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

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Abstract
Needles of Sitka spruce were illuminated on either the top or bottom surfaces or on both surfaces simultaneously, using parallel beam light. Sun needles had higher maximum net photosynthetic rates (Fmax), higher stomatal conductance (gs) and higher mesophyll conductances (gm) than shade needles. The largest differences in photosynthetic rates between sun and shade needles were obtained with bilateral illumination. The Fmax of sun needles was higher with bilateral illumination than with illumination on either the top or bottom surfaces alone. Illumination on the top surface resulted in higher values of Fmax than illumination on the bottom surface. These differences in Fmax were associated with large differences in gm whereas gs was not affected by the direction of illumination at high flux densities. The Fmax of shade needles was similar whether illuminated on the top surface or with bilateral illumination. Fmax was significantly lower when illuminated on the bottom surface. These differences in Fmax were again associated with differences in gm but not gs. Stomata on the bottom surface of shade needles appeared to be very sensitive to illumination on the top surface. The response of net photosynthesis to light departed from linearity, even at very low photon flux densities, especially when needles were illuminated unilaterally, indicating that the individual photosynthetic units in spruce saturate at a very low flux density. The photosynthetic light response curve was more convex in bilateral illumination than in unilateral illumination and in shade needles than in sun needles. It was less convex than in the leaves of other C3 angiosperm species. Small convexity indicates a wide range of photon flux densities at the chloroplasts and may lower productivity since photosynthesis of most needles in a canopy is not light saturated.