Scale of Physiological Processes Sensitivity to PEG-Induced Water Stress in Scots Pine Seedlings

The idea that water deficit strengthening induces concerted changes of plant physiological parameters is rather widespread. However, such changes are often difficult to identify due to challenges in establishments and maintenance of required water stress intensities using solid substrates. Therefore, we exposed Scots pine (Pinus sylvestris L.) seedlings to the range of water potentials from-0.15 to-1.5 MPa in PEG-water culture to identify the series of physiological parameters differently sensitive to water stress. We observed that even mild water stress (-0.15 MPa) inhibited root elongation, which could be one of the main pine seedlings vulnerabilities under drought. Active accumulation of osmolytes was already induced by mild water deficit and further increased with water stress severity. Plant fresh biomass growth sensitivity was more related to changes of relative water content (RWC) than to changes in tissue water content or dry weight accumulation. Plants were able to grow and accumulate dry weight down to-0.5 MPa, but lower medium water potentials (-1.0 and-1.5 MPa) suppressed growth and heavily damaged root cells, as judged from many-fold increase of Ca2+ content in roots. Chlorophyll a content was surprisingly sensitive to water stress, while carotenoids level was increased under severe stress conditions. In conclusion, the experimental system with stepwise water potential values allowed us to analyze the sensitivity scale of a range of P. sylvestris physiological processes to water stress. It was largely similar to those described earlier for other plant species, but its peculiarities were high sensitivity of root elongation, marked resistance of biomass growth to water deficit and well-developed ability to osmotic adjustment.