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ŠUMARSKI LIST 11-12/2020 str. 44     <-- 44 -->        PDF

vitality of oak stands due to limitations in water availability (Stojanović et al., 2014; Stojanović et al., 2015; Kostić et al., 2019). Pedunculate oak is an especially interesting species for breeding and selection programs, taking into consideration its ecophysiology. Dependence of this species on soil water properties plays an uncertain role in its survival and stress response and, therefore, a focus on ecophysiology of this valuable broadleaved species should be a priority in research, breeding and forest management.
Survival and distribution of sessile organisms such as plants depend strongly on their ability to adjust to environmental variation (Beikircher and Mayr, 2009). Water stress, especially drought stress, is the main restriction of plant growth and development (Hu et al., 2004) where competition for water with mature trees may exacerbate drought effects on understory tree seedlings (Aranda et al., 2005). Water shortage and drought periods present limiting factors for forest regeneration in modern silvicultural practice by weakening the seedlings and affecting their performance at several levels. Besides direct effects on whole plant water status, drought causes reduction in stomatal aperture that diminishes CO2 supply to mesophyll cells, thus causing a reduction in photosynthetic rate (Blödner et al., 2007). However, stomatal limitation is not the only reason for decreased photosynthesis in drought-affected plants. According to Gallé et al. (2007) and the references within, the stomatal limitation predominately affects photosynthesis in plants under moderate drought conditions, whereas in severe drought metabolic limitations become dominant. Various parameters can be assessed in order to be more precise about whether the drought-induced photosynthetic decrease results from stomatal or metabolic limitations, such as chlorophyll a fluorescence (Gallé and Feller, 2007; Arend et al., 2013; Arend et al., 2016; Vastag et al., 2020), chlorophyll content (Gallé et al., 2007; Arend et al., 2012), biochemical markers (Gallé et al., 2007; Stojnić et al.,2019a) or even different non-structural carbohydrates and carbon isotope signatures (Pflug et al., 2018).
Among a vast number of factors, insects present an important biotic stressor that affects plant growth and vitality (Drekić et al., 2019). Outbreaks followed by and combined with climate extremes (e.g. temperature, precipitation, drought) will become more frequent due to the increase of greenhouse gas levels in the atmosphere. Plants, as sedentary organisms, cannot escape from attacks and stress and have to adjust to the surrounding environment and biotic attacks through their life cycle (Niinemets et al., 2013). Decreases in total leaf number and area are not the only response of plants to defoliation (Meyer, 1998; Poljaković-Pajnik et al., 2019), as grazing injury may also include a vast spectrum of metabolic and physical changes in host plants (Oleksyn et al., 1998), including enzymatic activities, gas exchange processes or accumulation of different metabolites like proline. Considering it as a significant determinant of plant growth, yield and fitness (Welter, 1989), plant gas exchange and influence of insects on it, present a significant aspect for further research in selection programs. On the other hand, close correlation of nitrogen metabolism with photosynthesis (Marschner, 1995) highlights nitrate reductase activity (NRA) as a good parameter for investigation since it is the first enzyme in nitrogen metabolism (Kastori and Petrović, 2003). Also, nutrition levels and the presence of allelochemicals in plants determine plant suitability and resistance to insect herbivores (Chen et al., 2011).
Gypsy moth (Lymantria dispar L.) is the most significant pest of hardwood forests in the northern hemisphere (Elkinton and Liebhold, 1990). The species naturally occurs in Eurasia but is also introduced in Northern America. Gypsy moth populations have occasional outbreaks lasting 4 to 5 years (Mihajlović, 2008) when caterpillars cause damage during leaf feeding on almost all forest tree species, except ashes. For its development, gypsy moth mostly prefers pure oak stands, with emphasis on Turkey oak (Q. cerris L.) (Milanović, 2006). Defoliation often causes losses in increment and acorn yield, as well as physiological weakening and susceptibility to secondary pests. In the case of consecutive defoliation, increases of increment loss and mortality occur (Mihajlović, 2008), where increment loss may range from 30-70%, according to Mirković and Miščević (1960).
Considering the importance of pedunculate oak for forest biodiversity and ecosystem restoration initiatives, constant efforts for its ex situ and in situ conservation through research of genetic diversity are of profound importance in order to mitigate climate change (Stojnić et al., 2019b). Apart from these efforts, breeding of more resilient provenances presents one of the efficient ways for future adaptation of forest tree species to climate change, since adaptation enables plants to optimize their life processes in prevailing environmental conditions at an evolutionary scale (Beikircher and Mayr, 2009). In order to achieve these long-term goals of pedunculate oak conservation and selection projects, integrating different research fields such as plant physiology, genetics and entomology presents a holistic, interdisciplinary solution that is appropriate for current and upcoming environmental challenges caused by climate change. Also, success in tree breeding can be facilitated by increased understanding of the physiology of growth and survival during water supply limitations (Wikberg and Ogren, 2007). Therefore, the goal of this research was to determine the simultaneous effect of drought stress and/or attack by caterpillars on the physiological traits of different families of pedunculate oak seedlings, during the stress period and after recovery.