Seasonal changes in chlorophyll fluorescence of white spruce seedlings from different latitudes in relation to gas exchange and winter storability

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Variable chlorophyll fluorescence (Fvar) was investigated as a tool in detection of distinct seasonal physiological changes in 1+0 intact white spruce seedlings. The loss of the characteristic Fvar peak (Fp) between 0.8 and 1.0 s after illumination of dark adapted seedlings is an indication of regulation of photosynthetic activity in August. The peak represents excess photochemical water-splitting of photosystem II. We interpret its loss as a physiological indicator of the process of dormancy induction. Three dimensional (i.e. X[0-300 s], Y[rfu], Z[time of year] axis) Fvar curves of non-stressed seedlings measured over 300 s followed a three phase change over the growing season. In actively growing seedlings, the portion of the Kautsky induction curve between 60 and 300 s was 0.4 relative fluorescence units (rfu) in northern (i.e. >56° latitude) seedlots and 0.6 in the southern (i.e. <56° latitude) range seedlot from August until early September. About mid-September curve features between 60 and 300 s decreased sharply to approximately zero (rhu) by October. Freeze test data indicated seedlings became frost hardy during this time. The third, or inactive phase was seen as flat line from 40 to 100 s. The portion of the curve after 100 s was responsive to short term temperature changes. White spruce seedlots of northern and southern B.C. latitudes having curve fluorescence peak (Fp) values at about 1 s of 0.6, and 0.8 (rhu) respectively, plus curve minimum (Fmin) values at about 60 s which do not decrease further over a 2–3 week period represent stock which can safely be lifted for cold storage. The Fvar attribute at 5 s after the actinic light is turned on (F5s) correlates well with net photosynthesis (r2 =0.61) during the growing season.