Apical control of xylem formation in the pine stem. I. Auxin effects and distribution of assimilates

The effect of IAA upon cambial activity, xylem differentiation and translocation of assimilates from the lateral shoot was investigated in spring and late summer in decapitated and ring-barked young trees of <em>Pinus silvestris</em> in the forest stand. Decapitation interrupted cambial xylem production in the uppermost part of the main stem of decapitated trees in spring and late summer, regardless of whether lateral branches below were growing, dormant or disbudded, and the contact through phloem with the roots was maintained or severed. Auxin supplied to the decapitated stems caused an increasing stimulation of cambial xylem production in spring. It also stimulated cambial activity in August but was ineffective in September. Apical control of cambial xylem production was strongly dependent upon the continuity of phloem and/or cambial tissues of the decapitated main-stem-section with lower parts of the plant. Decapitation of the stem strongly reduced the daily rate of cell wall deposition in the cambial xylem derivatives which on the day the experiment started constituted the zones of radial enlargement and maturation. This reduction limited progressively secondary wall deposition in consecutive maturing tracheids even though the cells differentiated longer. Irrespective of the season, auxin prevented the effect of decapitation in cells which were already differentiating when the experiment started as well as extension of the maturation phase. The effect of auxin was somewhat reduced when the lateral branches were additionally decapitated in early summer. In early summer auxin caused a significant increase of the daily rate of cell wall deposition in cells of the cambial zone or the newly produced ones, thus resulting in formation of progressively thicker secondary walls. Late in summer assimilates were transported mostly to the lower part of the stem. Decapitation changed the intact tree pattern of assimilate distribution, increasing the transport in spring and reducing it later in the summer. Prevention of the contact with roots via phloem and cambium in spring (by ring-barking the stem at tree base) decreased decapitation-induced downward transport of assimilates. Application of auxin to the decapitated uppermost segment of the main stem resulted in a significant increase of assimilate translocation into the stem. At least two mechanisms of auxin involvement in regulation of the rate of secondary wall deposition in pine stem tracheids can be considered: (a) induction (or activation) of the cell wall metabolic potential which seems to occur during meristematic or early radial enlargement phases of tracheid differentiation, and (b) regulation of substrate availability during the phase of tracheid maturation.