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sub-stomatal CO2 concentration suggested a possible decrease of photosynthetic rate. The decrease in photosynthesis and stomatal conductance in our study was also followed by decreases of Ci under defoliation. Following analogy of the abiotic stress impact (i.e. drought) on the limitation of photosynthesis recorded in various papers (Gallé et al., 2007; Gallé and Feller, 2007; Haldimann et al., 2008; Arend et al., 2013; Arend et al., 2016), increases in Ci could be defined as a metabolic limitation of photosynthetic process of gypsy moth attacked leaves.
The decrease in photosynthesis of young oak seedlings under drought treatment is difficult to interpret and generalize because, besides the factor of treatment and date, the origin of seedlings plays a significant role given genetic effect on the physiological performance of the plants. Correlation between the genetic background and leaf gas exchange parameters is very well documented (Orlović et al., 1998), which often causes different physiological responses of the individuals within the same species to unfavorable conditions (Pilipović et al., 2014; Pointeau and Guy, 2014; Bojović et al., 2017; Vastag et al., 2019). Alterations of photosynthetic parameters differed between the families, where decreases in A in family 2 were not followed by increases in Ci like in family 7. Such results indicate possible higher drought sensitivity of family 7, compared to family 2. Similar results were observed by Haldimann et al. (2008) in Quercus pubescens L. who hypothesized that drought-induced metabolic limitations and drought-dependent increases in mesophyll resistance to the diffusion of CO2 were present. As mentioned previously, sub-stomatal concentrations of carbon dioxide (Ci) together with stomatal conductance (gs) indicate mechanisms and severity of photosynthetic inhibition where decreased Ci is related with stomata and increased Ci is described as a metabolic limitation of photosynthesis. Different studies (Lawlor, 1995; Gallé et al., 2007; Mathobo et al., 2017) confirmed that an intense drought leads to an increase in the values of Ci, while the conditions of a moderate drought result in reduction of Ci. Considering the results of this study, drought treatment affected Ci values in different manners, emphasizing the importance of genetic background in understanding drought responses of tested families. Opposite to photosynthesis, drought did not affect transpiration of investigated oak families. Bréda et al. (1993) concluded that oaks were drought-tolerant species due to their ability to maintain significant transpiration intensity under reduced water availability in the soil. This corroborates results obtained in this study (i.e., transpiration did not decrease under drought treatment).
As expected, the presence of stress factors affected metabolic processes in both investigated oak families. On the other hand, expected synergistic effects of both drought and defoliation were not observed in this study, probably due to the various expressions of simultaneously occurring stressors. According to Copolovici et al. (2014), one type of stress could weaken or enhance the effects of another simultaneous stress factor by direct physiological cumulative or interactive effects. In contrast, La Spina et al. (2010) emphasized a lack of expected parabolic response of herbivore performance to tree water status.
Despite chlorophyll a and b being highly sensitive to decreased soil moisture (Farooq et al., 2009) and that drought-induced reductions in pigment contents were previously found in many woody plant species (Lei et al., 2006; Gallé and Feller, 2007; Guerfel et al., 2009, Arend et al., 2013), chlorophyll contents of tested families did not respond similarly. In family 7 D treatment decreased chlorophyll content, while a significant decrease of pigments in family 2 was recorded only under gypsy moth defoliation (GM). Similar observation also was noted during summer drought in Q. robur and Q. petraea (Epron and Dreyer, 1993). According to Rahdari and Hoseini (2012), an increase in chlorophyll levels under conditions of environmental stress is one of the resistant symbols in plants that are proportional to stress. Since family 2 had a higher content of pigments under drought treatment than family 7, the results indicated that those plants may have had a higher drought tolerance and provided a stronger photoprotective system against drought stress than plants from family 7.
It is very well documented that NRA generally decreases in leaves of plants subjected to water stress (Kaiser and Förster, 1989; Foyer et al., 1998; Garg et al., 2001). During water deficit, NRA decreases more rapidly than most enzymes (Huffaker et al.,1970) and often presents more sensitive physiological indicators of water stress than either stomatal closure or photosynthesis (Bardzik et al., 1971; Hsiao et al., 1976). The negative effect of water deficit on the activity of this enzyme may result from decreased nitrate reductase protein or decreased activation of the existing protein (Correia et al., 2005). In our experiment, compared to the control, NRA appears to vary significantly in the leaves of family 2 in response to drought treatment. On the other hand, family 7 did not express decrease in any of the treatments. One of the reasons for this may be the fact that compatible solutes may contribute to the maintenance of enzyme activity. According to Smirnoff et al. (1985), accumulation of proline may facilitate the continued synthesis of nitrogenous compatible solutes using excess photochemical energy available when stomata are closed, as recorded in family 7. In addition to carbon metabolism, nitrogen metabolism is also affected by drought stress.
Recovery period still showed a significant decrease of net photosynthesis and transpiration in both investigated