Leaf isoprene emission declines in Quercus pubescens seedlings experiencing drought - any implication of soluble sugars and mitochondrial respiration?

摘要

Previous studies suggest that the sensitivity of leaf mitochondrial respiration and the pool of soluble sugars to water stress could influence the response of leaf isoprene emission to drought by affecting the availability of extra-chloroplastic carbon for isoprene synthesis. We measured rates of isoprene emission and CO₂ exchange, and the concentration of nonstructural carbohydrates in leaves of Quercus pubescens Willd. seedlings subjected to either normal watering (control plants, C) or drought (droughted plants, D). Stopping of watering caused predawn leaf water potential ( Psi _pd) to decline between -2.3 and -5.1 MPa among D plants, whereas Psi _pd remained higher than -0.45 MPa in C plants. Isoprene emission (I_s), net CO₂ assimilation (A_n) and dark mitochondrial respiration (R_d) decreased with increasing water deficit, with declines in these variables relative to the respective means of C plants being A_n > I_s > R_d. This resulted in positive pairwise correlations between the three variables. The concentration of nonstructural carbohydrates did not change between treatments, but the concentration of soluble sugars increased and that of starch decreased in D plants as compared with C plants. As a consequence, there was a negative correlation between I_s and the concentration of soluble sugars, which supports a limited use of cytosolic sugars in sustaining isoprene synthesis at high to severe water stress. Our data also indicate that competition between I_s and R_d for the same carbon substrates had little importance for isoprene emission at high to severe water stress, as compared to the overall constraint on isoprene metabolism probably imposed by the shortage of photosynthetic carbon, energy and reducing equivalents.