Benzyladenine (BA) stimulated 5‐aminolevulinic acid (ALA) accumulation in the presence of levulinic acid during illumination with 43 μmol m−2 s−1 light in excised etiolated cotyledons of cucumber (Cucumis sativus L. cv. Aonagajibai). A short dark‐pretreatment (6 h) with BA eliminated the lag phase of ALA accumulation. The rate of ALA accumulation during the steady‐state phase in cotyledons pretreated with BA for a long period (14 h) was considerably accelerated compared to that in cotyledons pretreated with BA for 6 h. The rate of ALA accumulation during the lag phase was saturated at a very low light fluence (<1.4 μmol m−2 s−1) in both BA‐pretreated and water‐control cotyledons. The steady‐state rate of ALA accumulation increased with increasing light fluence up to 43 μmol m−2 s−1 (parallel to that of Chl formation) in water‐control cotyledons. In contrast, in cotyledons pretreated with BA for either 6 or 14 h, the steady‐state rate reached a plateau at a very low light fluence. Based on the above results together with our finding that there are two components of Chl formation (M. Dei, 1984. Physiol. Plant. 62: 521–526) possible intermediate steps of Chl biosynthesis pathway affected by BA and light intensity are discussed.

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Treatment of 3‐day‐old excised etiolated cotyledons of cucumber (Cucumis sativus L. cv. Aonagajibai) with benzyladenine (BA) in the dark stimulates chlorophyll (Chl) formation during the lag phase (designated as ‘lag elimination’) and accelerates the steady‐state rate of Chl formation under subsequent continuous illumination with white light. The separation of this two‐fold effect is possible using two different methods of BA treatment in darkness: a brief BA treatment followed by various periods of water treatment in darkness, or various periods of continuous dark BA treatment. In either treatment, BA rapidly eliminates the lag phase (the fast‐appearing effect) and after a longer time period accelerates the steady‐state rate (the late‐appearing effect). With a brief BA treatment, both effects decay rapidly. In contrast, with continuous BA treatment, none of the effects decay after reaching their maxima, particularly in cotyledons excised 2 days after sowing and aged for a long period before the onset of BA treatment. These facts indicate that BA acts as a trigger in stimulating Chl formation. The relationship between the actions of BA and light is discussed.

Inter-organ control of greening in etiolated cucumber (Cucutnis sativus L. cv. Aonagajibae) cotyledons was investigated. Four-or six-day-old excised or intact etiolated cucumber cotyledons were illuminated under aerobic conditions. Excised cotyledons without hypocotyl hooks produced chlorophyll without a prolonged lag phase and the rate of chlorophyll formation was not depressed if they were illuminated immediately after excision. If the excised cotyledons were incubated in the dark before illumination, chlorophyll accumulation at the end of 6 h of continuous illumination was remarkably lowered as the dark period lengthened, especially in 6day-old cotyledons. The rapid loss of chlorophyll-forming capacity of excised cotyledons during dark preincubation suggests a stimulatory effect of hypocotyls on the greening in cotyledons. The treatment of excised cotyledons with bleeding sap in the dark for 18 h resulted in the promotion of chlorophyll formation during subsequent continuous illumination. Partial fractionation of bleeding sap with organic solvents and paper chromatography indicates that the active substances showed the same behavior as cytokinins. These facts add weight to the hypothesis that cytokinins from roots fiow into cotyledons and stimulate greening.

Excised etiolated cotyledons of cucumber (Cucumis sativus L. cv. Aonagajibai) were continuously irradiated under various intensities of white light. The rate of chlorophyII (Chi) formation during the lag phase reaches a plateau at fluence rates above 1.4 urmol m−2 s−1. This is true in both water‐control and benzyladenine (BA)‐pretreated cotyledons. In cotyledons pretreated for 14 h with BA in darkness (in which case, Chl formation is stimulated by BA during both the lag and the steady‐state phases), the increase in the steady‐state rate of Chl formation with increasing light in tensity is stimulated compared to that of the water control over the range of fluence rates, 0. 25‐43 urmol m−2 s−1. In cotyledons pretreated for 6 h with BA in darkness (only Chl formation during the lag phase is stimulated), only an increase in fluence rate from 0.25 to 1.4 umol m−2 s−1 causes a higher increase in the Chl formation in the BA‐treated cotyledons than in the water control. The time course of Chl formation shows that the BA‐induced late‐appearing effect (stimulation of the steady‐state rate) is almost absent at low intensity illumination, but the BA‐induced fast‐appearing effect (elimination of the lag phase) is effective at all intensities. From this evidence, the Chl‐forming process apparently consists of two components, whose periods of operation or light‐intensity requirements are different. BA stimulates the rates of the respective components in both the fast and the late‐appearing effects.