Hydraulic constraints on the morphology of Acacia s.str. phyllodes

Abstract

Acacias occur across a diverse array of habitats and display great variation in the size and form of their primary foliage, phyllodes. I explored how phyllode anatomy affected variation in function of Australian Acacia. Two key climatic variables differentially affected functional diversity in phyllodes of Acacia. Traits associated with flux of gas or heat across surfaces were aligned with temperature. Traits related to phyllode mass per area were aligned with precipitation. Where other studies have found density to be the primary driver of greater mass per area in foliage, thickness was the primary driver of greater mass per area in phyllodes. This in turn was driven by differential changes in the thickness of the palisade and water storage layers with variation in precipitation, such that the carbon cost of water transport and storage was greater in phyllodes of acacias from drier habitats. One of the greatest sites of resistance to flow of water through plants is the foliage. Effects of variation in phyllode anatomy on hydraulic conductance were further explored in acacias that were differentially distributed along a precipitation gradient. Phyllode hydraulic conductance was greater in phyllodes with higher primary nerve density. Phyllodes with higher primary nerve density also had greater mass per area. I suggest that higher primary nerve density may provide a shorter and less tortuous pathway for water movement through high mass per area phyllodes thus accounting for their higher phyllode hydraulic conductance. High mass per area phyllodes were more common in acacias from areas receiving lower annual precipitation. Given the relationships between nerve patterns and precipitation, I examined the spatio-temporal variation in photosynthetic responses to water-stress in two Acacia species with contrasting nerve patterns, A. floribunda and A. pycnantha. A. floribunda had greater primary nerve density than A. pycnantha and also showed greater spatial homogeneity in photosynthetic function with drought than phyllodes of A. pycnantha. A. pycnantha had lower maximum quantum efficiency of photosystem II in phyllode tissue further from primary nerves consistent with its lower primary nerve density. Further, A. floribunda phyllodes maintained function of the photosynthetic apparatus with drought for longer and recovered more swiftly from drought than A. pycnantha. These findings suggest that the species with greater primary nerve density showed greater drought tolerance and are consonant with the observed predominance of acacias with high primary nerve density in areas with lower precipitation. Phyllodinous Acacia provide novel insights into the role of foliage in plant survival and productivity with changing temperature and precipitation. Show more