Tolerance of peat-grown Scots pine seedlings to waterlogging and drought: Morphological, physiological, and metabolic responses to stress

Depending on the soil preparation method applied and peat characteristics, Scots pine (Pinus sylvestris L.) seedlings planted in prepared spots on forestry-drained peatlands may become more susceptible to extreme weather events such as drought or flooding. Only by studying its coping strategies can we eventually design methodology better suited for Scots pine regeneration on peat soils. In this study, we evaluated the tolerance of two-year-old seedlings to the two extremes of water-associated stress, drought and waterlogging, in unprocessed peat. Over one growing season in controlled conditions, drought distinctly reduced root and shoot growth in addition to photochemical efficiency (Fv/Fm) particularly in previous-year needles, whilst wet stress had little discernible impact. Drought also influenced polyamine metabolism by increasing free putrescine and spermine concentrations especially in current-year needles, whereas no impact was discerned in the wet treatment. Furthermore, reduced root hydraulic conductance (Kr) was observed in drought-stressed root systems. Apparently, waterlogging does not modify Scots pine seedling growth or vitality immediately, but rather in the longer term. However, no fatalities occurred in either of the stress treatments, this despite water availability in the Sphagnum peat reaching its lower (permanent wilting point) and upper (10% soil air content) limits. Maintenance of rather high photochemical efficiency despite severe drought stress would seem to indicate a potential for seedling recovery if water availability in the peat substrate improved.