The control of plant development by light quality.

An investigation has been made of the physiology underlying the increased rates of stem extension induced by simulated shade light quality. Vegetational shade light quality was simulated by the addition of supplementary far red light (FR) to background white light (WL). FR was given either to the whole plant, or to specific parts of the plant using interference filters combined with fibre optics to determine the site of FR photoperception. For the whole plant irradiations, supplementary FR was given for several days, throughout the main photoperiod. For the short FR treatments used in the fibre optics experiments, small rapid changes in extension rate were measured using a linear displacement transducer. The results indicate that supplementary FR can modulate stem extension rate in fully light grown plants following perception by either the leaves or the stem. Neither the apical nor the axillary meristems are required for the response to FR. Extension rate is most rapidly controlled by the light environment of the stem itself; supplementairy FR given to the stem will induce an increase in extension rate after a lag period of only 13 min. The rapid effect of FR to the stem is only partially reversed by a return to WL. Supplementary FR given to the leaves stimulates stem extension only after a lag period of approximately 3 h, and there is no rapid reversal of the increased extension rate induced by FR irradiation of the leaves on return to WL. In the physiologically simpler system of the de-etiolated pumpkin seedling at the cotyledonary stage, hypocotyl extension is directly related to the estimated phytochrome photoequilibrium established in the cotyledon. In fully light grown plants, the effect of long term supplementary FR on stem extension can be reduced by the inhibitor of gibberellin synthesis, CCC. Furthermore, long term supplementary FR produces an increase in endogenous gibberellin levels in both the leaf and stem, compared with plants grown in WL. A model is proposed to account for the action of FR, operating through Photosystem I via effects on cyclic photophosphorylation, and through phytochrome, controlling the release and synthesis of gibberellin.