Coaction of blue/ultraviolet-A light and light absorbed by phytochrome in controlling growth of pine (Pinus sylestris L.) seedlings.

Photomorphogenesis is a conspicuous feature in conifers. In the case of the shade-intolerant Scots pine (Pinus sylvestris L.), control of stem growth by light is well expressed at the seedling stage and can readily be studied. The present data show that hypocotyl growth is controlled by the far-red-absorbing form of phytochrome (Pfr). However, the Scots pine seedling requires blue or ultraviolet (UV-A) light to become fully responsive to Pfr. Blue/UV-A light has no direct effect on hypocotyl growth and its action appears to be limited to establishing the responsiveness of the seedling to Pfr. This type of coaction between phytochrome and blue/UV-A light has been observed previously in a number of angiosperm seedlings. With regard to the 'high irradiance reaction' of phytochrome in long-term far-red light the pine seedling deviates totally from what has been observed in etiolated angiosperms since continuous far-red light has no effect on stem growth.