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

Physiological condition of yellow and visually green Scots pine (Pinus sylv estris L.) needles was followed in the Finnish sub-arctic during May and Ju ne 1996. The five greenest and five yellowest saplings were chosen for need le sampling at weekly intervals. The chlorophyll fluorescence, chlorophyll and water content, fine structure of the mesophyll cells and frost hardines s were determined at weekly intervals. Intracellular, extracellular and cel l-membrane resistance were estimated by impedance spectroscopy and the dist ribution of free and bound water were determined by magnetic resonance imag ing for needle samples collected on June 11, 1996. The chlorophyll a and b contents, the ratio between the maximum variable fl uorescence and the maximal fluorescence yield (F-v/F-m) and the thickness o f 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 o f green color was accompanied by an increase in the chlorophyll a and b con tent, 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 conten t still decreased even though the chlorophyll content of yellow needles beg an to increase and approached that of the green needles. Based on the magne tic resonance imaging, the water was distributed differently and in a more mobile form in the yellow than in the green needles. The extracellular resi stance and the cell-membrane resistance were lower in yellow than green nee dles indicating impaired ability of yellow needles to maintain a high intra cellular ion concentration. The results show that the yellow color of needl es is an indication of a deeper state of photo-inhibition and slower deaccl imation and is not directly related to the desiccation stress. (C) 2000 Els evier Science B.V. All rights reserved.