Effect of the hypocotyl hook on greening in etiolated cucumber cotyledons.

Four-day-old etiolated cucumber cotyledons (Cucumis sativus, L.) were excised and allowed to green in white fluorescent light at 28 C. Cotyledons excised with a full hypocotyl hook exhibited a lag phase of 1 hour before entering the rapid greening phase, whereas cotyledons excised without any hypocotyl hook exhibited a lag phase of 6 hours. Cotyledons excised with varying lengths of hypocotyl hook accumulated chlorophyll roughly in proportion to the hook length. When cotyledons were excised with a full hook and were partially or totally shielded from light with aluminum foil, the samples with the hook covered accumulated more chlorophyll than the wholly exposed samples. The samples with the cotyledons covered showed no net accumulation of chlorophyll irrespective of hook's exposure to light. These data suggest the contribution of some factor or factors by the hypocotyl hook which reduce the lag phase during greening.