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

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 (F_v/F _m), phyllode dark respiration (R_dark), stomatal conductance to water (g_s), light-saturated photosynthesis (A _sat) were monitored. Drought significantly reduced RWC, g _s, F_v/F_m, and A_sast; 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 R _dark but increased the R_dark/A_sat 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 R _dark/A_sat ratio, with drought-mediated increases in R _dark/A_sat 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 R_dark under drought is particularly relevant within a climate change scenario where more severe and frequent drought episodes are predicted to occur.