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Four-year-old seedlings of Abies fraseri [(Pursh) Poir] (fraser fir) were grown in semicontrolled 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 to drought stress. Drought stress was monitored by measuring predawn (Cpd) and midday (Cmd) potentials in a subset of plants from each treatment. Physiological variables monitored were chlorophyll fluorescence (Fv/Fm), Chl a, Chl b, total carotenes, and total carbohydrate concentrations. Morphological characteristics including height growth, root collar diameter, and terminal shoot growth were also 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 Cpd and Cmd compared with nonirrigated treatments. Photosynthetic pigments (Chl a, Chl b, and carotenes) decreased midseason (14 July) and increased toward the end of the season (25 Aug.) in predawn and midday measurements. There was a significant effect (P < 0.05) of drought stress on photosynthetic pigment concentrations in predawn and midday samples in the late-season measurements (25 Aug.). These results were accompanied with a similar significant difference in Fv/Fm between non-irrigated and irrigated trees. We concluded that significant effects observed on photosynthetic pigment concentrations in some of the treatments did not affect carbohydrate concentrations. Exposure of A. fraseri to water stress did not cause a reduction in supply of metabolic carbohydrates; consequently, the decline and mortality in water-stressed plants can only be the result of hydraulic failure caused by xylem cavitation leading to cessation of water flow in tissues, desiccation, and cellular death. Further studies are needed to confirm these preliminary conclusions. Fraser fir [Abies fraseri (Pursh) Poir.] is widely planted for Christmas tree production in the midwest and eastern United States. The species has a unique natural distribution, restricted to the southern Appalachian Mountains of southwestern Virginia, western North Carolina, and eastern Tennessee, characterized by high elevations, prevalent fog cover, lower summer temperature extremes, and regular summer rainfall. The species has been intensively planted elsewhere in the United States as a result of its desirable attributes (Beck, 1990; Nzokou and Leefers, 2007); however, the physiological consequences of planting this species under summer drought stress-susceptible conditions are not well understood. Drought stress is characterized by reduction of water content, increased closure of stomata, and decrease in cell enlargement and growth. Severe water stress may result in the arrest of photosynthesis, progressive suppression of photosynthetic carbon assimilation, disturbance of metabolism, and finally the death of the plant (Deltoro et al., 1998; Jaleel et al., 2008). After stomatal closure, there is a decrease in CO2 intake and intercellular CO2 partial pressure, thereby a reduction in CO2 assimilation and net photosynthesis (Deltoro et al., 1998; Dubey 1997; Farquhar et al., 2001). Stomatal closure is the result of either hydropassive or hydroactive mechanisms (Dubey, 1997). In the hydropassive mechanism, guard cells loose water so rapidly that the loss cannot be balanced by water movement from adjacent cells (Shope et al., 2008). In the hydroactive mechanism, stomatal closure is the result of the reduction in stored abscisic acid from the mesophyll chloroplast to the apoplasts (Hartung, 1983). In addition, the rate of CO2 assimilation is depressed at very moderate water deficits or even before the plant’s water status changes in response to the drop in water pressure deficit (Bunce, 1981) or soil potential (Gollan et al., 1986). Therefore, it can be expected that diurnal changes in plant water potential caused by daily variation associated with environmental conditions will interact with water stress to affect the photosynthetic system. Direct consequences of these processes are inhibition of cell expansion, which adversely affects crop growth and yield. On the other hand, water stress causes ultrastructural changes in the chloroplasts that adversely affect photosynthesis (Dubey, 1997). Under drought stress, contents of photosynthetic pigments (Chl a, Chl b, and carotenes) diminish(Pukackiand Kaminska-Rozek,2005; Terzi et al., 2010; Yordanov et al., 2000). For example, in fully active leaves of Xerophyta scabrida submitted to drought stress, the CO2 assimilation, thylakoid activity, and respiration rate declined, whereas chlorophyll and carotene contents were successively broken down (Deltoro et al., 1998). When drought coincided with high radiation, it led to a decrease in carotenoid content in evergreen woody species Quercus ilex, Quercus coccifera, Pinus halepensis, and Juniperus phoenicea (Baquedano and Castillo, 2007). However, photosynthetic systems have the ability to recover from drought stress when water becomes available (Nar et al., 2009). This process involves a complex of signals comprising metabolites produced during rehydration. For example, re-accumulation of Chl (a + b) and carotenoid synthesis was observed in stressed plants once leaves reached 91% of the maximum leaf water content (Schwab et al., 1989; Tuba et al., 1996). This underscores the need to understand not only diurnal patterns, but also seasonal changes in photosynthetic pigments caused by variations in stress conditions that Received for publication 7 June 2012. Accepted for publication 14 Aug. 2012. Assistant Professor. Associate Professor. Professor. To whom reprint requests should be addressed; e-mail nzokoupa@msu.edu. 1512 HORTSCIENCE VOL. 47(10) OCTOBER 2012 plants are subjected to during a growing season. Another possible physiological response to drought stress is the alteration in the synthesis of both starch and sucrose in stressed plants (Vassey and Sharkey, 1989). Impairment of photosynthesis resulting from drought stress can result in the utilization rate of photosynthates exceeding their production rates, thus making carbon and energy metabolism dependent on non-structural carbohydrate reserves (Guehl et al., 1993). However, in some species, drought can enhance carbohydrate accumulation (Guehl et al., 1993; Thomas, 1990). In that situation, the increase in soluble sugars in response to water stress can be attributed to less translocation from the leaf, slower consumption resulting from decreased growth, and other changes such as starch hydrolysis (Kameli and Losel, 1996). We are hypothesizing that under increased drought conditions, trees will respond by decreasing their photosynthetic abilities resulting from stomatal closure, leading to a decrease in quantities of photosynthates produced. We also expect diurnal variation with a midday depression in measurements resulting from environmental conditions. The aim of this study was to determine diurnal and seasonal patterns in photosynthetic pigments (Chl a, Chl b, carotenes), carbohydrate accumulation, and growth of Abies fraseri under controlled drought stress created by varying irrigation regimes. The results of this study could help in understanding the drought tolerance and adaptation potential of this species to drought-prone environments. Material and Methods The study was conducted in four hoop houses running east to west at the Tree Research Center (lat. 42.65 N, long. 84.42 W) on the campus of Michigan State University. The hoop houses were designed to keep rain from falling directly on to treatment plots. Each house was divided into five sections containing the various treatments by waterproof dividers (oriented strandboard wrapped with a plastic cover) inserted about 1 m into the ground to prevent lateral movement of soil moisture. All hoop houses were open at both ends and 15 cm from the ground on both sides to allow free airflow and avoid rises in temperatures as a result of a greenhouse effect. This process was successful, although inside air temperatures were only 1 or 2 C higher than outside temperatures in the middle of very warm days. Each of the hoop house sections was randomly assigned one of the five irrigation treatments with all five treatments replicated in each of the four hoop houses (blocking) in a randomized complete block design. Each plot was physically separated from the adjacent plot by 2 feet wide 3/4-inch board wrapped with a plastic cover to prevent lateral movement of moisture and maintain the integrity of each irrigation treatment. These boards were vertical inserted between plot boundaries before the experiment began. Water was supplied during the growing season (10 May to 30 Aug. 2009) through an irrigation system equipped with Uniram (1.55 cm i.d. and 1.1 mm wall thickness) drip line manufactured by Netafim (Netafilm Irrigation Inc., Fresno, CA). The drip emitters used were equipped with pressure regulators to deliver 1.6 L per hour (0.42 gallon per hour) and spaced 60 cm (24 inches) apart. The irrigation system was fully automated and operated by an ACCLIMA SC 24 controller (Acclima Inc., Meridian, ID). One digital time domain reflectometry moisture sensor model ACC-SEN-TDR (Acclima Inc.) was inserted in each treatment for monitoring of soil moisture content and soil temperatures. Variability in soil moisture content and soil temperature from midday readings obtained from soil moisture sensor readings confirmed clear differences between irrigated and non-irrigated treatments (Fig. 1). The layout and tubing were designed to simultaneously apply water to all four replicate plots for each irrigation level (0, 0.62, 1.25, 2.5, and 3.75 cm/week). Irrigation depths were calculated using the emitter flow rate, the area covered by each emitter, and the run time controlled by the controller. These irrigation depths correspond to 0, 12, 18, 25, and 38 min of run time per

hortscience

Growth and Physiological Response of Fraser Fir [Abies fraseri (Pursh) Poir.] Seedlings to Water Stress: Seasonal and Diurnal Variations in Photosynthetic Pigments and Carbohydrate Concentration