Whilst juvenile during vegetative growth, plants are incapable of initiating flowering when grown under floral inductive conditions. Understanding juvenility is important since it influences flowering time, which in turn impacts on scheduling of crop production. Daily light integral (DLI or LI) is the total photosynthetically active radiation received in one day and is calculated from light intensity and light period (Korczynski et al., 2002). It is one of the factors that could regulate juvenility. The project aims are to determine the effect of light integral on (i) phase development, (ii) photosynthetic assimilation and partitioning in relation to juvenility, (iii) the expression of key flowering time genes, including FLOWERING LOCUS T (FT) and genes involved in carbohydrate metabolism and (iv) the expression of microRNA156 in relation to juvenility. Transfer experiments have been carried out to generate material differing in juvenility length for molecular and biochemical analyses. These utilised antirrhinum plants grown under different light integrals and Arabidopsis mutants defective in specific carbohydrate related genes. cDNAs representing genes involved in starch synthesis (PHOSPHOGLUCOMUTASE(PGM), PHOSPHOGLUCOISOMERASE(PGI), SUCROSE PHOSPHATE SYNTHASE(SPS) ) and degradation (STARCH EXCESS1(SEX-1), BETA AMYLASE3(BAM-3), ALPHA AMYLASE(AMY)) have been isolated from antirrhinum plants. Antirrhinum plants grown under reduced light integrals had extended juvenile and adult vegetative phases and reduced levels of AmFT, AmPGM, AmPGI, AmSEX-1, and AmBAM-3 during juvenility. Juvenility was shown to be extended in mutants defective in both starch synthesis and starch degradation. This indicates that light integral influences the time plants need to accumulate enough level of carbohydrate to support phase change and levels of oligosaccharide, released during starch degradation, may influence the length of juvenility. Consistently, in both Arabidopsis and antirrhinum, the timing of FT induction was shown to correlate with the end of juvenility and interestingly, light integral has a greater effect on AmFT induction and phase transition than photoperiod. Moreover, the current study shows that LI also affects the length of JP when there are highly expression of miR156 which means that miR156 possibly acts downstream of LI in the regulation of flowering.