Drought resistance and water use efficiency of conifer seedlings treated with paclobutrazol

Growth and drought resistance of Douglas fir (Pseudotsuga menziesii (Mirb.) Franco), lodgepole pine (Pinus contorta (Loud.)), and white spruce (Picea glauca (Moench) Voss) container seedlings, treated with paclobutrazol at different times and with concentrations up to 2.8 mM, were examined over their second growing season. In two experiments all three species were planted in sand beds in the open and subjected to different levels of moisture stress. Paclobutrazol drenches of 0.3 and 0.6 mM made twice in October, prior to planting the following March, reduced dry weight growth the least, or increased growth in lodgepole pine under dry conditions. Paclobutrazol generally decreased survival under drought stress, particularly if treatment was applied immediately before planting, rather than the previous October. In a third experiment paclobutrazol treated white spruce were grown, in 3.1 L capacity containers filled with peat and vermiculite, at different moisture supply levels and temperatures in growth chambers. Water use efficiency (WUE) was calculated from water use and increases in dry weight of these seedlings over 83 days. Paclobutrazol treatment increased WUE most strongly over the first 31 day measurement period, under all temperature and moisture conditions, but the effect was negligible during the last 24 days of the experiment, and height growth was reduced by treatment. Mineral nutrient concentrations were affected by paclobutrazol, and it was concluded that these and other responsed depended partly on seedling condition at the time of treatment.