GAS-EXCHANGE AND WATER RELATIONS OF 3 SIZES OF CONTAINERIZED PICEA-MARIANA SEEDLINGS SUBJECTED TO ATMOSPHERIC AND EDAPHIC WATER-STRESS UNDER CONTROLLED CONDITIONS

Containerized black spruce (Picea mariana [Mill] BSP) seedlings of 3 s izes (heights of 18, 35 and 45 cm) were placed in growth chambers and subjected to conditions of low evaporative (20 degrees C, 60% RH) or h igh evaporative (30 degrees C, 40% RH) demand, with 3 levels df soil w ater availability in each environment. The large seedlings had the hig hest rate of net photosynthesis in the cooler environment, but showed the greatest reduction in net photosynthetic rate in the warmer and dr ier environment, under conditions of limited water supply. The small s eedlings were least affected by the warmer and drier environment in wh ich they maintained photosynthetic rates higher than those of the larg er seedlings. The decrease in net photosynthesis experienced by the la rge seedlings in the warmer and drier environment under conditions of limited water availability was associated with a large decrease in sto matal conductance. However, the maintenance of a high level of interce llular CO2 concentration suggests that most of the limitations to net photosynthesis were of non-stomatal origin. Water content of the root plug was also reduced by increased seedling size, but the differences were least evident under conditions that produced the largest differen ces in net photosynthetic rates. These results, obtained under control led conditions, suggest that after outplanting, large seedlings would experience greater reduction in growth than smaller ones only under co nditions of high evaporative demand and low water availability.