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We determined the degree to which families differ in seed and germination characteristics and examined the extent to which these characteristics influence the early growth of 75 open-pollinated white spruce families. Seed characteristics (1000-seed weight, length, width, area, volume) were measured for 400 seeds per family. Germination variables (germination capacity, peak value, germination value) were determined for each of the 75 families under controlled conditions and germination patterns were modelled using the Weibull function. Seedling characteristics (height, diameter, shoot and root dry weights) were measured at the end of the first and second growing seasons under standard nursery cultural practices. Statistically significant family variation (p < 0.0001) was found for all seed characteristics and germination variables measured. The between-family variance explained 23% to 98% of the total variance of morphological and physiological seed characteristics. Family differences at the seed stage explained up to 33% (root dry weight) and 12% (shoot dry weight) of the family differences observed at the one-year and two-year seedling stages, respectively. Since, in this study based on a comparison of family means, a maximum of only 12% of the family differences observed at the two-year seedling stage were explained by the effect of seed size, a selection for families with better juvenile characteristics could be envisaged without considering the maternal effect of seed size.

The root systems of containerized seedlings must be sufficiently developed and have adequate root plug cohesion to permit handling and the planting of the seedlings with minimal root damage. Genetic variability in morphological and physiological seedling characteristics of 75 open-pollinated white spruce families was estimated to determine whether genetic selection for improved seedling root systems is possible. Seedlings were grown for 2 years under standard cultural practices in a forest nursery. Gas exchange measurements and seedling morphological characteristics (height, diameter, shoot and root dry mass, root to shoot ratio) were measured at the end of the two growing seasons whereas seedling mineral (N, P, and K) status was assessed at the end of the first growing season. Genetic parameters (heritabilities-h 2 -and genetic correlations) were estimated for every seedling characteristic and a strong genetic control associated with a large genetic variation was observed at both family (0.20≤h 2 f ≤0.88) and individual (0.21≤h 2 i ≤0.97) levels. A single, late-season measurement of physiological characteristics did not reveal physiological basis for family variability in seedling root growth. Nevertheless, the family variation was large enough to permit genetic improvement of 2-year-old seedling juvenile morphological characteristics. Strong, positive genetic correlations enable us to foresee using root collar diameter as an effective method for indirectly selecting white spruce families with heavier root systems.

This study examined the impact of increased irrigation efficiency on the hardening and frost tolerance of 2-year-old containerized white spruce seedlings in the context of groundwater protection, irrigation management and the maintenance of seedling quality in northern climates. The seedlings were grown under three different irrigation regimes (IR 030%, 40% and 55% v/v; cm 3 H 2 O/cm 3 substrate) and were hardened under conditions of natural photoperiod and temperature. After being subjected to artificial frost tests on four sampling dates during autumn, the seedlings were compared for bud development and frost tolerance. IR had no influence on frost tolerance as determined by measurements of physiological (electrolyte leakage, root water loss) and morphological (shoot damage, root initiation) variables. At the end of the second growing season, there was no significant difference between IRs in seedling height, root collar diameter, shoot dry mass and root dry mass. The results indicate that the amount of water applied to large-dimension 2-year-old white spruce seedlings during the growing season can be significantly decreased without prematurely impeding their growth or hindering their acquisition of frost tolerance.

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