Growth and physiology of loblolly pine (Pinus taeda l.) seedlings as affected by genetics of the root system

by 1961- Grissom, James Edwin

GRISSOM, JAMES EDWIN. Growth and Physiology of Loblolly Pine (Pinus taeda L.) Seedlings as Affected by Genetics of the Root System. (Under the direction of Steven E. McKeand.) Effects of root and shoot genotypes on productivity and physiology of loblolly pine (Pinus taeda L.) seedlings were evaluated in contrasting nutrient regimes. Twelve-week-old seedlings from contrasting provenances were grafted reciprocally to facilitate distinction of rootstock and scion effects. Five open-pollinated families each from a mesic region (Atlantic Coastal Plain) and from a xeric region (Lost Pines Texas) were planted in a split-plot design on a nutrient-poor site in the Sandhills region of central North Carolina. A total of 1800 seedlings were used, including ungrafted trees as controls. Half of the plots were fertilized annually, and after one and two growing seasons, seedlings were harvested for component biomass determinations. Total biomass production among families was positively related to proportional biomass allocation to roots. Generally, mesic sources produced more total biomass and allocated proportionally more biomass to roots. This finding may reflect that soil resources of water or nutrients were limiting tree growth on the site. When fertilized, mesic rootstocks were always associated with greater mass in aboveground components, regardless of scion genotype. Proportional biomass shifts between aboveground and belowground parts suggested that, in some instances of root and shoot genotypes, root system genotype was more influential in determining root:shoot allocation. This effect may also have been accentuated by the growth-limiting edaphic conditions of the site. Rootstock did affect stem growth efficiency, in that the xeric rootstock was associated with increased proportional allocation to stem, regardless of scion type. The main trade-off in that case, as in most others, was between stem versus root allocation. It was concluded that different root genotypes were associated with subtle changes in biomass allocation that may accrue over time to substantial productivity differences. Effects of root system genotype on foliar physiology of selected families were evaluated and related to whole-plant growth of genotypes. In four families (two from each provenance), midday light-saturated net photosynthesis (An) and stomatal conductance to water vapor (gs) were measured monthly during the summer of the second growing season. Leaf carbon isotope discrimination (?) was analyzed to estimate water use efficiency (WUE) of genotypes. Provenances differed in gs but not in An, averaged over all months. The mesic sources had lower gs and higher intrinsic WUE (WUEi; An/gs). Rootstock affected gs but not An nor WUEi of scions. Rootstocks were associated with lower gs when paired with scions of the other provenance. Rootstocks also affected leaf ?, in that xeric rootstocks were associated with lower ?. Photosynthesis and WUEi were limited more by stomatal factors than by nonstomatal factors during the measurement periods, which varied from mild to severely droughty. It was evident that stomatal behavior was pre-conditioned by factors inherent with root genotype. Leaf tissue ? and WUEi were not correlated over all grafted genotypes but were correlated among within-provenance graft types. Further, low leaf ? was associated with higher stem growth efficiency and with less biomass in roots among families, but was unrelated to total biomass production. The correlations between ? and growth traits are consistent with the supposition that low gs is a limiting agent in this population. The results show that root system genotype can substantially influence physiological performance in leaves, which can have large impacts on seedling growth. The findings may have utility in genotype selection and breeding for environments where soil water intermittently or chronically limits tree growth. Growth and Physiology of Loblolly Pine (Pinus taeda L.) Seedlings as Affected by Genetics of the Root System by