Abstract Six 20-year-old Pinus resinova Ait. trees were grown under two extremes of soil moisture, simulated drought and irrigation, for one growing season. Elongation of shoots and needles was measured weekly on main stem terminals and terminals of the two uppermost whorls of branches. Budset and cessation of elongation were not influenced by soil moisture treatment, as all trees set terminal buds at approximately the same time and ceased elongation within a few days of one another, in mid-July. The magnitude of shoot elongation was increased 40 percent by irrigation over thedrought treatment in the main stem terminals, to a lesser degree farther down the crown, and not at all at the base of the crown. Needle elongation was "inhibited" by rapid shoot elongation and reached maximum rates of extension as shoot elongation ceased, regardless of soil moisture treatment.Needles at the ends of shoots inhibited elongation of those farther down the shoot. Irrigation increased average needle elongation by 40 percent over dry trees, and extended the period of measurable needle elongation into September, whereas dry trees ceased needle elongation in mid-August.Results are interpreted in terms of the effect of current water deficits on both current and subsequent growth of red pine.
Lotan et al. (Sun,) studied this question.