THE ANATOMIC RESPONSES OF DAUCUS CAROTA TO THE ASTER YELLOWS VIRUS

STRUCKMEYER, B. ESTHER. (U. Wisconsin, Madison.) The anatomic responses of Daucus carota to the aster yellows virus. Amer. Jour. Bot. 50(9): 959-963. Illus. 1963.-The leaves, petioles, and roots of carrots (Daucus carota) displaying aster yellows virus in the field and those infected with the aster yellows virus inoculated by the 6-spotted leafhopper were examined anatomically. Compared to the uninoculated, the young infected leaves displayed fewer layers of palisade cells and larger spongy parenchyma cells with a more compact arrangement. Mature leaves of infected plants sometimes were undulated and had few chloroplasts, many of which appeared fragmented. Hypertrophy and hyperplasia in the phloem tissue were associated with some necrosis and obliteration of cells. Long, needle-shaped crystalline inclusion bodies were present in the phloem in the leaves and roots. Most of the vascular bundles of the petiole were abnormal. Malformations included proliferating phloem cells, which in some instances almost encircled the bundle, hyperplasia of the phloem, hypertrophy of the parenchyma, and considerable necrosis and obliteration of these cells. Other responses included the division into 3 or 4 rows of the large outer phloem parenchyma by parallel walls so that a cambium-like layer was simulated. The tissue enclosed by this layer divided and underwent considerable necrosis and gummosis. Lacunae were found between the phloem bundle cap and the older phloem. Some of the cells in the phloem tissue differentiated into bundles with xylem. Numerous, short, lateral roots were conspicuous on the main root. The most noticeable response of the root tissue to this malady was hyperplasia and hypertrophy of the phloem followed by cellular disorganization, necrosis, and obliteration of cells. Some sieve tubes, companion cells, and parenchyma contained a gumlike deposit. THE EFFECTS of aster yellows virus on the anatomical structure of plants have received less attention than comparable effects on plants infected with the curly top virus. Although their anatomic responses have been reported to be similar, Girolami (1955) and Rasa and Esau (1961) have shown some differences. The aster yellows virus of carrot is associated with phloem tissue, and belongs to the yellows group which produces pathologic effects primarily in the phloem tissue (Esau, 1956). Kunkel (1926) reported the transmission of the aster yellows virus by the leafhopper in more than 50 plant families. Later (1953) he described the external symptoms through the flowering stage for a number of species. The developmental anatomy of the normal edible storage organ of carrot has been described by Esau (1940). A study was made of field-grown plants, stored roots forced in the greenhouse, and plants infected with the 6-spotted leafhopper to determine if the internal symptoms were associated with the phloem tissue, and if crystalline inclusion bodies, as described in some other aster-yellows-infected plants, were present. MATERIALS AND METHODS-In July, 1959, plants showing normal growth and others with aster yellows symptoms were brought from the field and prepared for anatomic studies. In December of the same year, harvested roots of different 1 Received for publication April 19, 1963. Published with the approval of the Director of the Agricultural Experiment Station. lines grown in the field were forced in the greenhouse for pollination studies. After shoot development, some lines displayed the aster yellows virus symptoms and others remained normal. From these lines normal and infected plants were selected for anatomic comparison with field-grown plants. To check further the anatomic responses to the virus, seeds of carrot (Nantes, Ferry-Morse Seed Company) were planted. Approximately 8 weeks later, the plants were exposed for 1 week to 6-spotted leafhoppers, Macrosteles fasifrons (Stal), which had been previously infected with the aster yellows virus. An equal number of plants were kept in the greenhouse as uninoculated controls. Leaves, petioles, and roots were sampled and fixed in formalin-acetic-alcohol. The plant material was dehydrated with n-butyl alcohol and embedded in 56-58 C tissuemat. Sections were cut IOIA in thickness on a rotary microtome and stained with safranin 0 and fast green FCF. RESULTS-Plants of carrot infected with the aster yellows virus were severely stunted, the abundant foliage was small, curled and a lightyellow color. Many short, thick, lateral roots were differentiated on the fleshy roots. Changes in the mesophyll-The normal carrot leaf had 2-3 layers of palisade cells with numerous chloroplasts, round and loosely arranged spongy parenchyma celis, and an upper and lower epidermis consisting of cells of different sizes and shapes. Stomata were common in the lower epidermis; some also were found in the upper epidermis (Fig. 1).