The Flowering Plants of Hawaii
Part 10 Caricaceae - Caryophyllaceae
Caricaceae are a small family of mostly tropical species, four genera with about 55 species according to the authors of the Manual, six genera with 34 species according to Mabberley (p. 152). The only genus in the family of interest to us here is Carica with C. papaya the subject of this section. Historically the genus Carica was thought to consist of as many as 50 species. Recent DNA-based research, however, has reduced the number to one, C. papaya being the sole survivor (Aradhya et al, 1999). Former members have found new homes in the Mexican genus Jarilla (3 species), and the two tropical American genera, Jacaratia (7 species), and Vasconcellea (20 species). These three genera are not represented in the Hawaiian Islands.
The papaya is known in Hawaiian as mïkana, hë`ï, milikana, or papaia (the last one is the Hawaiian spelling of papaya) and familiarly throughout the tropics as the pawpaw. The ripe flesh of the papaya has a soft texture and a rich flavor that can be enjoyed fresh (see image) or in preserves, the tangy seeds are used to flavor salad dressings, and all parts of the plant can be used as a source of papain, a proteolytic enzyme used as a meat tenderizer, and also to remove protein haze in the production of beer. All in all, a very useful plant indeed.
Papaya is an important element of commerce in many tropical areas including the Hawaiian Islands. The area around Kopoho, near the easternmost point on the Big Island is described as producing the islands’ sweetest papayas. These plantations (see image) can be seen by driving south east From Hilo to Pähoa on Highway 130 and then east on Highway 132. This is also an important access to beaches on the eastern coast of the island.
Visitors fortunate to be able to stay for a while on Moloka`i have an opportunity to visit Kumu Farms, a commercial garden a short distance west of the island’s airport. The farm is aptly named as the word kumu in Hawaiian has among its several meanings, base or foundation, source or origin, and teacher (Pukui and Elbert, p. 182). Fresh produce is on sale at the farm from Monday to Friday (subject to change), and an informal tour of the facilities is available, at no charge, on Thursday mornings. One of the featured crops at the farm is papaya, where one learns that the plant grows from seedling (see image) to fruit-bearing (see images) in 18 months. An interesting challenge for papaya growers is learning how to determine the sex of new plants. This is extremely important as the fruit produced by female plants is considered of poorer commercial quality than fruit from hermaphrodites, so it is normal practice to maintain only the latter. Males, of course bear no fruit. On a visit to Kumu Farms we were shown how to make the distinction between female and hermaphrodite by examining their flowers (see image); clearly, it is learned skill. Interested readers can visit the farm via their web site: www.KumuFarms.com.
A serious problem for papaya growers surfaced in the early 1990s with the appearance of papaya ring spot virus (PRSV) as a major threat to the crop. Although the virus had been known on the islands for many years, it had been kept under control. For reasons that are not relevant to the present account, the virus did not manifest itself fully until the early 1990s. By the late 1990s PRSV had decimated the crop—reckoned to be the fifth largest agricultural crop in the islands—resulting in a 40% drop in production. Fortunately for the industry, a transgenic line of PRSV-resistant papaya was under development by workers at Cornell University and the University of Hawai`i (Mänoa). Using a DNA segment from a weakened virus mutant, these workers produced a papaya that was resistant to the natural virus. Background for that work has been described in a report, published by the American Phytopathological Society, written by Dennis Gonsalves and colleagues (2004). A follow-up report written by Carol Gonsalves and coworkers (2004) described the reaction of papaya farmers to the availability of the PRSV-resistant fruit. A more technical description of the work appeared as Ferreira et al. (200). Additional information on various aspects of papaya biology, with specific focus on agricultural applications in Australia.
The four main varieties of papaya cultivated in the islands are Kapoho-Solo; Kamiya or Laie Gold; Sunrise/SunUp; and Rainbow. Kapoho-Solo is a long-standing workhorse of the industry characterized by high sugar content. This variety has not been genetically modified and continues to provide fruit for markets where transgenic organisms are illegal or unacceptable to the public. Kamiya or Laie Gold is a modified variety that is grown on O`ahu for local markets. Sunrise/SunUp, also a transgenic variety, is known as the “strawberry” papaya because of its red flesh. Rainbow, the major variety of commerce, is grown on the Big Island and on O`ahu and Kaua`i. Rainbow is the F-1 (first generation) hybrid between Kapoho-Solo and the genetically modified red-flesh SunUp.
Despite the obvious commercial success of Rainbow, there continues to be staunch resistance to the production of genetically modified food crops in the islands, including papaya. The list of concerns voiced by these groups include contamination of non-transgenic papaya by pollination with pollen from Rainbow or other genetically modified varieties. There has also been concern voiced over the possible allergenic problems that might arise from the protein component transferred from the mutant pathogen to papaya. As in the case of all genetically altered organisms, there is the fear that we do not know what, if any, impact genetically altered organisms will have on the ecosystem. These concerns, and others, have been presented in some detail in a 2006 position paper by M. Bondera and M. Query entitled, “Hawaiian Papaya: GMO Contaminated.”
It is not just papaya that has roused the ire of anti-GMO groups in the islands; these groups are opposed to the development and use of any genetically modified organisms. As the reader will see later in the discussion of taro in the chapter on Araceae (the arums), this issue came to a rather violent boil as a result of workers at the University of Hawai`i (Mänoa) developing transgenic lines of taro. This dispute was doubly heated owing to the fundamental importance of taro within the Hawaiian creation story.
Caryophyllaceae are a moderately large family with 2,000 species in 75 genera (Manual), or about 2,630 species arrayed in 85 genera (Mabberley, p. 156). The family enjoys a cosmopolitan distribution and includes several well known cultivated species, e.g., Dianthus caryophyllus, the popular sweet smelling carnation; Gypsophila paniculata, baby’s breath; and Silene coronaria, the rose campion. The common lawn weed Stellaria, chickweed, also belongs to this family.
In the original treatment in the Manual the family on the islands was thought to consist of two endemic genera, Alsinidendron, with four species, three of which are rare and/or endangered, while the fourth is thought to be extinct; and Schiedea, with at least 22, three of which are thought to be extinct. About half of the extant species are marked as rare and/or endangered. The most recent monographic treatment of Schiedea, however, places all species in the latter genus, with the islands’ species comprising subfamily Alsinidendroideae within Caryophyllaceae (Wagner et al. 2005). The example from Alsinidendron shown here is A. lychnoides (see image), known as kuawäwaenohu in Hawaiian. This rare species only occurs on the western edge of the Alaka`i Swamp, on Kaua`i. Two species of Schiedea are included to provide some idea of the range of morphology of the genus. Schiedea globosa (see image) is a coastal species known from O`ahu, Maui, Moloka`i, and the Big Island. [Authors of the Manual described this species as occurring on O`ahu, Maui, and Moloka`i; the more recent work by Wallace et al. (2009) report it from the northeastern coast of the Big Island as well.] Lisa Wallace, of Mississippi State University, and coworkers describe S. globosa as the most widespread member of the genus and one that serves well as a model for the study of evolution within the group. Their study of DNA sequence analysis of samples from 10 populations representing the known range of the species revealed that the species originated on O`ahu with subsequent migration to progressively younger islands: Moloka`i, Maui, and the Big Island. Small differences in some morphological features (Wagner et al. 2005) further suggest that divergence is continuing to occur within S. globosa on the islands. Schiedea kealiae (see image) occurs only on slopes in the northern Wai`anae Mountains, O`ahu.
Authors of the Manual suggest that Alsinidendron and Schiedea are likely derived from a common ancestor in the genus Arenaria (or closely related group). Arenaria is a mostly Northern Hemisphere genus of about 210 species with 54 European and nine North American species. An interesting side note is that one species, A. bryophylla, occurs in the Himalayas at an elevation of 6,180 m (20,276’) (Mabberley, p. 64) thought to be the highest elevation local for any flowering plant species.
Silene, the largest genus in the family with about 700 primarily Northern Hemisphere species, is represented in the Hawaiian flora by eight species seven of which are endemics. Of the endemic species two are extinct, four are rare and endangered, and only one, S. struthioloides (see image), appears to be in no immediate danger. Silene hawaiiensis (see image) occurs only on ash and decomposed lava on the Big Island.
The relationships among the island species of Silene have been the subject of speculation over the past few years. Authors of the Manual suggest that the present group of species is the result of two independent colonizations, one that resulted in the S. struthioloides-S. hawaiiensis pair, which was thought to be related as progenitor-daughter species. This hypothesis was tested by two Swedish botanists, Anna Westerbergh and Anssi Saura (1994), who demonstrated that levels of genetic variation within populations of S. hawaiiensis on the younger islands are extremely low and that there is very little morphological variation among them as well. However, populations of S. struthioloides on Haleakalä and Mauna Kea exhibit somewhat higher levels of both genetic and morphological diversification. These observations are typical of progenitor:daughter species pairs where the daughter species carries only a limited sample of the variation exhibited by the ancestor. This can be readily appreciated when one considers that colonization is a chancy business and that very few seeds—perhaps only one—might be successful in becoming established in the new habitat. As has been noted elsewhere, the source of seeds from which the Haleakalä populations would have arisen could have been either West Maui or eastern Moloka`i. Suitable sites—that is to say, appropriately young lava—are no longer available on either of those mountainous areas.
The related questions of colonization of the islands and the most likely ancestor of Silene were addressed by F. Eggens and coworkers (2007). Using several genes representing both the chloroplast and nuclear genomes these workers demonstrated that the island species are the result of a single colonization followed by differentiation. The data suggested that the widespread North American weedy Silene antirrhina most closely resembles the ancestral form. A single long distance dispersal event seems the most likely means by which colonization occurred.
January 9, 2012