Influence of Daylength on the Photosynthetic Capacity of Stone Pine (Pinus Cembra L.)

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Abstract
The effects of constant and varying daylengths on the photosynthetic capacity of some pine, the major tree forming timberline in the Eastern Alps, were studied from fall to spring on plants in the open under natural conditions and in the greehouse at 2,000 m altitude (timberline). CO2 uptake was measured using a CO2 gas analyzer on plants in a climatized wind canal under standardized optimal conditions. Photosynthetic capacity of plants in the open rapidly decreased in the fall, reaching zero in the early December, became positive in mid—March, and slowly increased in the spring. Plants in the greenhouse under natural day—length and photoperiods of 12 and 8 hr also had a definite midwinter depression in photosynthetic capacity. This winter depression is attributed to either an endogeneous rhythm in physiological activity, or to an induced change before the start of the test, or to the effect of changing quality and quantity of light in the wintertime.