RESPONSES OF DARK-PRECONDITIONED AND NORMAL LIGHT-GROWN CUTTINGS OF SYRINGA-VULGARIS MADAME-LEMOINE TO LIGHT AND WETNESS GRADIENTS IN THE PROPAGATION ENVIRONMENT

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 measure d using an artificially illuminated controlled propagation environment (CPE) constructed with perpendicular gradients of water deposition (m ean range 209 to 13 mu m h(-1)) and light (mean range 289 to 17 mu mol 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 bot h 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 cuttin gs in most environments, given that photosynthetic rate, and respirati on 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 envir onments, whilst among the dark-pretreated cuttings this was less serio us or absent in high light, but more frequent and extensive in low lig ht. The combination of high light with low wetting caused permanent wi lting of both types of cuttings. Callusing and subsequent rooting afte r three weeks reflected the viability of cuttings, with more dark-pret reated cuttings rooting in the high light, heavy wetting zones, but wi th a slight advantage for light-grown cuttings in the darker, drier en vironments. Differences in rooting between the two sources of cuttings were correlated with the accumulation of dry matter in the proximal s ection of stem between planting and the emergence of roots (days 0 to 10). This usually favoured the dark-pretreated cuttings with an initia lly low dry-matter content, improved rooting being associated with bot h 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 enviro nments 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.