ABSTRACT
This chapter reviews evidence linking developmental changes in the flux of endogenous nutrients and phytohormones with leaf photosynthetic capacity and senescence and the promotive and inhibitory effects of foliar applications of exogenous chemicals on leaf photosynthesis and senescence. It presents several lines of experimental evidence suggesting that age and/or deficiency-related declines in the photosynthetic capacity of individual leaves can be reversed or reduced by leaf uptake of exogenous chemicals such as mineral nutrients, cytokinins, and CO2. The net flux of endogenous solutes and water into leaf cells is related to relative rates of xylem and phloem transport in and out of the leaf. Wareing et al. suggested that partial defoliation reduced competition among remaining leaves for mineral nutrients and/or cytokinin phytohormones supplied by the roots, thus causing the increase in photosynthetic efficiency. When the soil-root complex provides a less than adequate flux of plant-essential micro-nutrients to the leaves, the leaves respond with progressive losses in functional capacity and eventual death.
