GAS-EXCHANGE AND WATER RELATIONS RESPONSES TO DROUGHT OF FAST-GROWINGAND SLOW-GROWING BLACK SPRUCE FAMILIES

To determine how different mechanisms of drought tolerance contribute to growth rate under drought, this study compared four full-sib black spruce (Picea mariana (Mill.) B.S.P.) families which differed in growt h rate when soil water became limiting, stomatal conductance, photosyn thesis, and water relations responses to drought. Repeated drought cyc les were imposed by withholding soil water in a nursery and physiologi cal responses were measured near the end of the first and third cycle. The most vigorous family under drought had greater osmotic adjustment and maintained higher rates of net photosynthesis during the first cy cle of drought and resumed higher rates of photosynthesis sooner upon stress relief, compared with two slow-growing families. Pressure-volum e analysis of drought-stressed shoot tissues indicated that the fast-g rowing family exhibited a larger degree of elastic enhancement (i.e., decrease in bulk modulus of elasticity), which would explain its highe r turgor pressure, compared with the two less vigorous families. Howev er, family differences in gas exchange and water relations largely dim inished when seedlings were exposed to repeated cycles of drought. The refore, fast-growing black spruce families under drought may gain sele ctive growth advantage by a better ability to tolerate, rather than po stpone, momentary dehydration. Short-term screening trials could be us ed to detect drought tolerant genotypes in black spruce.