STOMATAL AND MESOPHYLL LIMITATIONS OF PHOTOSYNTHESIS IN BLACK SPRUCE SEEDLINGS DURING MULTIPLE CYCLES OF DROUGHT

Container-grown black spruce (Picea mariana (Mill.) B.S.P.) seedlings were planted in trays containing a sand and pear mixture, and placed i n a climate-controlled greenhouse. One group of seedlings was kept wel l-watered, and another group was subjected to three cycles of drought. Gas exchange analysis showed that mesophyll photosynthetic function w as largely unimpaired by drought. In contrast, stomatal conductance wa s sensitive to drought, although it became less sensitive with each dr ought cycle. Both stomatal and mesophyll conductances increased with t ime in control and drought-stressed seedlings, but mesophyll conductan ce increased with time more rapidly than did stomatal conductance. Lim itation of photosynthetic rate was dominated by the mesophyll. In cont rol seedlings, relative stomatal limitation increased from 6 to 16% by the end of the experiment. In drought-stressed seedlings, relative st omatal limitation of photosynthesis reached 40% during the first droug ht, but decreased to near control values immediately after rewatering. Because the third, most severe drought had only a minor effect on sto matal conductance, relative stomatal limitation of photosynthesis was similar to that in control seedlings by the end of the experiment. Inh ibition of ontogenetic change during drought stress may be responsible for the apparent acclimation of mesophyll photosynthetic processes. W e conclude that it would be more effective to select for high photosyn thetic capacity than for reduced stomatal sensitivity when breeding fo r increased drought resistance in black spruce seedlings.