Contrasting Morphology and Ecophysiology of Co-occurring Broad and Terete Leaves in Hakea trifurcata (Proteaceae)

Abstract

We studied the morphology, anatomy, phyllotaxy and daily seasonal ecophysiology of the two leaf types (broad and terete) of Hakea trifurcata (Smith) R.Br., a widespread shrub in south-western Australia. Both leaf types may be present on the same branchlet, with one or two broad leaves forming first during the annual growth period (late winter) followed by many terete leaves in spring. Terete leaves were more xeromorphic than broad leaves, including greater thickness, denser tissues and fewer veins. Broad leaves fixed more carbon and transpired more water per unit mass than terete leaves, in a well ventilated leaf chamber, and had lower (more negative) xylem pressure potentials. Broad leaf temperatures only exceeded those of terete leaves under hot, dry conditions, with no relationship between transpiration rates and leaf temperature. Terete leaves possessed many structural and physiological characteristics commonly associated with drought-tolerant leaves, whereas broad leaves were characteristic of leaves which keep their stomates open during periods of water and heat stress. Both leaf types appear to increase the fitness of this species in a mediterranean climate, with broad leaves having the potential to supply extra photosynthates and nutrients to the new season's growth.