BLACK and HAIRY NIGHTSHADES: TAXONOMY, BIOLOGY, and Distributioli Jodie S
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BLACK AND HAIRY NIGHTSHADES: TAXONOMY, BIOLOGY, AND DISTRIBUTIOli Jodie s. Holt* Introduction The nightshades are a widespread group of economically important agricultural weeds in many parts of the world (1, 7, 10). In North America they are particularly troublesome in irrigated lands of the western United States. In California these weeds are found in many crops, including tomatoes, potatoes, peppers, melons, cotton, onions, garlic, beets, safflower, and grains. It has been estimated that weedy nightshade species occur in more than 73 countries worldwide in at least 37 crops (4). Although several species of the genus Solanum are recognized as nightshade weeds, the abundant literature on this group is inconsistent in species identification, and overlaps of their common names are frequent (7). Misidentifications that occur as a result of the confusion in the published literature may lead to difficulty in understanding how to manage these weeds and may account for the observed variability in herbicide response among nightshades in general (7). The purpose of this talk, therefore, is to clarify the taxonomy of the group known as nightshades, to present an overview of the characteristics and North American distribution of these species, and to describe important features of their biology that may be useful in their control. Taxonomy and Distribution Four of the nightshade species that occur in North America, black and hairy nightshades, are especially weedy and will be considered here. These species are Solanum. americanum (American black nightshade), s. nigrum (black nightshade), s. ptycanthum (eastern black nightshade), arid s. sarrachoides (hairy nightshade) (10). These weeds are all classified in the "Solanum niqrum complex", so named because it is composed of species most closely related to s. niqrum (10). For several reasons this complex is very difficult to work with taxonomically (3). First, the members of the group are all herbaceous broadleaved plants and are very similar in gross morphology. Second, these species exhibit a high degree of phenotypic plasticity, i.e., when grown under different environmental conditions, they vary in many of the characteristics frequently used for taxonomic identification. Even on a single plant certain features may vary with season. Third, some species in this complex possess a high degree of genetic variation and, therefore, many ecotypes within a single species. Because of these biological factors that complicate identification, historically there has been much nomenclature confusion wi~hin this group in the literature (3, 11). •Assistant Professor, Department of Botany and ~lant Sciences, University of California, Riverside. 209 The Solanaceae, or nightshade family, is a large family of vascular plants with approximately 90 genera and over 2000 species (2, 3, 10). Most members of this family are of Central and South American origin. The largest genus in this family, Solanum, is also one of the largest genera in the .plant kingdom, and is divided into 7 subgenera and 50 sections (10, 11). Solanum section Solanum is one of the sections in this genus, known also as the ~olanum niqrum complex. This section contains 30 species total, 11 of which occur in North America. Representatives of this section are found on all continents in tropical and temperate environments, from sea level to 3000 meters in elevation. All are annual or short-lived perennial herbaceous species. In general, species are frequently defined on the basis of genetic isolating mechanisms, (i.e., whether or not two individuals will hybridize determines whether or not they are consldered the same species). For the species of the Solanum nigrum complex, however, crossing experiments demonstrated that many intraspecific (within a species) as well as interspecific (between species) hybrids were sterile (12); thus, this method cannot be used conclusively to separate them. The fact that most species of this section are autogamous, or self-pollinating, suggests that little gene flow or mixing occurs between distinct populations in the field; crossing experiments may not be relevant to separation of these species, therefore. Of the 11 species of Solanum section Solanum that occur in North America, 6 are diploid and 5 are polyploid (11). Among the diploid species, American black nightshade (S. americanum) and eastern black nightshade (S. ptycanthum) are native to North America while hairy nightshade (S. sarrachoides) was introduced from South America. Among the polyploids, black nightshade (.§.... niqrum) is the weedy species, while two of the others are sometimes cultivated. The most recent taxonomic surveys of these species have concluded that s. americanum will replace the name~. nodiflorum,and thats. ptycanthum will replaces. pericanum in the northeastern United States (11). Since s. ptycanthum is not found in California, it will not be considered in detail here. American black .nightshade is common in the southeastern and western us near the coasts and is abundant in California. It possesses many morphological types and is most similar in appearance to eastern black nightshade. Plants of this species are annuals or short-lived perennials. Black nightshade is considered to be one of the world's worst weeds (4), but in the US is locally common only in the far west (10). In California, black and American black nightshade occur together, but black nightshade is distinguishable by its larger seeds and pollen. It is also an annual or short-lived perennial plant. Hairy nightshade is widespread in North America, including Canada (1). This annual species is unique in many characteristics and relatively uniform morphologically throughout its range, making it easily distinguishable from the black nightshades. 210 Althouqh the three weedy niqhtshades in California, American black nightshade, black niqhtshade, and hairy nightshade, exhibit much variability, their flower and fruit characteristics are somewhat consistent and may be used to distinquish them. Those characteristics are listed in the following table, summarized from Rogers and Ogg (10), which uses easily recognizable field characteristics as a basis for identification. CHARACTERISTICS OF SPECIES IN THE FIELD BERRIES American black Black or sometimes brownish black; o to 4 niqhtshade sclerotic granules per berry; white flecks on immature berries. Black nightshade Black, rarely yellow green; no sclerotic granules; no white flecks. Hairy nightshade Brownish to olive green; calyx expanded 1/4 to 1/2 over berry; 2 or 3 sclerotic granules per berry; no white flecks. SEEDS American black Small, 1.4 to 1.6 mm long, 50 to 110 per berry; nightshade usually yellow or nearly white. Black nightshade Large, 1.8 to 2.2 mm long, 15 to 60 per berry; usually yellow or nearly white. Hairy nightshade Large, 1.8 to 2.3 mm long, 10 to 35 per berry; tan. FLOWERS American black White, rarely tinted purple with greenish niqhtshade yellow star; stellate. Black nightshade White with qreenish yellow star; stellate. Hairy nightshade White with greenish yellow star with purple flecks; rotate. PUBESCENCE American black Not obviously hairy. niqhtshade Black nightshade Not obviously hairy. Hairy nightshade Usually obviously hairy, especially young leaves; frequently sticky. POLLEN American black 15 to 25 um in diameter. nightshade Black nightshade 26 to 35 um in diameter. Hairy nightshade 24 to 27 um in diameter. 211 Biology There are several notable features of the nightshades that will be addressed in other papers of this session, therefore only mentioned briefly here. First, these species are frequently associated with a broad spectrum of nematodes and phytopathic microorganisms, serving as alternate hosts and, thus, disease vectors (10). Nightshades are also poisonous to humans and livestock, due to steroidal alkaloids in their tissues. Nontheless, in many areas of the world, they serve as a minor food source, as an herb, and as an ingredient in dyes (2, 3, 10). Seed physiology and ecology are important areas of nightshade biology with implications for their management. Much research has been conducted on the germination behavior of black nightshade (Solanum nigrum) and hairy nightshade (S. sarrachoide~) (5, 6, 8, 9). In experiments conducted in the us (5, 6) and in England (8, 9), both species exhibited strong seasonality of germination and emergence. From field plantings of black nightshade by Keeley and Thullen (5), seeds emerged from March through October; however, plants from the May through July plantings were significantly larger and produced many more berries and seeds per plant than those from earlier or later plantings. Plants from March through June plantings flowered in 7 to 9 weeks, while those from July through September plantings flowered in 5 to 6 weeks (5). While the ~onth of earliest seedling emergence varies with soil temperature, similar seasonal patterns of emergence were found for hairy and black nightshade in Washington (6) and in England (8, 9). Fresh seeds of both species were dormant but would germinate after a period of storage or burial (8, 9). During the months of maximal germination (approximately April through June), the temperature range of germination was widest. As the season progressed, dormancy was induced in ungerminated seeds and the range of temperatures over which germination would occur narrowed (8). Thus, the seasonal emergence patterns of black and hairy nightshade are associated with cyclic changes in the dormancy status of buried seeds. In experiments to