QUANTITATIVE OBSERVATIONS ON THE PATTERN OF SYNERGID DEGENERATION IN BARLEY

Serial sections of 75 ovules taken from two pollinations were examined in order to determine the pattern of early synergid degeneration in Hordeum vulgare(Bonus). It was found that synergid degeneration occurs in various patterns in ovules from successful pollinations: either both synergids degenerate, or only one degenerates; in those ovules where only one synergid degenerates, the degenerate synergid is nearly always located either proximal or distal to the placental attachment of the ovule. The results suggest that 1) synergid degeneration depends upon successful pollination, 2) degeneration of both synergids is equally as likely to occur as degeneration of only one synergid, and 3) in those cases where only one synergid degenerates, there is a greater tendency for the degenerate synergid to be located toward the placenta. DEGENERATION of one or both synergids near the time of fertilization in angiosperms has been well documented in the early literature (Maheshwari,1950). Modern investigations of this phenomenon have shown that the typical case involves the degeneration of one synergid initially and that it is this synergid which consistently contains the pollen tube tip and its discharge (Pluijm, 1964; Van Went and Linskens, 1967; Diboll, 1968; Jensen and Fisher, 1968; Schulz and Jensen, 1968; Cocucci and Jensen, 1969; Cass and Jensen, 1970, Van Went,1970a, b; Mogensen, 1972,1978, 1982; Maze and Lin, 1975; Mogensen and Suthar, 1979; Wilms, 1981). A common question in the minds of angiosperm embryologists has been: "is it always the same synergid that degenerates, or does degeneration occur at random between the two synergids?" To my knowledge, no published account has addressed this question in a quantitative way, probably because in most plants it is technically difficult to distinguish one synergid from the other in an oval-shaped ovule. In barley, however, a natural marker is present which allows the positional recognition of the two synergids, thus making it relatively easy to score synergid degeneration. The present investigal Received for publication 13 June 1984; accepted 16 August 1984. This work was supported by Grant No. 83-CRCR-11270 from the United States Department of Agriculture. Partial support from the Organized Research Fund, Northern Arizona University and the Carlsberg Laboratory, Copenhagen, Denmark is gratefully acknowledged. Special appreciation for helpful suggestions on the manuscript is extended to Drs. David L. Mulcahy, Dieter von Wettstein J. Heslop-Harrison and Robert A. Finch. tion, therefore, was undertaken to obtain quantitative data on the positional pattern of synergid degeneration in barley and to provide information that might aid future studies on the question of synergid dimorphism in this plant. MATERIALS AND METHODS-Plants (Hordeum vulgare, cv. Bonus) for this study were the same as those used for a previous study on double fertilization in barley (Mogensen, 1982). Spikes were emasculated and bagged two days before hand-pollinations were performed. Two pollinations were carried out. In both crosses, the same female plants were used, but in each case an unknown, but not necessarily different, male was used as the pollinator. The positional orientation of the synergids in barley (and probably all grasses) is such that when the ovary is sectioned sagitally (parallel to the long axis and through the shortest distance front and back), the two synergids are observed side by side. One synergid can then be referenced according to its location either toward (proximal) or away from (distal) the placental attachment of the ovule to the carpel wall. Also, the extent of the outer integument on either side of the ovule can be used as a reference point; i.e., on the side away from the placenta, the outer integument does not extend as close to the micropyle (formed by the inner integument) as it does on the side of the ovule toward the placenta (Fig. 1). By trimming the ovule before fixation in such a way as to retain this orientation, it is possible to consistently section the ovule in the desired plane. Ovules, along with some carpellary tissue, were fixed in glutaraldehyde and osmium tetroxide and processed according to standard techniques for