SummaryBlue light responses in higher plants can be mediated not only by specific blue light receptors, but also by the red/ far-red photoreversible phytochrome system. The question of interdependence between these photoreceptors has been debated over many years. The availability of Arabidopsis mutants for the blue light receptor CRY1 and for the two major phytochromes phyA and phyB allows a reinvestigation of this question. The analysis of photocontrol of seed germination, inhibition of hypocotyl growth and anthocyanin accumulation clearly demonstrates that (i) phyA shows a strong control in blue light responses especially at low fluence rates; (ii) phyB mediated induction reactions can be reversed by subsequent blue light irradiations; and (iii) CRY1 mediates blue light controlled inhibition of hypocotyl growth only at fluence rates higher than 5 µmol m -2 s -1 and independently of phytochrome A and B.

Abstract— Two non‐photosynthetic photoreceptors (phytochrome and the usual blue/UV light photoreceptor) were previously found to be involved in light‐mediated anthocyanin synthesis in the mesocotyl of Sorghum seedlings (Drumm and Mohr, 1978). The decisive point is that phytochrome can act only after a blue/UV light effect has occurred. On the other hand, the expression of the blue/UV light effect is controlled by phytochrome (‘obligatory sequential action’). A strong positive interaction between the blue/UV‐A and the UV‐B part of the spectrum was found, in addition to the above sequential action: an inductive effect of blue or UV‐A light can only express itself fully if short wavelength UV (approximately 300–320nm. UV‐B range) is also given, either after the blue/UV‐A light or simultaneously. Since even small amounts of the UV‐B are strongly effective it is probable that this effect plays a role under natural conditions and may not be considered as a mere laboratory artifact.

Drumm -Herrel, H, and Mohr, H, 1985, Photosensitivity of seedhngs differing in their potential to synthesize anthocyanin, -Physiol, Plant, 64: 60-^66, in the present report the suggestion (Paech, K, 1950, Biochemie und Physioiogie der sekundaren Pflanzenstoffe, -Springer, Berlin, pp, 201-203) was tested that the photosyntbetic apparatus requires light protection during the early phase of its development and that this is the reason (in a teleonomic sense) for the transient formation of large amounts of juvenile anthocyanin in outer tisstie layers of seedlings and young leaves of deciduous trees and shrubs. Seedlings of two species {Sinapis alba L, and Sesamum indicum L,) which differ in their potential to produce anthocyanin were compared under identical light conditions. The results obtained are compatible with the idea that juvenile anthocyanins are involved in photoprotection. However, the experimental results also indicate that full photostability of the plastid is attained -irrespective of the presence or absence of anthocyanin -once a certain amount of chlorophyll has been accumulated. Thus, photosensitivity of a seedling under natural light eonditions is restricted to an early phase of development prior to intense greening.

Abstract— The hypocotyl of the tomato (Lycopersicon esculentum) seedling synthesizes large amounts of anthocyanin if exposed to prolonged light. Single light pulses are totally ineffective. The involvement of phytochrome can be shown by light pulse treatments following a prolonged light exposure. It is predominantly the action of blue/UV light which leads to a high responsiveness of anthocyanin synthesis towards phytochrome. Moreover, the data suggest a phytochrome‐independent action of blue/UV light, in particular of UV‐B, on anthocyanin synthesis.

The rate of hypocotyl longitudinal growth in seedlings of Sesamum indicum L. is strongly inhibited by continuous blue light (cBL)† and slightly by continuous far‐red light while continuous red light (cRL) or red light pulses are hardly effective from 60 h after sowing onwards. Between 36 and 60 h after sowing the growth rate responds to red light pulses the effect of which is fully reversible by long wavelength far‐red light. When seedlings are kept in cBL for 3 days and then treated with red light hypocotyl growth rate responds strongly. However, RL effectiveness decreases with time after transfer from BL to RL. BL → darkness transfer experiments with different levels of Pfr established at the beginning of darkness show that after a BL pretreatment phytochrome (Pfr) alone is capable of fully controlling growth rate. When white light (WL) is given no BL effect is detectable in weak WL. Only high light fluxes maintain a typical BL growth rate. At medium WL fluxes elongation rate returns gradually to the dark rate. The simplest explanation of the data is that light absorbed by a separate BL photoreceptor is necessary to maintain responsivity to Pfr. With increasing age of the seedlings the requirement for BL increases strongly. On the other hand, brief light pulses—given to demonstrate photoreversibility of phytochrome—remain equally effective provided that responsivity to Pfr exists.