MODERATE WATER-STRESS ALTERS CARBOHYDRATE CONTENT AND COLD TOLERANCE OF RED SPRUCE FOLIAGE

Red spruce (Picea rubens Sarg.) has been tested for response to many e nvironmental stresses and found to be relatively tolerant. An explanat ion for this tolerance is its maintenance of high levels of carbohydra te reserves, mainly soluble sugars and starch, which change with alter ations in photosynthetic capacity, rate of growth and degree of cold t olerance through the year. Red spruce saplings responded to moderate w ater stress in early September by depleting foliar starch and maintain ing foliar soluble sugars. Foliar carbohydrate contents of water-stres sed trees were restored to control levels within 1 week of rewatering. During and immediately after the water-stress period, foliage of wate r-stressed trees was more cold tolerant than foliage of well-watered t rees as measured by relative electrolyte leakage. Two months after ter mination of the water-stress treatment, starch content of fine roots a nd cold tolerance of foliage of well-watered trees were significantly higher than those of water-stressed trees. Before the trees became ful ly cold tolerant, differences between treatments in the degree of foli ar cold tolerance were positively associated with differences in solub le sugar content. As trees became more cold tolerant, soluble sugar co ntents were not related to the degree of cold tolerance. These results suggest that species with substantial reserves withstand periods of r educed carbon fixation by utilizing reserves for maintenance; however, lowered reserves could result in increased susceptibility to subseque nt stresses.