Since the seedling phase is the most vulnerable stage in the life cycle of a tree, their hydraulic system needs to be protected against the harsh climate found, for example, on boreal clearcuts. There are commonly-used methods of hardening plants. But these are difficult to apply, limited in their effectiveness, and tend to slow the growth of plants. To identify simple, yet effective, methods of protecting plants, their own natural, phenylpropanoid defences were activated. This could be done using either antioxidants or ultra-violet light (UVB) to provide a kind of ‘immunization’ against the damaging effects of stress. Low to ambient levels of UVB were found to increase phenylalanine ammonia lyase (PAL) production, and the resulting increase in flavonoid synthesis enhanced hardiness and seedling vigour. To study how seedlings acclimate to chronic stress, a computerized root misting system was used to supply a series (mild to severe) of root dehydrations. Drought acclimation varied with the species, ecological niche and number of stress cycles, when species from drier (jack pine) and moister (black spruce) regions of the boreal forest were compared. Black spruce showed earlier stomatal closure and the more rapid decline in hydraulic conductance (K) indicated the earlier onset of cavitation in black spruce. Static features in jack pine postponed dehydration (e.g., a larger root area and smaller needle area). Although jack pine was unable to adjust turgor, or K, it was more resistant to cavitation. While a high K is necessary for rapid growth, cavitation can limit K, which could aid survival when drought becomes severe.

19-29