Photoperiodic Control of Natural Growth Substances and Wood Formation in Larch (Larix deciduaD.C.)

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Abstract
One-year-old larch plants were submitted to one of the following photoperiodic treatments in greenhouses: C.L.(H) = Continuous light, which included 16 hours high-intensity illumination/24-hour cycle. C.L.(L) = Continuous light, which included 12 hours low-intensity illumination/24-hour cycle. S.D. = Short day. The investigation of the wood anatomy and the content of growth-promoting and growth-inhibiting substances in different parts of the plants at different times of plant collection showed remarkable correlation between the formation of thick-walled tracheids in S.D.- and C.L.(L)-treated plants and the accumulation of water-soluble inhibitors in the cortical tissues. Plants under C.L.(H) conditions did not produce thick-walled tracheids and no significant accumulation of inhibitors was observed. The photoperiodically controlled changes in the content of promoters could not be correlated with the thickening of cell walls of tracheids. These changes were, however, correlated with the extension-growth of the shoot. The results are discussed in relation to the possible influence of these two groups of growth substances on the thickening of cell walls and on the increase in the radial diameter of cells differentiating into the conducting elements of wood in the cambial zone.

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