of the reported early growth behaviour of the species in coping with seed predation and shading are discussed.