Characterization of the photosynthetic induction response in a Populus species with stomata barely responding to light changes

The photosynthetic induction response is constrained by stomatal and bioche mical limitations. However, leaves in some plants like Populus koreana x tr ichocarpa cv. Peace (a hybrid clone) may have little stomatal limitation be cause their stomata barely respond to changes in photon flux density (PFD). We examined the induction responses of leaves of well-watered and dehydrat ed P. koreana x trichocarpa plants grown in a high-light or a low-light reg ime. With an increase in PFD from 50 to 500 mu mol m(-2) s(-1), steady-stat e stomatal conductance (g(s)) increased by only 0.25-8.2%, regardless of th e initial g(s), but steady-state assimilation rate (A) increased by 550-181 0%. Photosynthetic induction times required to reach 50% (IT50) and 90% (IT 90) of A at high PFD were 60-90 s and 210-360 s, respectively. Examination of the dynamic relationships between A and g(s), and between A and intercel lular CO2 concentration, indicated that the induction limitation was impose d completely by the biochemical components within 30-40 s after the PFD inc rease. Values of IT50 and IT90, were significantly higher in low-light leav es than in high-light leaves, whereas the induction state at 60 s and the i nduction efficiency at 60 and 120 s after the increase in PFD were lower in low-light leaves than in high-light leaves. Dehydration reduced leaf water potential (Psi) significantly, resulting in a significantly decreased init ial g(s). Leaf water potential had no significant effects on induction time in high-light leaves, but a low Psi significantly reduced the induction ti me in low-light leaves. We conclude that the photosynthetic induction respo nse was limited almost completely by biochemical components because the sto mata barely responded to light changes. The biochemical limitation appeared to be higher in low-light leaves than in high-light leaves. Mild water str ess may have reduced steady-state A and g(s), but it had little effect on t he photosynthetic induction response in high-light leaves.