Responses of Scots pine seedlings to low root zone temperature in spring

Abstract

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 seedlings of Scots pine (Pinus sylvestris L.) were studied for 10 days during flushing in spring. Nursery‐grown seedlings were transferred to cold storage (−5°C) in mid‐October, thawn in a cold room at 5°C in May, and transferred to hydroponic cultures at an air and root zone temperature of 5°C for a 3‐day adjustment period. The experiment started when the RZT was changed to either 5, 12 or 20°C and the air temperature was increased to 20/15°C (day/night). RZTs of 5 and 12°C were suboptimal for root growth, and also shoot growth was suppressed at 5°C. The degree and rate of phospholipid fatty acid saturation in the PM of roots was highest at RZT of 20°C and intermediate at 12°C, while no change in the degree of saturation occurred at 5°C. PM‐ATPase activities, measured at 5 and 12°C (real activities) were severely temperature‐limited, but the increasing potential activities (measured at 38°C) at these RZTs indicated delayed deacclimation of the root system. At RZT 20°C, the decline of C18:2/C16:0 ratio in combination with decreasing potential and real PM‐ATPase activities indicated, instead, fast deacclimation of the root system. Net photosynthesis of the seedlings was limited by non‐stomatal factors at the beginning of the experiment, but recovered from winter inhibition and later became limited by low stomatal conductance at RZTs of 5 and 12°C. Instead, at 20°C the net photosynthesis increased with increasing 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, limits net photosynthesis and, thus, the availability of photosynthates for 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.