Effects of calcium on chlorophyll synthesis and stability in the early phase of greening in cucumber cotyledons.

The effects of calcium on chlorophyll accumulation and its stability in the early phase of greening in cucumber cotyledons were investigated. Chlorophyll accumulation was hardly affected by dark preincubation of cotyledons with 10 millimolar calcium solution, but was inhibited almost completely when 50 or 100 millimolar solution was used. On the other hand, 50 millimolar calcium inhibited delta-aminolevulinic acid formation in the light by only 75%. Calcium had little effect on the lag for initiation of protochlorophyllide(650) regeneration, but slowed down the rate of accumulation of protochlorophyllide(650). In calcium-treated cotyledons, the chlorophyll formed by primary photoconversion was quickly decomposed in the dark. The present results show that calcium inhibited chlorophyll accumulation by inhibiting delta-aminolevulinic acid formation in the light and by stimulating the decomposition of newly formed chlorophyll, both effects being completely prevented by potassium.