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Given the findings of previous research and the theoretical framework of loblolly pine genetic improvement, we proposed several hypotheses. Our first hypothesis was that uniformity in growth and physiology would increase as genetic homogeneity increased. Our second hypothesis was that genotype differences in leaf-level physiological process rates over time would be correlated with productivity differences. Our third hypothesis was that genotype productivity would be associated with differences in growth efficiency and biomass partitioning, and genotypes with greater specific hydraulic conductivity (k s), canopy conductance (Gs), and transpiration (E), would be less sensitive to changes in vapor pressure deficit (D), and more susceptible to cavitation. Our fourth hypothesis was that productivity and foliar concentrations of carbon-based secondary compounds (CBSC; total phenolics, proanthocyanidins (PA), starch and soluble sugars) would be negatively related. To test our hypotheses, we measured productivity, physiological traits, biomass partitioning and production, and concentrations of CBSC in nine loblolly pine genotypes (3 clones, 3 full-sib families, and 3 open-pollinated (half-sib) families) growing under field conditions.
Our results suggested that uniformity in growth and physiology within less genetically diverse genotypes was not greater than within more genetically diverse genotypes growing in an operational plantation setting. More productive genotypes had more rapid rates of height and diameter growth between May and August, produced longer stem growth flushes and more biomass at the stand-level, and tended to have higher Asat and WUEi. Also, genotypes with higher maximum rates of specific conductivity had lower stomatal sensitivity (--dGs/dlnD ) during the spring, and genotypes that reached the full embolism point at a lower pressure potential and had a more rapid loss of conductivity tended to have higher reference canopy conductance (G s-ref or Gs at D = 1 kPa) in the spring. Finally, concentrations of CBSC were affected by both site and genotype, and the relationship between productivity and concentrations of CBSC was also dependent upon site and genotype. Our results showed that higher productivity clones exhibited less variation in allocation to CBSC across sites while low productivity clones exhibited greater variability in allocation to CBSC across sites. Therefore, genetic differences in C allocation relative to tree growth may impact stand-level defense against disease and insect infestation, carbon and nutrient cycling.
In conclusion, less genetically diverse genotypes may show equal or higher amounts of variation relative to more genetically diverse genotypes within an operational plantation setting where resources and competition may vary greatly. These results demonstrate the importance of understand