Floral Determination in the Short Day Plant, Pharbitis nil

A shoot apical meristem is florally determined if following its removal from an induced plant it can make a flower when cultured in non-inductive conditions. We have measured floral determination times in the short day plant, Pharbitis nil, to examine the extent to which floral whorls are determined simultaneously or sequentially. Shoot apices were excised at daily intervals following a 48 h dark inductive treatment, cultured in non-inductive conditions for 4 weeks in continuous light and then the number of floral organs were scored. The culture medium was White's supplemented with either sucrose, glucose, fructose or 1:1 glucose : fructose (G:F) all at 2% (w/v) but no added plant growth regulators. The determination time for sepals was one day regardless of carbohydrate added and it was 4 days for the petals/stamens in the sucrose treatment and 3 days in the fructose and G:F treatments. However, carpel determination times varied remarkably from 5 days on sucrose, 3 days on fructose, 2 days on G:F to 1 day on glucose (for 20% of the apices). A minimum determination time of 1 day was also observed for the petals/stamens on glucose. Hence, by culturing the apices on sucrose, we observed sequential determination times but on glucose coincident determination times with the fructose and G:F treatments resulting, generally, in an intermediate response relative to glucose and sucrose. Based on the remarkable shortening of carpel determination time in the glucose treatment we suggest a specific role for this carbohydrate in floral morphogenesis.