Root regeneration potential in Douglas fir and Western hemlock seedlings : the role of environmental factors and current photosynthesis

Graduation date: 1980Three experiments were conducted on containerized seedlings to examine the short-term and long-term effects of environment on the root regeneration potential, and to examine the fate of the current photosynthate and its relationship to the root regeneration potential. The short-term effects were studied first in a 3x3x2 factorial experiment comprising photoperiod, air temperature, and soil type.\ud Treatments were repeated on two sample dates, each lasting 28 days with root growth measurements both during and following the test period. Measurements during the test period showed that sample date differences\ud in total length of new roots were clearly visible in five days and were due to differences in average growth rate of individual roots. The roots of dormant seedlings that received ten weeks of chilling grew faster than those that received only five weeks. All the environmental factors tested had a significant effect on root growth. Plants grown at 20°C produced more roots than 15° or 25°C. Root growth increased with\ud increasing photoperiod (16 hr > 12 hr > 8 hr) and was greater in the sandy loam than in the perlite for the Douglas-fir while the hemlock roots grew significantly better in the perlite. These results supported the initial hypothesis that futher standardization of the RRP evaluation environment was warranted. In the second experiment, the hypothesis that long-term environmental treatments cause changes in the RRP in direct proportion either to the amount of chilling or the dormancy status of the bud was examined. A series of 11 treatments was employed incorporating three photoperiods, three air temperatures regimes and six dates of chilling initiation. Seedlings were sampled from these treatments at one or two-month intervals from June 1977 to March 1978, and at each sampling, RRP and bud dormancy were evaluated. The results showed a definite seasonal pattern of root growth for the three parameters measured; total length, number and average growth rate of new roots. Bud dormancy was only weakly correlated to RRP. Chilling when the buds were in a dormant state led to a consistent\ud increase in the number of new roots regenerated. The occurrence of a seasonal pattern of number of new roots in the continuously warm environment and of the elongation rate of the roots under all treatment environments suggested the existence of some endogenous controls. The possibility of using current photosynthate translocated to the root as a rapid measure of RRP was explored by incubating seedlings\ud with ¹⁴CO₂ and measuring the distribution of radioactive marker after 24 hours. The results indicate that although the percent\ud ¹⁴C-photosynthate translocated to the root was not predictive of the RRP, it was significantly correlated with both the dormancy status of the bud and the elongation rate of the roots. Comparison of the results of the two experiments indicate that seedlings age is an important factor in determining how the seedlings will respond to environmental factors. In general, younger seedlings are more dependent on current photosynthate for root growth than older ones