Cellular and Molecular Studies on the Shoot Terminal Meristem of Pharbitis Nil Chois. CV. Violet During Floral Evocation.

Morphological, cell cycle and molecular events were studied during floral evocation in the shoot terminal meristem of the short-day plant Pharbitis nil. The haploid genome size of P.nil was 3.7 X 10e8 bp. Morphological and cell doubling time observations demonstrated that the apex widened and flattened follOwing an inductive dark treatment, the cell doubling time decreased in the peripheral zone and increased in the central zone. An inductive treatment comprising 48 h darkness given to 4 d old plants produced. 100~ flowering at the shoot terminal meristem. An inhibitory treatment was developed using 5 min RL breaks during the 48 h dark period. This treatment prevented flowering at the shoot terminal apex and discriminated between events essenttat for flowering and changes resulting from shifts from light to darkness and vice versa. It was shown using shading experiments that the shoot terminal apex may be directly involved in the reception of red-light.
Maximal differences in the proportion of cells in G2/G 1 (G2/G 1 ratio) were recorded at the light/dark transition (i.e. at the end of the 48 h dark period). Apices were sampled at this time to construct floral and vegetative cDNA libraries. Less growth occurred in the central zone of the shoot apex. For example, the mitotic index of the central zone was consistently lower than that of the peripheral zone in both vegetative and induced apices. Unusual PLM curves were generated which meant that the component phases of the cell cycle were determined from microdensitometric data. The duration of the cell cycle in the peripheral zone was two-fold longer in vegetative apices compared to floral apices. However, the proportion of cycling cells (the growth fraction) was similar in the peripheral zones of both vegetative and floral apices. Following exposure to RL the duration of G2 in the peripheral zone increased five-fold. mRNA was separately extracted from 40 floral and vegetative shoot apices. The mRNA was copied into cDNA and amplified using the polymerase chain reaction. The amplified cDNA was then cloned into two A phage cDNA libraries (floral and vegetative). These libraries will form the foundation of future work on the molecular biology of floral evocation.