Control of cambial activity and dormancy in Picea sitchensis by indol-3-ylacetic and abscisic acids

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Abstract
The combined gas chromatography – mass spectrometry (GCMS) technique of single-ion current monitoring was used to measure indol-3-ylacetic acid (IAA) and abscisic acid (ABA) levels in diffusible, acidic, and conjugated fractions obtained from the cambial region of Sitka spruce (1) during the annual cycle of cambial activity and dormancy in trees reared under natural and controlled environmental conditions, and (2) after debudding, girdling, defoliating, applying exogenous IAA, droughting, or changing the photoperiod. Seasonal changes occurred in the level of IAA and ABA in each fraction, but not obviously and consistently in conjunction with specific changes in cambial activity and dormancy. Debudding and girdling halted tracheid production, decreased the radial enlargement of the last-formed tracheids, and abruptly reduced the content of IAA and ABA in the diffusible and acidic fractions without affecting the ABA level in the conjugated fraction. Applying exogenous IAA to debudded shoots maintained tracheid production and differentiation, and largely prevented the decrease in IAA and ABA content. Elevating the internal water stress inhibited apical and cambial growth, increased the contents of diffusible, acidic, and conjugated ABA, and decreased diffusible and acidic IAA levels; the foliar concentrations of acidic IAA, acidic ABA, and conjugated ABA were similarly altered. Short-day photoperiod induced apical and cambial dormancy and reduced the levels of diffusible and acidic IAA and diffusible, acidic, and conjugated ABA; the decline in hormone content occurred mainly after the cambium ceased activity. Exogenous IAA could not prevent the cambium from ceasing activity under natural field conditions or short-day photoperiod. It is concluded that (1) there is an intrinsic seasonal pattern of change in IAA and ABA levels in the cambial region, governed primarily by seasonal changes in foliar hormone production; (2) this pattern is readily and variously perturbed by changes in the environment, particularly by factors affecting the internal water stress; (3) the evidence accumulated to date casts doubt on a role for ABA in controlling cambial dormancy; (4) cambial activity and the completion of the springtime transition from quiescence to activity depend on a continuous supply of basipetally transported IAA; (5) the changeover from activity to rest at the end of the cambial growing period is due to the development of an inability to respond to IAA, not to a deficiency in IAA supply; and (6) the transition from rest to quiescence during the cambial dormant period involves regaining the ability to respond to IAA.