Effects of shading on growth, leaf gas exchange and chlorophyll fluorescence of three container-grown shrubs

The aim of this work was to find the optimal level of irradiance for growth and leaf gas exchange in three container-grown widely-used ornamental shrubs (Rhododendron hybr. 'Orion', Choisya ternata H.B.K. and Viburnum ×pragense Vik). One-hundred and twenty rooted cuttings per each species were grown in shadehouses which provided different light environments (full sun, 30% shade and 60% shade). Light intensity was reduced by using commercially available shade material consisting of black woven polypropylene fabric that acted as a neutral filter to PAR. Standard commercial nursery production for fertilization, irrigation and pest control was followed. Plant height, dry weight, and shoot o root ratio were measured at the end of the growing season in 2005 and 2006. Leaf gas exchange and chlorophyll fluorescence were measured in 2006. For Viburnum ×pragense the total leaf number per plant, mean area per leaf, Leaf Area Index (LAI) and Relative Growth Rate (RGR) were measured in 2005 and 2006. Rhododendron hybr. responded to shading by decreasing its transpiration and stomatal conductance. Shaded plants also had lower shoot and root dry weight. Shading increased carbon assimilation of Choisya ternata; plants of this species were also taller and had higher total plant shoot and root dry weights. Viburnum ×pragense showed a great adaptability to the different light environments. Plants grown in full sun and under 60% shade had similar plant dry weight and leaf gas exchange. Our results confirm that response to shade is species-specific. Rhododendron performed better in full sun. Choisya performed better under 60% shade, which may lead to water saving in nursery production of the species, without compromising plant quality. Viburnum can grow both in full sun and 60% shade. In this species, the most shaded plants also showed a higher number of leaves/plant, average leaf area, LAI and LAR especially in the second year.