ANAEROBIOSIS AS A STIMULUS TO GERMINATION IN TWO VERNAL POOL GRASSES

Tuctoria greenei and Orcuttia californica are two aquatic annual grasses in the small tribe Orcuttieae. They are endemic to seasonal pool habitats in Califomia and Baja Califomia. In years of poor rainfall their seed banks remain dormant until a season of rainfall which is adequate to fill the pool basins. This study attempts to elucidate the factors responsible for cuing germination. Tuctoria greenei germination is almost entirely dependent upon a combination of anaerobic conditions and light. It is suggested that in nature, such cues would be relatively specific to the uppermost soil substrate overlain by water. Orcuttia californica germination exhibits far less dependence upon anaerobic conditions. There is some indirect evidence that fungi may play an important role in stimulating germination of this species. CALIFORNIA VERNAL POOLS are a unique ecosystem with many endemic plant species. Such pools are widely distributed throughout the state on relatively level sites underlain by an impervious hardpan (Holland and Jain, 1977). The mediterranean climate leads to winter and spring filling of the pools followed by summer dry-down. Due to urban expansion and agricultural development, much of the vernal pool habitat has been destroyed and it is estimated that only 10% of the pools are extant (Holland, 1978). This is a matter of some concern in that a number ofthe species occurring in these pools are considered to be rare and endangered. The majority of vernal pool species are annuals, and a sizable proportion of the species are endemic to California (including Baja California) vernal pools (Holland and Jain, 1977). A remarkable example of this narrow endemism is in the grass tribe Orcuttieae. There are nine species included in the genera Neostapfia, Tuctoria, and Orcuttia, and all are endemic to pools in this region (Reeder, 1982). The species in this tribe are annuals which germinate under water in the spring and grow submerged for weeks. During this period, the five species of Orcuttia produce a set of leaves that float on the pool surface, but such leaves are absent in the monotypic Neostapfia and the three species of Tuctoria. After the pools have dried down, all of these grasses initiate a new set of foliage I Received for publication 1 June 1987; revision accepted 13 October 1987. This research was supported in part by National Science Foundation grants RII-8304046 and BSR-8206884, and a John Simon Guggenheim Memorial Fellowship. I thank Cheryl Swift and Paul Zedler for helpful comments on the manuscript. which lasts for one to two months until flowering and fruiting are completed. Due to the unpredictable California climate, pool size varies annually. In years of very depauperate rain, pools may not fill at all. In such years, germination may be limited or in extreme years the entire seed bank may remain dormant. Griggs (1980) described one population of Orcuttia which was abundant one year, depauperate the next year and then completely absent for the following three years before it finally returned in abundance in the sixth year. Such demographic patterns suggest that the seeds of these grasses possess dormancy mechanisms capable of distinguishing years with adequate precipitation from years which are unsuitable for growth. Griggs (1976) noted that germination of several Orcuttia species could be induced by burial in "water saturated mud" for 30 days at low temperature. He suggested that these conditions provided an anaerobic environment and that such a germination response would have selective value as a means of timing germination to years in which the pools had filled sufficiently to insure survival and reproductive success. Additionally, Griggs (1980) found that for species of Tuctoria and Orcuttia, "naked" seeds (i.e., seeds removed from the infructescence) would not germinate regardless of treatment. However, seeds which remained attached to the stalks could be induced to germinate if the infructescences were submerged and incubated for a month, during which time they became covered with a dark mycelium belonging to species of Alternaria and Curvularia. It was suggested that this germination requirement would also be adaptive as a means of cuing germination to periods