Physiological changes in red spruce seedlings during a simulated winter thaw

We evaluated net photosynthesis, respiration, leaf conductance, xylem pressure potential (XPP) and cold hardiness in red spruce (Picea rubens Sarg.) seedlings exposed to either a continuous thaw (CT) or a daytime thaw with freezing nights (FN) for 8 days during mid-winter. Physiological differences between CT and FN seedlings were evident for all measured parameters. However, the temporal expression of treatment differences varied among parameters. When compared to FN seedlings, CT seedlings had higher rates of respiration following 24 h of treatment, and a higher net photosynthetic rate, leaf conductance and XPP after 48 h of treatment. The CT seedlings were significantly less cold tolerant than the FN seedlings following 4 days of thaw, whereas FN seedlings did not deharden over the 8 days of treatment. Examination of temporal trends among thaw-associated changes in physiology suggested that, although greater carbon exchange occurred as stomatal conductance increased, the transition from negative to positive net photosynthesis was not the result of increases in conductance, but may have been associated with thaw-induced increases in XPP. Because thaw-associated changes in gas exchange and cold hardiness were offset in time, we conclude that, if changes in these processes are physiologically linked, the linkage is indirect.