Photosynthesis in Sitka Spruce (Picea sitchensis (Bong.) Carr.). IX. The Relative Contribution Made by Needles at Various Positions on the Shoot

Publisher Website
Google Scholar
Abstract
Rates of net photosynthesis of detached sun and shade shoots of Sitka spruce were measured using parallel beam illumination normal to the top surface, bottom surface or both surfaces of the shoot simultaneously. The insertion angle of the needles, their position on the stem and width had large effects on the rates of photosynthesis of shoots. Rates of net photosynthesis of shoots at 1.2 mEinstein m-2 s-1 were 40-70% of those of unshaded needles normal to the incident light beams. The direction of illumination had much larger effects on the rate of photosynthesis of shoots than of needles. In bilateral illumination the rate of photosynthesis of shoots was less than that of needles because some needles were at angles of < 90% to the light beam and because of shading of needles by the stem and by other needles. Rates of net photosynthesis of shoots were higher with illumination on the top surface, of the shoot than on the bottom surface, partly as a direct result of shoot structure. Shade shoots were structurally better acclimated to achieving near maximum rates of net photosynthesis with non-saturating illumination on their top surface than sun shoots, because the needles were at angles close to the horizontal plane. Shading by the stem and of one needle by another was small in shade shoots illuminated on the top surface because of the angles of insertion of the needles and because the needles were narrow.