Vol. 70 No: 6

Title:
Responses of Dark-Preconditioned and Normal Light-Grown Cuttings of Syringa Vulgaris ‘Madame Lemoine’ to Light and Wetness Gradients in the Propagation Environment

Authors:
B.H. HOWARD and R.S. HARRISON-MURRAY

pp: 989-1001

Abstract:
Leafy cuttings of Syringa vulgaris ‘Madame Lemoine’ were prepared from shoots grown normally in the light or pre-treated by starting growth in darkness. Their response to the propagation environment was measured using an artificially illuminated controlled propagation environment (CPE) constructed with perpendicular gradients of water deposition (mean range 209 to 13 µm h^-1) and light (mean range 289 to 17 µmol m^-2 s^-1), creating a matrix of light and wetness. The main effect of dark-preconditioning was to reduce stem dry matter by reducing both stem diameter and dry-matter percentage. This in turn almost doubled the ratio of leaf area to cutting stem dry weight, which appeared to determine the relative rooting advantage of dark-preconditioned cuttings in most environments, given that photosynthetic rate, and respiration rate per unit of stem tissue, were broadly similar for light-grown and dark-pretreated cuttings. By day 10, just prior to root emergence, basal necrosis was evident in some light-grown cuttings in most environments, whilst among the dark-pretreated cuttings this was less serious or absent in high light, but more frequent and extensive in low light. The combination of high light with low wetting caused permanent wilting of both types of cuttings. Callusing and subsequent rooting after three weeks reflected the viability of cuttings, with more dark-pretreated cuttings rooting in the high light, heavy wetting zones, but with a slight advantage for light-grown cuttings in the darker, drier environments. Differences in rooting between the two sources of cuttings were correlated with the accumulation of dry matter in the proximal section of stem between planting and the emergence of roots (days 0 to 10). This usually favoured the dark-pretreated cuttings with an initially low dry-matter content, improved rooting being associated with both enhanced dry-matter accumulation at the stem base in the high light, and less loss of dry matter compared with light-grown cuttings at low light. From a practical viewpoint it is important to note that environments likely to be found in commercial propagating houses do not have the necessary high wetting in high light or low wetting in low light shown to support rooting in these experiments.

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