Physiological vs. morphological traits controlling the productivity of six aspen full-sib families

For investigating genotypic differences in the production potential of Populus tremula L., we grew aspen plants of six full-sib families under optimal water and nutrient conditions and analysed more than 20 physiological and morphological traits with a potential impact on productivity. The six families were produced from controlled crossings of two male and four female trees. Despite genetic distances of 2-28%, the families showed no significant differences in photosynthetic and leaf water status parameters (photosynthetic capacity, leaf water potential and others), even though productivity differed up to twofold between the families. Hence, growth rate was not related to photosynthetic activity but showed a close association with several morphological traits, most closely with the leaf number (L) and total leaf area. Variation in L explained 70% of the growth variation across the six families, and the start of bud burst (BB) correlated with the leaf number (early-starting families produced more leaves). The between-family variation in growth-related morphological traits was much larger than that in physiological traits (coefficient of genetic variation 4-29% vs. 0-4%). Even though the genetic constitution had a significant effect on eight morphological (leaf and root-related) traits, we found no relation between the genetic differences between any two families and the corresponding growth differences. We conclude that the timing of bud burst and the resulting total number of leaves developed are the determinants of growth in P. tremula. Selection programmes should focus on the considerable intraspecific variation in L and BB in order to increase yield.