Photosynthesis in Sitka spruce VIII. The effects of light flux density and direction on the rate of net photosynthesis and the stomatal conductance of needles.
[See FA 38, 1151] Needles of Sitka spruce were illuminated on either the top or bottom surfaces or on both simultaneously, using parallel beam light. 'Sun' needles had higher max. net photosynthetic rates (Fmax), higher stomatal conductance (gs) and higher mesophyll conductances (gm) than 'shade' needles on the same plant. The Fmax of 'sun' needles was highest with bilateral illumination and lowest with bottom illumination. Differences in Fmax were associated with large differences in gm but not gs. Fmax of 'shade' needles was similar for top and bilateral illumination and lowest for bottom illumination. These differences were again associated with differences in gm but not gs.