Size and age: Intrinsic confounding factors affecting the responses to a water deficit in black spruce seedlings

The resistance to stress of seedlings during the initial phases after planting is fundamental for assuring fast establishment and long-term survival of artificial regeneration. Although needing less storage space and handling during their production and planting, small seedlings are considered to be less efficient in terms of water uptake and more sensitive to a water deficit than bigger seedlings. The responses to a water deficit produced by a suspension of irrigation for 14 days were assessed in black spruce (Picea mariana [Mill.] BSP) seedlings of different sizes, with height ranging between 13 and 71 cm. Seedlings growing in containers with cavity volumes of 25, 50, 110, 350 cm3 were tested. During the treatment, the seedlings attained Ψpd of between -1.71 and -2.28 MPa, indicating that a severe water stress was reached. Smaller seedlings exhibited similar or higher water potential and gas exchanges than bigger seedlings both during and after the treatment. Although root biomass was higher in bigger seedlings, the growth rates of roots were similar between seedling sizes and were not affected by the water stress. The initial hypothesis that small seedlings are more sensitive to water stress was rejected. The delayed stomatal closure and higher CO2 assimilation rate of smaller seedlings during the treatment could be attributed to a lower shoot:root ratio and greater ability of roots to sustain the evaporative needs of needles, which could attain higher performances in carbon assimilation. The potential effects of confounding factors such as age and pre-treatment preventing to identify the main factor affecting drought tolerance in black spruce seedlings were discussed