Seasonal changes in light transmission by bud scales of spruce and pine.

A bundle of optical fibres and a photometer were used to measure light transmission through bud scales of Norway spruce [Picea abies (L.) Karst.] and Scots pine (Pinus silvestris L.). Percentage transmission increases with wavelength from none below 480 nm to 3% the far-red. Transmission of all wavebands measured was lowest in late summer and winter and highest in the autumn and spring. For most of the year in pine, and only from July to September in spruce, there is a depression of red light (660-670 nm) transmission. This affects the ratio of 660/730 nm light penetrating the apical domes. This ratio is lowest in July and August, at the time of initiation of female strobili. The seasonal variation in transmission of light by bud scales is discussed in relation to the possible consequences for the control of flowering.