Primary nerve (vein) density influences spatial heterogeneity of photosynthetic response to drought in two Acacia species

We examined the relationship between variation in phyllode nerve density and the spatio-temporal response of the photosynthetic apparatus to water-stress in two Acacia s.str. species with contrasting nerve patterns: Acacia floribunda (Vent.) Willd and Acacia pycnantha Benth. 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 PSII 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 greater primary nerve density may enhance drought tolerance and are consistent with the observed predominance of acacias with high primary nerve density in areas with lower precipitation.