Physiological Responses To Sunlight Of Several Ornamental Foliage Plants

Photosynthesis process and chlorophyll contents were determined for eight common ornamental foliage plants, e.g. Photinia×frasery, Aucuba japonica var. variegata, Buxus megistophylla, Osmanthus fragrans, Pittosporum tobira, Ligustrum lucidum, Fatsia japonica, and Viburnum odoratissimum var. awabuki. Leaf anatomical characteristics of Ph. frasery and A. japonica var. variegata were compared under different shading treatments. This study was aimed at providing a scientific basis for evaluation of photosynthesis characteristics and shading tolerance of the eight ornamental foliage plants.The conclusions were described below:(1) I determined three ecophysiological indices, e.g. transpiration rate, stomatal conductance, and intercellular carbon dioxide concentration, as well as six environmental factors, e.g. photosynthetically active radiation, air temperature, relative humidity, air concentration of carbon dioxide, leaf temperature, and leaf water deficit. The correlation coefficients between ecophysiological indices and net photosynthetic rates, as well as between environmental factors and net photosynthetic rates were calculated to understand related factors of photosynthesis for eight ornamental foliage plants. The main environmental factors affecting photosynthesis examined were light, temperature, humidity, and atmospheric carbon dioxide concentration. Diurnal and seasonal regularity presented in environmental factors resulted in regular diurnal and seasonal changes in net photosynthetic rates. Top chlorophyll contents occurred in summer, whereas the lowest value of chlorophyll a/b content presented in summer.(2)Compared to Ph. frasery, A. japonica var. variegata was featured by lower values in specific leaf weight and numbers of stomata per unit area, but higher values in stomata size. Though with higher leaf area, A. japonica var. variegata had lower values in leaf thickness and plasticity indices compared to Ph. frasery. Ph. frasery featured by higher plasticity index was a light-requirement species, while A. japonica var. variegatea characterized by lower plasticity index was a shading-tolerant species. The results showed that A. japonica var. variegata was more shading-tolerant than Ph. frasery.(3)Under light shading treatments, both A. japonica var. variegata and the Ph. frasery maintained at the highest level in rates of net photosynthesis and transpiration, this state was even higher than the full light treatment. Ph. frasery had no midday depression, while A. japonica var. variegata had a weak midday depression. Therefore, the two ornamental foliage plants had a certain ability to adapt to low light. When the two ornamental foliage plants shading-treated exposed again to full sunlight, they got improved light-harvesting capacity and net photosynthetic rates, indicating that a degree of shading treatment can increase their photosynthesis rates. Under the same conditions, transpiration rates of full light treatment were always lower than the shading treatments.Therefore, shading treatments enhanced ecophysiological plasticity of the two plants, and widened their range of ecological adaptation. When the leaves adapted to low light environment returned to full sunlight, the metabolic activity was significantly higher than the normal leaves.The effect of shading treatment on pigment content strengthened when the shading degree increased. Shading treatment raised contents of chlorophyll a, b and a+b of two species, and reduced the values of chlorophyll a/b. The ratio of chlorophyll/carotenoid content also increased under shading treatment.(4)This article defines the shade-tolerance threshold for the first time, it can provide the evaluation for shade tolerance of ornamental foliage plants.