Water stress in Eucalyptus pauciflora: comparison of effects on stomatal conductance with effects on the mesophyll capacity for photosynthesis, and investigation of a possible involvement of photoinhibition.

Seedlings of Eucalyptus pauciflora Sieb. ex Spreng., grown in 4-1 pots, were stressed by withholding water while relationships between net assimilation rate (A) and intercellular partial pressure of CO2 (pi) in selected leaves were obtained repeatedly throughout the stress cycle. Water stress at first caused stomatal closure without any decline in the A(pi) relationship. As stress became more severe, the A(pi) relationship was affected as well. This always affected assimilation rate at both high and low intercellular partial pressures of CO2. It was then tested whether water-stressed leaves were more prone to photoinhibition than unstressed ones. Plants were water-stressed while at the same time subjected to strong photon flux area density (2000 μmol quanta·m(-2)·s(-1)). A possible light-induced inhibition was assessed by comparing quantum yields of photosynthesis with light directed onto one or the other surface of the leaf. A decline in quantum yield was observed, and the decline on the previously irradiated side was more pronounced than on the previously shaded side, but the effect was small and disappeared entirely within 1 d of rewatering the plants. It is concluded that photoinhibition can play a role, but not an important one, in the effect of water stress on the A(pi) relationship in leaves of E. pauciflora.