Abstract:
Plants pass through series of changes in growth habit, leaf size, and shape during ontogeny. In some plants these changes are gradual (homoblasty), while in others changes are dramatic between seedling and adult stages (heteroblasty). I predicted that by having fixed leaf morphologies at seedling and adult stages, heteroblasty could be an adaptation to changes in light environment from understory to overstory. Specifically, I tested whether heteroblastic seedling leaf morphology is associated with better survival and growth in low light, and if heteroblastic plants were less plastic within a life stage relative to that of homoblastic seedlings. I grew seedlings of four pairs of congeneric homoblastic (Hoheria Iyallii, Aristotelia serrata, Pseudopanax arboreus, Melicope ternata) and heteroblastic species (H. sexstylosa, A. fruticosa, P. crassifolius, M. simplex) in deep shade and full sun light environments in a glasshouse, and in forest understory and canopy gaps in the field (Hoheria and Aristotelia only). I measured foliar responses (morphology, physiology, anatomy) and growth attributes (above and below ground growth and allocation) among homoblastic and heteroblastic species. I found that heteroblastic species had lower survival relative to homoblastic species in deep shade, but had 100% survival in full sun enclosures and canopy gaps.
Heteroblastic species, which initially had lobed/trifoliate leaf morphology, produced entire/simple leaves in the shade, but retained their initial leaf morphology in full sun. In the shade, leaf physiological measures of net photosynthesis and stomatal conductance were lower for heteroblastic species than homoblastic species. In comparison to homoblastic species, heteroblastic congeners produced thicker leaves in full sun but varied in the shade. However, both in sun and shade heteroblastic species had greater stomatal pore area (stomatal density x stoma aperture length) than their homoblastic congeners. Heteroblastic species were lower in total leaf area and growth than homoblastic relatives in sun and shade with the exception of growth in the understory sites in the field. Biomass allocation to seedling parts varied between glasshouse and field experiments. However, in both experiments heteroblastic species had greater allocation to shoots in full sun enclosures and in gaps. Both homoblastic and heteroblastic species had similar plastic responses to changes in light environment. I conclude that heteroblastic seedling leaf morphology is not associated with deep shade and is not less plastic relative to their homoblastic congeners.
