Changes in Chlorophyll a and b Content in Dark-Incubated Cotyledons Excised from Illuminated Seedlings

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Abstract
Cucumber seedlings were illuminated for various time periods, cotyledons excised, placed in the dark, and changes in chlorophyll a and b content monitored. During the dark periods, chlorophyll b content decreased while chlorophyll a did not. When the illumination time was lengthened, the percentage of chlorophyll b decomposition from initial levels decreased. Ca2+ at 50 millimolar prevented the decrease in chlorophyll b and caused a decrease in chlorophyll a. The effect of Ca2+ decreased with increased illumination time. Cycloheximide and chloramphenicol inhibited chlorophyll b decrease, but did not induce chlorophyll a decrease. When Ca2+ was applied to cotyledons, excised from 4 hour illuminated seedlings and preincubated with water in the dark until chlorophyll b content had decreased to a low level, chlorophyll b began to accumulate in the dark. Cycloheximide and chloramphenicol had no such effect. Ca2+ also induced an accumulation of chlorophyll b in cotyledons excised from seedlings treated with periodic light (2 minutes light followed by 98 minutes dark) during dark incubation. Our results indicate that there is unstable chlorophyll present in the early phase of greening, and that Ca2+ induces chlorophyll b synthesis from chlorophyll a in the dark.