Physiological changes in Pinus sylvestris needles during early spring under sub-arctic conditions

Physiological condition of yellow and visually green Scots pine (Pinus sylvestris L.) needles was followed in the Finnish sub-arctic during May and June 1996. The five greenest and five yellowest saplings were chosen for needle sampling at weekly intervals. The chlorophyll fluorescence, chlorophyll and water content, fine structure of the mesophyll cells and frost hardiness were determined at weekly intervals. Intracellular, extracellular and cell-membrane resistance were estimated by impedance spectroscopy and the distribution of free and bound water were determined by magnetic resonance imaging for needle samples collected on June 11, 1996. The chlorophyll a and b contents, the ratio between the maximum variable fluorescence and the maximal fluorescence yield (F-v/F-m) and the thickness of the grana stacks were higher in the green than yellow needles during the first three samplings in May. The yellow needles maintained a higher level of freezing-stress resistance compared to the green needles. The recovery of green color was accompanied by an increase in the chlorophyll a and b content, F-v/F-m and the thickening of the grana stacks. The water content was slightly higher in the green than yellow needles in June. The water content still decreased even though the chlorophyll content of yellow needles began to increase and approached that of the green needles. Based on the magnetic resonance imaging, the water was distributed differently and in a more mobile form in the yellow than in the green needles. The extracellular resistance and the cell-membrane resistance were lower in yellow than green needles indicating impaired ability of yellow needles to maintain a high intracellular ion concentration. The results show that the yellow color of needles is an indication of a deeper state of photo-inhibition and slower deacclimation and is not directly related to the desiccation stress. (C) 2000 Elsevier Science B.V. All rights reserved.