National Inst. of Biology, Vecna pot 11 , 1000 Ljubljana, Slovenia

Background and objectives
A virus infection can strongly modify growth and development of a plant. Pathological symptoms appear as a consequence of modifications of host metabolism caused by the infection. When the interaction between virus and host results in the formation of chlorotic or yellow leaf tissue, there is an almost inevitable associated effect on many aspects of photosynthesis [1]. The potato cv. Igor grown in soil is highly sensitive to potato virus YNTN, which causes potato tuber ring necrotic disease. After primary infection, the virus evokes severe symptoms: mosaics, yellow senescent leaves and leaf drop. On the other hand, secondarily infected potato plants of cv. Igor grown in tissue culture were not severely affected; this was also reflected in photosynthetic pigment metabolism [1]. The aim of this work was to investigate the effect of PVYNTN on photosynthetic pigments of the potato cv. lgor after different periods of primary infection.

Materials and methods
Potato plants cv. Igor were grown by a stem node segmentation procedure and transferred into soil. After 4 weeks, plants were mechanically wounded and blind (inoculated with healthy plant sap) or PVYNTN inoculated. The pigments were measured 3, 6, 9, 12, 24 h and 5 days after inoculation, before visible symptoms, and 16 days after inoculation, when symptoms were already observed. The amount and composition of leaf photosynthetic pigments of PVYNTN-infected, blind inoculated, and intact plants were studied. Samples were frozen and lyophilised. They were extracted with acetone, separated by high performance liquid chromatography (HPLC) and identified by their light absorbance characteristics, hypochromic shifts and retention time [2].

Results and conclusions
Six major pigments were identified in all samples: 9'-cis neoxanthin, all-trans violaxanthin, lutein, chlorophyll a, chlorophyll b and 11-carotene. A trend of carotenoid and chlorophyll decrease was detected 9 h after of inoculation. Sixteen days after inoculation, when symptoms were expressed, a significantly lower amount of chlorophyll was found in infected plants in comparison to blind inoculated plants. The decrease was much more pronounced in yellow than in green mosaic leaves. In infected yellow leaves the amount of chlorophylls represented only 32% of chlorophylls found in blind inoculated plants. In contrast to chlorophylls, an increase of carotenoids was detected in yellow leaves, which was also reflected in a decrease of the ratio between chlorophylls and carotenoids.

In conclusion, the results presented in this study clearly indicate the effect of virus infection on photosynthetic pigments in the later stage of infection, when mosaic symptoms were expressed. At this stage the amounts of chlorphylls and carotenoids were significantly reduced. The increase of carotenoids in yellow leaves is most probably the result of senescence. In the first 5 days after inoculation, the similar decrease of photosynthetic pigments in blind and virus-inoculated plants, might be the plant response to wounding.

1. Mandahar CL, 1990. Plant Viruses, Vol. 2, Pathology. CRC Press, Boca Raton, Florida, pp. 30-33.
2. Antlovar S, Kovac M, Ravnikar M, 1996. Phyton Annales Rei Botanicae 36, 221-230.