Abstract
Quercus robur L. is one of the most valued tree species of deciduous temperate forests. However, in the last decade, serious oak declines and loss of adaptation plasticity have been reported throughout Europe as a consequence of drought. Therefore, the aim of the present study was to define the adaptation potential of five Q. robur half-sib lines from the UNESCO Biosphere Reserve Mura-Drava-Danube to drought, using physiological, anatomical and biometrical traits. Half-sib lines that exhibited drought tolerance had particular suites of trait expression regarding biometrical traits (seedling height, root length, root to shoot ratio of dry mass and specific leaf area), leaf stomatal traits (stomatal density per mm2, stomata guard cell length and width, stomatal aperture length and width) and leaf structural traits (adaxial epidermis thickness, palisade parenchyma thickness, spongy parenchyma thickness, lamina thickness). All of the observed parameters of chlorophyll a fluorescence were shown to be good indicators of short-term and severe drought. For the selection of drought-tolerant half-sib lines, all studied chlorophyll a fluorescence parameters associated with the heat dissipation of light energy (coefficient of non-photochemical quenching, quantum yield of regulated energy dissipation, Stern-Volmer type non-photochemical fluorescence quenching) and one parameter related to photochemical dissipation of light energy (effective quantum yield (efficiency) of PS II photochemistry) were proven to be suitable. On the other hand, the coefficient of photochemical quenching, coefficient of photochemical fluorescence quenching assuming interconnected photosystem II antennae and electron transport rate were not suitable for distinguishing the different responses of the studied half-sib lines under drought. The importance of results of the present study is in the selection of drought-tolerant Q. robur half-sib lines for future reforestation programs, particularly in protected areas with sensitive forest management and restricted activities for mitigation of the adverse effects of climate changes.
Highlights
Drought has been recognised as a major abiotic stress, limiting the geographical distribution, growth, and productivity of plants [1,2]
In the autumn of 2017, a total of 750 acorns were collected below the canopy of five phenotypically dominant pedunculate oak (Quercus robur L.) mother trees, whose crowns were located within the upper canopy (150 acorns for each mother tree), originating from the native population situated in the UNESCO Biosphere Reserve Mura-Drava-Danube (N 45◦ 91’, E 18◦ 88’)
A similar trend was observed for R:S parameters, with half-sib lines 1 and 5 (2.19 ± 1.14 and 2.22 ± 0.10) having the highest values (F = 5.4992, p ≤ 0.01) and belonging to the same homogenous group, half-sib line 4 having the intermediate values (1.16 ± 0.85), while half-sib lines 2 and 3
Summary
Drought has been recognised as a major abiotic stress, limiting the geographical distribution, growth, and productivity of plants [1,2]. Q. robur is widely distributed in European deciduous temperate forests [11]. As a consequence of its high timber quality, pedunculate oak forests are among the most valued in Europe [12]. The distribution of this species is characterised by fertile and moist soil conditions; its drought tolerance in the predicted forthcoming conditions will become essential [11]. As a consequence of disturbed conditions with declining levels of groundwater in South-East European forests, a reduced plasticity of Q. robur adaptation to drought was observed presuming even greater rates of mortality in the future [16]
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