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 (Pukacki and 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 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.
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