Moa ghosts exorcised? New Zealand's divaricate shrubs avoid photoinhibition

Abstract

1. An intriguing feature of the New Zealand flora is the high frequency of 'divaricate' shrubs and tree juveniles. These plants have numerous interlacing wide-angled branches with small leaves and no or few leaves in the outer canopy. They comprise 10% of native woody species, and have evolved in 18 different plant families. 2. We tested the hypothesis that the leaves of divaricate plants are sensitive to cold-induced photoinhibition, and that self-shading by outer branches reduces light intensities enough to prevent photodamage. In a field experiment, leaves of three divaricate species (Aristotelia fruticosa A. Cunn., Corokia cotoneaster Raoul and Coprosma propinqua Hook. f.) inside the north (sunny) side of the shrubs were exposed to one of three experimental treatments over winter: (i) control leaves which were not manipulated; (ii) exposed leaves which had their outer screen of branches pruned away leaving them open to full sun; or (iii) shaded leaves which were exposed by pruning, then sheltered from direct sunlight with shade cloth. 3. Experimental removal of the shielding branches in winter led to rapid (< 20 days), large (23-31%) and persistent (> 3 months) reductions in the maximum photosynthetic capacity (Amax) and photochemical efficiency (dark adapted Fv/Fm) in exposed leaves, but not in shaded leaves. Full recovery of photosynthetic capacity and photochemical efficiency was displayed by Coprosma propinqua, but very little capacity for recovery was displayed by Aristotelia fruticosa or Corokia cotoneaster. 4. When the effects of self-shading and photoinhibition were combined, control leaves photosynthesized faster than exposed leaves at all ambient irradiances in A. fruticosa, and in bright light (> 500 μmol m-2 s-1) in C. cotoneaster. 5. Our data show that by shielding their leaves within an outer screen of branches, the three divaricate species studied reduce photoinhibition of photosynthesis. We contend that this architectural self-shading in divaricate plants maximizes potential carbon fixation by minimizing photoinhibition.