Observations on the growth and physiology of Pinus strobus L. seedlings grown under various conditions of soil moisture and nitrogen and phosphorus nutrition.

PART I White pine seedlings (Pinus strobus L.) were grown at high or low soil-moisture levels. The leader stem length, fresh weight of the seedlings, respiration, photosynthesis, transpiration, translocation of photosynthate from shoots to roots, and bio-electric potentials between the tip and the base of the stem were measured throughout the growing season from April to October. At both moisture levels the lowest translocation of recent photosynthate from shoots to roots occurred during early summer, or at the time when the rate of root growth was the lowest and that of the shoot the highest. During 14 early summer the specific activity of CC>2 respired by the shoots of such plants remained high throughout an 8 h experimental period, indicating a continuous utilization of recent photosynthate as a respiratory substrate. On the other hand, early and late in the growing season, when translocation of recent photosynthate from shoots to roots and the rate of root growth were high, the specific activity 14 of CO- respired by the shoots rapidly decreased during the ^ 8 h experimental period, indicating a drop in the utilization of recent photosynthate as respiratory substrate. The highest positive values for the potential difference between the top and the base of the main shoot also occurred in early summer or during the period of high rates of i canspiration. PART II Potted white pine (Pinus strobus L.) seedlings were grown on media containing different amounts of nitrogen (N) and phosphorus (P). The seedlings were grown either in controlled environment chambers or outdoor cold frames. Following periods of five, seven and thirteen weeks on treatment the seedlings were analyzed to determine rates of respiration, photosynthesis and the degree of translocation of recent photosynthate to the roots. Shoot and root fresh weights were recorded. Analyses were made to determine the metabolic fate of the translocated sugars. The best overall growth and the highest root/shoot ratios were found in seedlings receiving intermediate levels of N and P. The range of nutritional conditions employed was found to have no effect upon rates of shoot and root respiration or photosynthesis, even after thirteen weeks of treatment. Lateral root formation was depressed under conditions of high N and P. Mycorrhizal abundance showed a maximum at intermediate levels of nutrients. Translocation of recent photosynthate to the roots was depressed by high P, this depression was however, reversed to some extent by increasing N levels. The hydrolysis of sucrose recently translocated to the roots was increased with increasing N supply. The resultant hexoses being metabolised to amino and organic acids. However, sucrose continued to be the dominant form in which VI 14 recently translocated C occurred in the roots. High levels of P reduced the effect of N on the metabolism of translocated sucrose. Relatively very few of the metabolically active compounds, isolated from the soluble fraction of the roots, showed distinct patterns of change which could be correlated to either the different nutritional levels of N and P supplied or to the incidence of mycorrhizae.