Leaf development and leaf stress: increased susceptibility associated with sink-source transition.

Relationships between leaf age and leaf susceptibility to biotic and abiotic stress agents have been studied, but unifying concepts relating leaf ontogeny to stress susceptibility are not well developed. Leaves go through predictable and orderly physiological stages as they progress from metabolite sinks to metabolite sources and then become senescent. During this process, they may pass through a stage of maximum susceptibility to a given stress. It is proposed that, for many leaf stresses, this stage occurs at the time of the sink-source transition and can be related to anatomical, physiological and biochemical leaf ontogeny. This concept may be useful in relating host-plant growth habit and leaf production pattern to the distribution and abundance of herbivores and leaf pathogens.