Growth and physiological adjustments of conifer seedlings in response to drought and chilling

grantor: University of TorontoChanges in growth, water relations and ABA accumulation were studied in black spruce (Picea mariana (Mill) B.S.P.) and jack pine (Pinus banksiana Lamb.) seedlings under drought and root chilling. When soil dried gradually from the surface downward growth of the lower part of the root system increased, although a partial drought inhibited root elongation of black spruce in the upper soil layer. This redistribution of root growth under surface drought increased new root length and hydraulic conductivity, which allowed stomatal conductance and shoot growth to be maintained. By contrast, drying of the whole root system significantly decreased shoot growth and gas exchange. Although repeated drought treatments in root misting chamber did not increase stomatal sensitivity to another drought, preconditioning promoted stomatal reopening during recovery, which was unrelated to the rapid increase in shoot water potential. Levels of xylem ABA paralleled the reduction in shoot growth and stomatal conductance. A common mechanism also occurred under root chilling, similar to those observed under drought, because chilling-induced water deficits and ABA accumulation were correlated with changes in root growth, hydraulic conductivity, and gas exchange. ABA accumulation would explain the increase in hydraulic conductivity under both drought and chilling because ABA increased root growth in moist soil under drought and promoted root branching under chilling which would help to maintain shoot growth, stomatal conductance and gas exchange under stress. Changes in root growth and gas exchange, when unstressed seedlings were treated with exogenous ABA resembled those occurring under drought and chilling. Root growth could be inhibited, promoted, or remain unchanged, depending on the species and ABA levels applied. However, gradually increasing ABA concentration in the treatment solution stimulated both root elongation and initiation. After ABA treatment, jack pine adjusted root growth and stomatal conductance more rapidly than black spruce. The difference between species in the ability to tolerate stress was correlated to their responses to ABA suggesting that ABA is involved in the root and shoot adjustments under stress. Rather than a single change, physiological and morphological adjustments to stress were integrated in a manner that increased plant stress tolerance.Ph.D