Photosynthesis in Sitka Spruce (Picea sitchensis (Bong.) Carr.). II. Response to temperature.
[Cf. FA 33, 3807]. The response of net photosynthesis (F), and stomatal resistance (rs) and mesophyll resistance (rm) to CO2 flux, to a stepwise sequence of temperatures between 30 and -5 deg C was determined for shoots of P. sitchensis grown under different conditions (greenhouse, forest, and growth cabinets). The minimum temperature for F was ca. -6 deg , the maximum ca. 45 deg , and the optimum ca. 19 deg , varying from 10 to 20 deg according to the preceding growing conditions; rs increased below 5 deg and above 20 deg ; rm sometimes increased slightly above 20 deg but increased consistently below 20 deg and appreciably below 10 deg . The response curves were similar for seedlings grown under different conditions, except those grown under a 7 deg day/4 deg night temperature regime in a growth cabinet; for the last, the curve was flatter and the maximum rate of F much reduced because of high rm values. The recovery of F, rs and rm after exposure to temperatures below 0 deg was determined as the ratio between the initial value and the value 24 hours after exposure, both measured at 20 deg . Exposure of the shoots of greenhouse-grown seedlings to short periods of temperatures below 0 deg gave short-term hardiness only; hardiness in shoots from the forest increased from late Oct. to Jan., and F continued at an unreduced rate after exposure of hardened shoots to temperatures below 0 deg . Reduction of F in unhardened shoots was proportional to the duration and temperature of exposure below 0 deg . One hour at -5 deg gave total inhibition of F accompanied by large increases in rs and rm. The possible formation of intercellular ice, its effect on subsequent rates of F in hardened and unhardened shoots and the inter-relation between effects of intercellular and intracellular ice formation with an accompanying 'exotherm' are discussed.