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This article presents the results of laboratory examinations concerning the effects of soil bulk density on the growth of sessile oak Quercus petraea Liebl. seedlings. The soil for the study was collected from a nursery plot and then compacted into PCV pots in eight different bulk density levels ranging from 0.81 to 1.32 g cm -3 . Oak seedlings were cultivated in controlled conditions for 76 days after sowing. The growth and mass parameters of particular seedlings together with the parameters of the assimilation apparatus and roots were measured, taking into account the share of various diameter roots in the root system. For the purpose of the experiment, 120 acorns were sowed, of which 73 seedlings were cultured. An analysis of the growth of the sessile oak seedlings revealed that a change in soil density significantly affected root system development, total height of seedlings and dry mass. It was observed that the dry mass of the root system as well as the area and length of the roots decreased with an increase in soil density. Root system reduction was noted, the size of which determines the proper development of the tree and ensures its stability. Besides a decrease in the root system, an increase in soil compaction also resulted in a reduction in the number of roots of a specified diameter. The first to be affected were the smallest roots, i.e., those with diameters up to 0.2 mm, which are responsible for the uptake of mineral components. It was confirmed that even a slight increase in soil compaction might negatively result in the growth of young seedlings, impeding root system development.

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The physiological reasons for the differences in sensitivity of C 3 and C 4 plant species to environmental stresses have not been thoroughly explained. In this study the effects of drought stress on the growth and selected physiological traits were examined in the seedlings of 13 single cross maize (C 4 plant) hybrids and 11 spring triticale (C 3 plant) breeding lines and varieties differing in drought sensitivity. For plants in the seedling stage the results demonstrated a genetic variation in dry matter accumulation of shoots and roots (DW S , DW R ), number (N) and length (L) of particular components (seminal, seminal adventitious, nodal) of the root system, membrane injury by soil drought (LI D ), osmotic and high temperature stress (LI OS , LI HT ), water potential (w), water loss (WL), grain germination in osmotic stress (FG, PI), and seedling survival (SS). Seedlings grown under moderate soil drought showed a decrease in dry matter of the top parts and roots and a decrease in the length of seminal, seminal adventitious and nodal roots in comparison to seedlings grown in control conditions. The observed harmful effects of drought stress were more distinct in drought sensitive genotypes. Used in this paper drought susceptibility indexes (DSI GY ) were calculated in other experiment by determining the changes in grain yield (GY) under two soil moisture levels (irrigated and drought). The variation of DSI GY for maize ranges from 0.381 to 0.650 and for triticale from 0.354 to 0.578. The correlations between DSI GY and laboratory tests (LI, FG, SS) confirmed that they are good indicators of drought tolerance in plants. The highest values of genetic variation were observed in LI, DW S , SS and WL and the lowest in the measurements of w FG, PI, L S , L SA and L N . The correlation coefficients between LI OS and LI HT tests were, in most of the considered cases, statistically significant, which indicates that in maize and triticale the mechanisms of membrane injury caused by simulated drought or high temperature are physiologically similar. It can be concluded that an approach to the breeding of maize and triticale for drought tolerance using these tests can be implemented on the basis of separate selection for each trait or for all of them simultaneously. In that case, it would be necessary to determine the importance of the trait in relation to growth phase, drought timing and level, as well as its associations with morphological traits contributing to drought tolerance. The obtained values of the correlation coefficient between laboratory tests suggest that the same physiological traits may be applied as selection criteria in drought tolerance of maize and triticale genotypes.