Homeostasis of respiration under drought and its important consequences for foliar carbon balance in a drier climate: insights from two contrasting Acacia species

Abstract

Under drought, when photosynthesis (A) is impaired, foliar respiration (R) becomes crucial to estimate plant carbon balance. However, the effects of drought on R remain unclear and little is known about differences between congeners with divergent anatomy. In this study we compared the physiological response to imposed drought in plants of two Acacia species: Acacia pycnantha Benth. and Acacia floribunda (Vent.) Willd. in a controlled environment. We subjected half of the plants to two desiccation cycles. Relative water content (RWC), the ratio of variable to maximum fluorescence (Fv/Fm), phyllode dark respiration (Rdark), stomatal conductance to water (gs), light-saturated photosynthesis (Asat) were monitored. Drought significantly reduced RWC, gs, Fv/Fm, and Asast; increased the instantaneous water use efficiency in the species with higher foliage mass per area (FMA) (A. pycnantha) and did not have any significant effect on Rdark but increased the Rdark/Asat ratio. Although the shape of the response to drought of both species was similar, the two species differed in the magnitude in the increase of the Rdark/Asat ratio, with drought-mediated increases in Rdark/Asat being greater in A. floribunda than in A. pycnantha; the latter also showing greater recovery of photosynthesis. Collectively, our results highlight the extent to which drought alters the carbon balance of the two selected species. The homeostasis of Rdark under drought is particularly relevant within a climate change scenario where more severe and frequent drought episodes are predicted to occur.