Growth and Physiological Response of Abies Fraseri to Drought Stress

Four-year-old seedlings of Abies fraseri [(Pursh) Poir] (Fraser fir) were grown in semi-controlled conditions in hoop-houses with five watering regimes (0.00, 0.62, 1.25, 2.50, and 3.70 cm/week), with the goal of determining the seasonal variation in the physiological response of this species to drought stress. Soil moisture and temperatures were continuously measured with sensors connected to a datalogger. Drought stress was monitored by measuring predawn (Ψpd) and midday (Ψmd) levels in a subset of plants from each treatment. Physiological variables monitored were chlorophyll fluorescence (F_v/F_m), Chl a, Chl b, total carotenes, and total carbohydrates concentrations. Several morphological characteristics including height growth, root collar diameter, and terminal shoot growth were measured. Predawn stem water potential values were generally higher (-0.8 to -1.9 Mpa) than midday values (-1.3 to -2.9 Mpa). Irrigation consistently increased both Ψpd and Ψmd compared to the control treatments. However, the separation was less consistent in Ψpd and Ψmd measurements between trees irrigated at 0.62, 1.25, 2.50, and 3.70 cm/wk. This suggests a significant difference in stress between irrigated and control treatments, but low level of differentiation between irrigated treatments. Reducing drought stress resulted in increased plant relative height growth (RHG), relative root collar diameter (RRCD), and terminal shoot growth (TSG). Similar to stem water potential values, there was a clear contrast between control and irrigated treatments, and less variation between the various irrigation treatments. Photosynthetic pigments (Chl a, Chl b, and carotenes) decreased mid-season (Jul. 14), and increased towards the end of the season (Aug. 25) in both predawn and midday measurements. There was a significant effect (p<0.05) of drought stress on photosynthetic pigment concentrations in both predawn and midday samples, with a very clear dose response in the late season measurements (Aug. 25). These results were accompanied with a similar significant difference in F_v/F_m between control and irrigated trees. The seasonal variation in carbohydrate concentration was similar to photosynthetic pigments decreasing in the middle of the season and increasing late in the season. However, there was no clear contrast between irrigated and non-irrigated treatments. These results demonstrate that the drought response of A. fraseri seedlings include several physiological changes starting with the decrease in photosynthetic pigments caused by structural changes in the thylakoid membrane of the chloroplasts. However, the low differentiation in carbohydrate content suggests that there are parallel physiological and biochemical changes enhancing the capability of plants to survive and grow during drought periods.