Chlorophyll synthesis is usually coincided by growth promoting hormones GAs..... and ABA is growth inhibiting which can block the synthesis of GAs steps i ant-7kauronic acid. Besides ABA facilitate the generation of senescence hormone Ehylene. However, in many trails I found chlorophyll % increases in cells of low expansion rate, this in fact a paradox Increases of chlorophyll comes from sequestering chloroplasts in small cells. Therefore, pale plants looking occurred with high growth rate of cell owing to high temperature in summer. In contrast fall and winter plants being greener. Thanks