Photosynthetic and Morphological Response of Abies fraseri (Pursh) Poir to the Combined Effect of Irrigation and Fertilization in Controlled Environment

Three-year-old (plug+2) Fraser fir [Abies fraseri (Pursh) Poir.] seedlings were grown under three irrigation (375, 750 and 1500 mL/week) and fertilization levels (2.5, 5.0, 10 g N/L) in the greenhouse, with the goal of determining the interactive growth and physiological response to irrigation and fertilizer application rates. Physiological variables measured included chl concentrations (Chl a, b, total chl and carotenes), gas exchange parameters [photosynthesis (A), stomatal conductance (Gs), transpiration rate (TR), water use efficiency (WUE), and stem water potential (Ѱ_S)]. Growth variables included height growth (HG) and root collar diameter (RCD). Irrigation had the largest impact on HG and RCD.  Water stress had a significant impact (P < 0.05) on gas exchange parameters for all three-measurement dates. The interaction of irrigation and fertilization on photosynthesis was statistically significant on the September 18 measurement and not significant for the other two measurement dates (August 5, and August 28). Gs and WUE showed similar seasonal trend throughout the measurement period. Even though irrigation had a positive effect on A, Gs, WUE, and TR, fertilization had a negative correlation with those variables. As expected midday stem water potential (Ψmd) values were generally lower than the predawn (Ψpd) values. Photosynthetic pigments (Chl a, chl b, total chl, and carotenes) generally increased throughout the treatment periods. However, the interaction of irrigation and fertilization on photosynthetic pigments was not significant (P > 0.05). Our results suggest that the plant’s water condition is critical to physiological processes measured in the study.