Evidence of competition for water as a factor in the mechanism of root-bud inhibition in milkweed (Asclepias syriaca)

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Abstract
In experiments with seedlings of milkweed, Asclepias syriaca L., grown in sand culture under controlled conditions, growth of the root buds was significantly increased within 24 h of the excision of the parent shoot. However, only a few of the measured buds continued to grow, the growth of most of the remianing buds being arrested within 48 h of their release from inhibition. At low humidity (approx. 30–50% relative humidity (RH)), the decapitated disbudded stem, bearing only a single pair of leaves, promoted the early growth of the root buds but strongly inhibited their subsequent elongation and emergence as shoots. The mature stem itself, when completely defoliated, also exerted a significant degree of inhibition of root-shoot production. At high humidity (93–100% RH), which reduced transpiration by approximately 50%, this inhibiting effect of mature shoot tissue was either eliminated or greatly reduced, while the rate of emergence and subsequent growth of shoots produced from the root buds following the complete removal of the parent shoot were also significantly increased. The results suggest, as a working hypothesis, that transpiration-induced reduction in water potential (ψ) plays a major role in the mechanism of root-bud inhibition.