New Zealand Journal of Botany abstracts
The architecture of New Zealand’s divaricate shrubs in relation to light adaptation
Rochelle Christian*
Dave Kelly
Matthew H. Turnbull
School of Biological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand
*Present address:
School of Botany and Zoology, Building 116, Daley Rd, Australian National University, Canberra, ACT 0200, Australia.
rochelle.christian@anu.edu.au
Abstract The divaricating shoot habit is typified
by a suite of architectural traits, quantified here using phylogenetic
independent comparisons of three pairs of congeners, with or without
the habit. We consider the hypothesis that the habit evolved as a
structural photoprotection mechanism that maximises potential carbon
fixation by minimising photoinhibition. Plants were grown in pots in
full sun, or behind vertical screens transmitting c. 25, 52, or
73% sunlight. When shaded, all species shifted partitioning of biomass
from stem thickening to leaf area expansion and occupied a larger crown
volume for a given shoot biomass. Leaf numbers per stem length of
divaricates and non-divaricates were greater in the lower and upper
canopies, respectively, consistent with the view that in divaricates
outer branches protect inner leaves. However, leaf numbers per stem
length showed no response to variation in high light receipt.
Divaricates showed some traits typical of plants adapted to sunny
habitats: smaller effective leaf size, lesser fractional partitioning
of biomass to leaves, and greater foliage densities. Other traits of
divaricates were typical of plants adapted to shaded habitats: lesser
stem diameters, stem biomass per unit stem length, leader dominance,
leaf area index, and heights relative to crown diameters; and more
horizontal twig orientations. Compensation for high costs of support of
photosynthetic area in divaricates (leaf area per unit shoot biomass
c. 1.3 m2 kg-1) compared with non-divaricates (c. 5.5 m2 kg-1) would require a larger enhancement of net canopy photosynthesis than is likely to arise from avoidance of photoinhibition.
Keywords divaricate; morphological plasticity; penumbra; photoprotection; support costs
B05029; Received 11 July 2005; accepted 13 February 2006; Online publication date 2 May 2006
New Zealand Journal of Botany, 2006, Vol. 44: 171–186
0028–825X/06/4402–0171 © The Royal Society of New Zealand 2006
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