Abstract: 1College of Horticulture and Landscape, Hunan Agricultural University, Changsha 410128; 2Center of Analytical Service, Hunan Agricultural University, Changsha 410128; 3Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, Beijing 100081) Abstract: 【Objective】 The aim of this study was to investigate the effects of sustained weak light on growing conditions and photosynthetic characteristics of potato leaves at the seedling stage and to offer a theoretical basis for screening and utilization of new resistant potato resources to weak light.【Method】 The original cultivated potato Yan (S. tuberosum subsp. andigena var. yanacochense) and cultivated potato Favorita (S. tuberosum) were grown in a growth chamber at 50 μmol•m-2•s-1 and 350 μmol•m-2•s-1 light, respectively, using potato tuber (50 g) growing in medium. After 1 month treatment, the growth of the test materials was measured. Plant phenotype, the chlorophyll content, net photosynthetic rate, photosynthetic curve, the CO2 response curve were tested by LI-COR 6400 portable photosynthesis system. The stomatal morphological characters were measured using JEM-1200EX scanning electron micrography. The ultrastructure of chloroplast was observed by transmission electron microscope using the blade section after uranium acetate-citrate double staining.【Result】There was a difference between the treatment and the control in leaf size, morphology of plant. Favorita had weak vigour, slender seedling with smaller pale green foliage. Foliage and branch induction of Yan were very difficult and grew extremely slowly in or after treatment. Sustained light treatment significantly reduced the apparent quantum yield (AQE), light saturation point (LSP), maximum net photosynthetic rate (Pnmax), CO2 saturation concentration (CSP), and the chlorophyll content efficiency, and improved apparent carboxylation rate (EC), and CO2 compensation concentration (CCP) simultaneously. The light compensation point (LCP) of Yan was activated in the treatment. While the LCP and EC of Favorita were reverse. The EC, CSP of Yan had no difference between the treatment and the control. Growing under weak light, the stomatal density, chloroplast number declined. The numbers of grana and grana lamellae increased in Favorita. On the contrary, the number of grana lamellae decreased in Yan. 【Conclusion】 Foliage differentiation of potato was hypersensitivity to light. There was a significant genotypic difference. The development and growth of the sensitive genotypes were hindered when plants acclimated to low light (50 μmol•m-2•s-1). The damage could not be resumed by restoring strong light later. The sustained weak light treatment at seedling stage depressed photosynthetic rate (Pn) and dark respiration rate (Rday). Saturation point of plant fell off along with the light intensities reduced. Following the dark respiration rate (Rday) being decreased, Favorita potato could adapt to lower light intensity by reducing the light compensation point. On the contrary, Yan could not adapt to lower light intensity. Appeared organic matter accumulated difficulties because of the improved light compensation point (LCP), the narrowing effective intensity range and the increased dark respiration rate (Rday). Under sustained weak light stress, the mesophyll cell arrangement was changed , and the stomatal density, the stomatal apparatus and numbers of chloroplasts decreased significantly, the stomatal apparatus length-width ratio had a tendency to increase, and chlorophyll composition ratio changed. Favorita potato improved the capture ability of effective light sources by increasing the number of chloroplast grana, grana lamella and chlorophyll b. Yan potato had subdued capturing ability of effective light and CO2 affinity because the formation of chloroplast grana was affected and the number of chloroplast grana, grana lamella and stomatal density decreased.

Key words: potato , seedling , weak light , photosynthetic characteristics , chloroplast ultrastructure , stomata