Daylength Effects on Growth, Tracheid Development, and Photosynthesis in Seedlings ofPicea sitchensisandPinus sylvestris

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Abstract
Effects of change in daylength on tracheid expansion and tracheid wall thickening are considered in relation to shoot and root growth, assimilation rate, and starch content, in seedlings of Picea sitchensis and Pinus sylvestris grown in controlled enviroments. Rates of wall thickening decreased in the first tracheids to begin wall thickening after decrease in daylength. A sharp decrease in starch-grain size at the same time supports previous suggestions that wall thickness is related to available substrate; however, rates of assimilation were found to be far greater in short days than in long, suggesting that a surplus of carbohydrate may be produced in long days. It is pointed out that if available substrate does limit wall thickening it is likely to determine to al accumulation of wall material; wall material per tracheid would follow logistically from this depending on the number of tracheids around the xylem, and wall thickness would depend also on the radial tracheid diameter. Unexpectedly, rate of shoot growth accelerated after transfer to short days before its final cessation with terminal-bud formation. Root growth declined in short days, but later increased again after terminal bud formation. Rate of root growth did not appear to be associated with tracheid development in the shoot.

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