Photosynthesis in Sitka Spruce (Picea sitchensis (Bong) Carr). IV. Response to soil temperature.
[Cf. FA 34, 3826; 35, 7073] The fourth part of a study concerned entirely with photosynthesis in P. sitchensis. The CO2 and water-vapour exchanges of shoots of 2-year-old seedlings were measured at root temperatures between -8 and 20 deg C, and during the period of recovery with the roots at 20 deg . In unhardened plants (grown at 15 deg day/12 deg night temperatures and with 16-hour days) and hardened plants (transferred to 4 deg day and night temperatures and 8-hour days), photosynthesis and the transfer conductances for CO2 were not reduced during irradiation for 5.5 hours when soil temperatures were between 1 and 20 deg . A marked decrease in the rate of net photosynthesis and in stomatal and mesophyll conductances occurred at soil temperatures below 1 deg in unhardened plants and below -1 deg in hardened plants. Soil temperature had little effect on the rates of dark respiration of either unhardened or hardened plants. The rates of photosynthesis of hardened plants recovered fully overnight from a soil temperature above -4 deg , but recovered to only 58% of the initial rates from -8 deg . The photosynthetic rates of unhardened plants recovered fully overnight from soil temperatures above -0.5 deg , but did not recover fully in 17 days when exposed to lower soil temperatures. The role of shoot characteristics and hardening in the reduction of photosynthesis and transpiration at low soil temperatures is discussed.