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Root growth potential (RGP), the ability of a transplanted seedling to rapidly elongate existing roots and initiate new roots, is used as an indicator of seedling physiological quality. Three separate experiments were conducted on loblolly pine RGP during 1983-84 and 1984-85 using 1-0 nursery-grown seedlings. In the first study, seedlings were hand-lifted from September to April at 17-day intervals in 1983-84 and 7-day intervals in 1984-85. The RGP variation over time was related to environmental and seedling phenological changes. In addition, the new root systems were morphologically described. In the second study, changes in loblolly pine seedlings following different lift-date X cold storage duration treatments were described. These changes were compared to the RGP, shoot activity, and new root system morphology of fresh-lifted seedlings. Genetic variation in RGP on different lift-dates was quantified in the third study. Variation patterns over time among 14 half-sib loblolly pine families hand-lifted at one-month intervals were examined. Root growth potential was low in the autumn until the cessation of shoot activity. Seedlings stored poorly if put in cold storage during this time, or prior to the accumulation of at least 400 chilling hours. Seedling RGP increased in the early-winter to a late-February peak, and then decreased when spring shoot elongation occurred during the RGP tests. Similar trends over time were noted with seedlings removed from cold storage during these times. The RGP pattern over time was consistent among years and among the half-sib families, although there were some minor differences among half-sib families in the rate of increase to, and decrease from, the late-winter peak. Most new roots resulted from elongation of existing roots rather than initiation of new roots regardless of lift-date; however, when seedling metabolism was high (early autumn and late-winter) both root initiation and elongation of existing roots contributed to the new root system. In addition, the morphology of root systems of cold-stored seedlings was similar to fresh-lifted seedlings. Differences in numbers and lengths of new roots among half-sib families were due to the differential ability to produce new roots through both root initiation and elongation. / Ph. D. / incomplete_metadata

The effect of moisture stress conditioning on the drought tolerance and performance of three open-pollinated families of loblolly pine (Pinus taeda I.) during water stress were investigated. Seedlings were subjected to prolonged sublethal drought treatments which included a watered control, a moderate stress treatment (MWS, seedlings watered only when needle water potential reached -0.8 MPa) and a severe stress treatment (SWS, seedlings watered only when water potential reached -1.4 MPa). After the conditioning period, numerous physiological and morphological parameters were measured, and performance of seedlings during water stress evaluated. Significant decreases in needle osmotic potential occurred in moisture stress conditioned seedlings. As a result, turgor in conditioned seedlings was equal to or greater than control seedlings even at lower needle water potentials. Photosynthesis vas decreased greatly with reduced needle water potential. However, the MWS and SWS seedlings maintained photosynthesis to water potentials 0.15 and 0.45 MPa lower than control seedlings, respectively. This response is likely the result of both osmotic adjustment, and an acclimation of the photosynthetic process resulting in less non-stomatal inhibition of photosynthesis at low needle water potentials. Initial needle conductance and transpiration, but not photosynthesis, were reduced greatly by the conditioning treatments, and resulted in improved water-use efficiency in conditioned seedlings. The response of stomata to changing vapor pressure deficit was increased through moisture stress conditioning. Boot growth was affected more by moisture stress than shoot growth, causing a decrease in root/shoot ratio. Changes in root morphology as a result of conditioning are not likely to improve the drought tolerance of loblolly pine seedlings. SWS conditioning significantly improved the height growth increment and resulted in slightly greater shoot and root biomass of outplanted, containerized seedlings after the first growing season, despite these seedlings being much smaller at the time of planting. Differences between seed sources did occur in the experiments, with a source from Texas generally showing the least response to moisture stress conditioning. / Ph. D.

The effect of date and method of lifting on ethylene production by 1-0 loblolly pine (Pinus taeda L.) seedlings during cold storage, the dose-response relationship between ethylene and loblolly pine seedlings during cold storage, and the effect of water stress on ethylene and aminocyclopropane-1-carboxylic acid (ACC) production in two half-sib loblolly families were investigated. Seedlings stored in Kraft-Polvethvlene (K-P) bags showed a general trend of increasing ethylene concentrations from November through February, with an abrupt drop in March. Production rates may be related to the level of dormancy of the seedlings, with the peak in production corresponding to fulfillment of the chilling requirement. Ethylene concentrations within the K-P bags generally declined over the twelve weeks in cold storage. Roots produced significantly higher levels of ethylene while stored in the K-P bags; however, when incubated under light, the needles produced higher concentrations. Roots of machine-lifted seedlings produced significantly higher levels of ethylene than roots of hand-lifted seedlings. However, rates tended to moderate during storage and differences in production between HL and ML whole seedlings were not significant, which suggests that mechanical lifting is not a source of increased ethylene production. A dose-response study indicated that ethylene fumigation during cold storage tended to slightly enhance growth of outplanted seedlings. The effects of water stress on a Virginia Coastal Plain (CP) and an East Texas Drought Hardy (DH) loblolly family were also investigated. Ethylene production during severe stress (-2.8 MPa) appeared to be related to drought hardiness, with the CP seedlings producing much higher levels. Roots of both families produced greater levels of ethylene than the needles and may be due to an enhanced ability to convert 1-Aminocyclopropane-1-carboxylic acid (ACC) to ethylene. / Master of Science

M.S.