EFFECT OF SHOOT SIZE ON THE GAS-EXCHANGE AND GROWTH OF CONTAINERIZED PICEA-MARIANA SEEDLINGS UNDER DIFFERENT WATERING REGIMES

Containerized black spruce (Picea mariana [Mill.] B.S.P.) seedlings of three different sizes (small, medium, and large) were planted in rais ed sand beds maintained under wet, moderately dry or dry watering regi mes during the growing season. The small seedlings were of a conventio nal stock type. The two larger sizes were novel stock types grown in l arger containers. Physiological measurements during the summer showed that the small and medium seedlings maintained nearly similar levels o f gas exchanges and water status, but that the large seedlings had red uced net photosynthesis and stomatal conductance under all watering re gimes. Analysis of dry masses showed comparable relative growth rates in the small and medium seedlings, but a small to null growth in the l arge seedlings. Examination of root relative growth rate under wet con ditions revealed significant root growth in small and medium seedlings , but negligible growth in the large seedlings. It was concluded that increasing the shoot size of containerized seedlings can be achieved w ithout increasing the susceptibility of the seedlings to water stress, as long as the vigour of the root system is maintained.