No abstract

In a three-factor analysis the rate of chlorophyll a (Chl) accumulation in excised mustard cotyledons was studied as a function of kinetin, light (operating through phytochrome, P fr) and an excision factor. It was found that the three factors operate additively provided that the P fr level is high enough. When the P fr level is below approximately 1 per cent (ϕλ<0.01) the effectiveness of the excision factor decreases while the effect of kinetin remains additive. The observed additivity is explained by a model where the three factors operate independently through a common intermediate (presumably 5-aminolevulinate) in the biosynthetic chain leading to Chl. With regard to the coaction of the excision factor and phytochrome it is concluded that the production of the excision factor requires the operation of phytochrome (even though saturated at a low P fr level) while the action of the excision factor is independent of phytochrome. This conclusion was confirmed by experiments in which the rate of light-mediated anthocyanin synthesis was measured in excised mustard cotyledons. The effect of excision in the case of anthocyanin formation differs kinetically from the effect of excision on Chl formation.

A pretreatment with light prior to continuous illumination with high intensity white light eliminates the lag phase in chlorophyll a accumulation and increases the steady-state rate of chlorophyll a accumulation. In mustard seedlings (Sinapis alba L.) the effect of a pretreatment can be fully attributed to phytochrome. The effect of phytochrome on chlorophyll a accumulation is twofold. It is possible to separate the effect on the lag phase from the effect on the steady-state rate of accumulation. While the effect on the lag phase is a relatively fast process (occurring within less than 3 h) the effect on the rate requires a considerable period of time (at least 12 h) to become manifest.

Within the temporal pattern of "primary differentiation" the capacity of chlorophyll - a biosynthesis in the cotyledons ofSinapis alba L. seedlings is controlled by phytochrome (in continuous light) or by releasing the circadian rhythm either with lightdark cycles or by a light→dark transition. The sensor pigment for this process is phytochrome. It is very probable that in continuous light as well as under conditions under which the circadian rhythm plays the major part, the capacity of chlorophyll a biosynthesis is limited by the capacity of the biosynthetic step which produces 5-aminolaevulinate.

Intact mustard seedlings were treated with zeatin and photomorphogenetically active light in different ways: (1) hormone treatment preceding light treatment, (2) light treatment preceding hormone treatment, (3) hormone and light applied simultaneously. Under all experimental conditions the effect of the hormone treatment is multiplicative to the light effect with regard to the increase of cotyledon area. However, the hormone effect is additive to the light effect with regard to increases of the level of NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13) and carotenoid contents. Anthocyanin synthesis is inhibited by exogenous zeatin whereby the concentration response curves are similar, irrespective of the extent of anthocyanin formation mediated by light. However, an interaction was found in the sense that the responsiveness toward zeatin is decreased somewhat by the action of phytochrome. Our results show that the responsiveness to light (via the far-red-absorbing form of phytochrome; P fr) is not changed by a preceding or simultaneous hormone treatment. Moreover, the responsiveness of the plant to exogenously applied zeatin is not affected - except in anthocyanin synthesis - by a preceding or simultaneous light treatment. We conclude from our results that the action of phytochrome on the developmental processes is not related to cytokinin levels.