PHYSIOLOGICAL CONTROL OF FINE ROOT TURNOVER IN DOUGLAS-FIR (STARCH, DROUGHT, CARBOHYDRATES)

To determine whether shading increases drought-induced root mortality by changing the chemical composition of roots, I conducted an experiment in which high and low light treatments in combination with high and low soil moisture treatments were applied to seedlings of Douglas-fir (Pseudotsuga menziesii Mirb. Franco). Because synthesis of suberin (a water-repellent compound found in roots) is metabolically expensive, I hypothesized that suberin synthesis might be reduced in roots of a shade-stressed tree and thereby, susceptibility to desiccation would be increased. As expected, the drying treatments increased root mortality for the shaded seedlings, however, no differences in suberization were found. I measured starch and sugar concentrations of the roots and found that carbohydrate reserves of roots in the shaded treatments were nearly exhausted. I concluded that root mortality induced by shading and drought was probably caused by depletion of carbohydrate reserves in the root rather than by differences in resistance to desiccation.; I then set up an experiment to determine whether accumulation and depletion of carbohydrate reserves were indeed the physiological mechanisms underlying fine root turnover. Douglas-fir seedlings were grown at soil temperatures of 10(DEGREES)C, 20(DEGREES)C, and 30(DEGREES)C, at uniform air temperatures, with and without light. It was necessary to develop a starch extraction procedure using 35% perchloric acid in order to consistently extract all starch from the tissue. Sequential harvests were weighed and analyzed for starch and sugars. Starch reserves were exhausted in the roots of the dark-grown seedlings at all temperatures and were also exhausted in the roots of the light-grown seedlings at 20(DEGREES)C and 30(DEGREES)C. In the light-grown seedlings from the 10(DEGREES)C treatment, however, significant accumulations of starch were observed in association with pulses of root growth. Based on this study and on others from Sweden and from Oregon, I suggest that root lifetimes may be determined by the amount of starch with which a newly grown root is endowed and the rate at which this starch supply is respired. A simple means of estimating root biomass production and turnover based on root starch and soil temperature is described.