Relationship between photoconvertible and nonphotoconvertible protochlorophyllides

Two forms of protochlorophyllide are found in dark-grown bean (Phaseolus vulgaris, var. Black Velentine) leaves, one ($\text{protochlorophyllide}_{650}$) which is directly photoconvertible to chlorophyllide and another ($\text{protochlorophyllide}_{632}$) which is not. Dark-grown leaves placed in solutions of δ-aminolevulinic acid accumulate $\text{protochlorophyllide}_{632}$. $\text{Protochlorophyllide}_{650}$ and $\text{protochlorophyllide}_{632}$ can be partially separated on sucrose density gradients. A nitrogen atmosphere blocks chlorophyll synthesis in light or the regeneration of $\text{protochlorophyllide}_{650}$ in the dark, even in the presence of excess δ-aminolevulinic acid, except when a stockpile of $\text{protochlorophyllide}_{632}$ is present in the leaf. Under the latter conditions chlorophyll synthesis or $\text{protochlorophyllide}_{650}$ regeneration is accompanied by a decrease in $\text{protochlorophyllide}_{632}$. These experiments suggest that $\text{protochlorophyllide}_{632}$ may be converted to $\text{protochlorophyllide}_{650}$. Cycloheximide inhibited greening only after an "action-dependent" delay, requiring a predictable minimal period of illumination. This inhibition could be relieved for a time by feeding δ-aminolevulinic acid.