RESPONSES OF SCOTS PINE-SEEDLINGS TO LOW ROOT-ZONE TEMPERATURE IN SPRING

The effects of root zone temperature (RZT) on growth, gas exchange, H-ATPase (EC 3.6.1.3: PM-ATPase) activity and fatty acid composition of plasma membrane (PM) phospholipids in the roots of one-year-old seedl ings of Scots pine (Pinus sylvestris L.) were studied for 10 days duri ng flushing in spring. Nursery-grown seedlings were transferred to col d storage (-5 degrees C) in mid-October. thawn in a cold room at 5 deg rees C in May. and transferred to hydroponic cultures at an air and ro ot zone temperature of 5 degrees C for a 3-day adjustment period. The experiment started when the RZT was changed to either 5, 12 or 20 degr ees C and the air temperature was increased to 20/15 degrees C (day/ni ght). RZTs of 5 and 12 degrees C were suboptimal for root growth, and also shoot growth was suppressed at 5 degrees C. The degree and rare o f phospholipid fatty acid saturation in the PM of roots was highest at RZT of 20 degrees C and intermediate at 12 degrees C, while no change in the degree of saturation occurred at 5 degrees C. PM-ATPase activi ties, measured at 5 and 12 degrees C (real activities) were severely t emperature-limited, but the increasing potential activities (measured at 38 degrees C) at these RZTs indicated delayed deacclimation of the root system. At RZT 20 degrees C, the decline of C18:2/C16:0 ratio in combination with decreasing potential and real PM-ATPase activities in dicated, instead, fast deacclimation of the root system. Net photosynt hesis of the seedlings was limited by non-stomatal Factors at the begi nning of the experiment, but recovered from winter inhibition and late r became limited by low stomatal conductance at RZTs of 5 and 12 degre es C. Instead, at 20 degrees C the net photosynthesis increased with i ncreasing stomatal conductance during the experiment. We conclude that low RZT suppresses growth of roots and such changes in the PM as are needed for efficient uptake of water and nutrients. This, in turn, lim its net photosynthesis and, thus, the availability of photosynthates f or root growth in spring. On the other hand, a rise in RZT can rapidly induce the structural and functional changes in PM of the roots that are needed for the efficient gas exchange and growth of the Scots pine seedlings.