LIGHT REQUIREMENT FOR PROPAGATING HARDY ORNAMENTAL SPECIES FROM LEAFY CUTTINGS

Leafy cuttings of Forsythia × intermedia 'Lynwood' and Weigela florida 'Variegata' were propagated in controlled environment chambers to minimise variations in leaf and air temperature. Four irradiance levels, viz. 20, 40, 60 and 80 W m-2 PAR for 16 h at 20°C and 0.1 kPA vapour pressure deficit were used. Rooting was best at 20 and 40 W m-2 and decreased at greater irradiance. At first, dry weight gain in the cuttings was greatest at high radiation but subsequently it slowed at 60 and 80 W m-2. Rooting was positively correlated with the moisture content of the cuttings but negatively related to dry weight gain and total ethanol-soluble sugar content. Accumulation of sugars to unusually high concentrations was associated with reduced relative growth rate and also reduced root initiation. Later, high irradiance favoured root growth. These results suggest that light levels in glasshouses are super-optimal for root initiation in cuttings through much of the year and support the use of automatic, light-regulated shading-curtains. © 1979.