Experiments with milkweed (Asclepias syriaca), conducted under controlled conditions, were designed to investigate the inhibiting effect of expanded leaves on the growth of their axillary buds. Precise measurements of bud growth (± μm), using a specially constructed strain gauge transducer, showed that excision of the apical bud and partly expanded leaves (shoot decapitation) or excision of only the expanded leaves both induced an almost immediate, but transient, bud growth response whereas excision of the expanded leaves on decapitated shoots permitted the axillary buds to escape from inhibition. Experiments providing measurements of long-term bud growth substantiated these results and showed that the rapid bud response to expanded leaf excision was associated with a 35% increase in the xylem water potential in the stem (ψ xy ). There was also some evidence that increasing the mineral nutrient supply may reduce the inhibiting effect of the leaves. The rapidity with which bud growth was initiated in response to leaf excision and the associated increase in ψ xy are consistent with the hypothesis that the reduction in ψ xy produced by transpiration limits the uptake of water by the lateral buds and thus plays a major role in the correlative inhibition of bud activity.
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McIntyre et al. (1990) studied this question.