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

Tissue culture and radicle excision techniques were employed to evaluate salt tolerance in alfalfa (Medicago sativa L.). Plant suspension cultures of either seedling root or shoot origin were studied in media with or without supplemental NaCl (3.54 g liter⁻¹). In most cases, the growth rates of root-derived cultures were stimulated by this low level of supplemental NaCl while most shoot-derived cultures were not stimulated by NaCl. Excised radicles of three populations of alfalfa which possessed widely differing ranges of germination salt tolerance were screened in four salts (NaCl, KCl, Na₂SO₄, and K₂SO₄) at six varying concentrations. As was observed in the tissue culture experiments, low levels of NaCl (7.09 g liter⁻¹) stimulated radicle elongation of all populations as compared to the elongation levels of the control solutions (no supplemental salts). In general, for NaCl, the population that posessed the highest degree of germination salt tolerance (Az-St 1982) also displayed the greatest rates of radicle elongation especially in the highest salt concentrations. Additionally, this population along with the moderately germination salt tolerant population (Az-ST 1979) maintained higher rates of elongation in KCl, K₂SO₄ and Na₂SO₄ than did the control germplasm which has little germination salt tolerance (Mesa Sirsa Control). Examinations of each individual population in all four salts simultaneously, indicated that the sulfate salts reduced radicle elongation to a greater extent than did the chloride salts. Evaluation of both osmotic effects and specific ion effects showed that the specific ion effects attributed to the anions were more detrimental to radicle elongation than were the osmotic effects.