| Abstract: | Chloroplast replication has been studied in discs cut from thebase of young spinach leaves and cultured on sterile nutrientagar. In discs grown in a growth cabinet chloroplast numbersper cell increased logarithmically with time over a 7-day cultureperiod. Chloroplast replication proceeds in a similar way incultured discs and in intact leaves. Cytokinins do not affect chloroplast replication in this systembut they stimulate the fresh-weight growth of discs. Chloroplastreplication is temperature dependent, having an optimum at 25°C. By contrast chloroplast size is at a maximum in discscultured at 12 °C. Light stimulates chloroplast replication, a linear relationshipoccurring between chloroplast number per cell and the dailyquantity of light given to discs up to a saturating value of250 J d–1. Daylength does not affect chloroplast formationin spinach. In a number of experiments a general relationship was establishedbetween chloroplast number per cell and cell size but no evidenceis available to suggest that this correlation is causal. Theresults of experiments in which discs were transferred fromdark to light suggest that some of the events which precedechloroplast replication may occur at similar rates in both lightand dark. |
