Plant and Soil 192: 277±283, 1997. 277
c 1997 Kluwer Academic Publishers. Printed in the Netherlands.
Purslane (Portulaca oleracea L.): A halophytic crop for drainage water reuse systems
C.M. Grieve and D.L. Suarez
USDA-ARS, U. S. Salinity Laboratory, Riverside, CA 92507-4617, USA
Received 3 July 1996. Accepted in revised form 18 March 1997
Key words: leaf areas, salinity, sand cultures, selenium, water use
Abstract
Drainage water reuse systems have been proposed for the west side of the San Joaquin Valley of California in order to reduce the volumes of water requiring disposal. Implementation of this system requires development of a cropping system with successively higher salt tolerance. A major limitation is the need to identify alternate species that will be suitable as the ®nal, most salt tolerant crop in the series. These crops must be productive when irrigated with waters that are typically high in sulfate salinity and may be contaminated with potentially toxic trace elements. This study was initiated to evaluate the interactive effects of sulfate salinity and selenium on biomass production and mineral content of purslane (Portulaca oleracea). Plants were grown in greenhouse sand cultures and irrigated
four times daily. Treatments consisted of three salinity levels with electrical conductivities (ECi) of 2.1, 15.2, and
28.5 dS m 1, and two selenium levels, 0 and 2.3 mg L 1. In the initial harvests, shoot dry matter was reduced by
15 to 30% at 15.2 dS m 1 andby80to90%at28.5dSm 1. Regrowth after clipping above the ®rst node was vigorous and biomass from plants irrigated with 15.2 dS m 1 water was nearly double that from the 2 dS m 1 treatment. Purslane appears to be an excellent candidate for inclusion in saline drainage water reuse systems. It is (i) highly tolerant of both chloride- and sulfate-dominated salinities, (ii) a moderate selenium accumulator in the sulfate-system, and (iii) a valuable, nutritive vegetable crop for human consumption and for livestock forage.
Introduction has resulted in elevated groundwater levels, soil salin- ization, and the construction of evaporation basins to Drainage water reuse has been promoted as an dispose of drainage water. Presser et al. (1990) esti- environmentally-sound method for the disposal of mated that there were 2704 ha of evaporation ponds saline drainage waters, particularly in the San Joaquin and that expansion to 13360 ha would be required by Valley of California (Rhoades, 1989). In the San the year 2000. Expansion of evaporation ponds has not Joaquin Valley, construction of a master drain to occurred due to environmental concerns and regulato- the Sacramento-San Joaquin Delta (discharging to the ry restraints, thus the need to reduce drainage volumes Paci®c Ocean) was halted in 1975 due to funding lim- remains acute. itations and concerns over the environmental impact The multiple water use approach is advocated to of disposing of this water into the delta (Ohlendorf conserve water, to reduce drainage water volume, and Santolo, 1994). Kesterson Reservoir, as the ter- and to minimize the use of otherwise arable land minal point of the existing drainage system, received for the construction of temporary disposal facilities, drainage water until 1986,when all drainage was halted such as evaporation ponds. In this system, selected after embryo deformities in aquatic birds were related crops would be grown and irrigated in sequence start- to elevated selenium concentrations in the Reservoir ing with very salt sensitive species (melons, onions, (Ohlendorf and Santolo, 1994). Insuf®cient drainage almonds) and continuing with increasingly salt toler- ant crops (e.g. sweet corn, wheat, cotton, sugar beet).
FAX No:19093424963. E-mail:[email protected] The drainage water becomes progressively more saline
JS: PIPS No. 137449 BIO2KAP
*137449* plso6398.tex; 1/10/1997; 19:44; v.7; p.1 278 as each successive crop is irrigated. As conceptually for livestock. It is widely used as a vegetable in var- proposed, the sequence ends with irrigation of halo- ious Mediterranean and Central American countries phytic species. In addition to the requirement of high and in the Philippines. Many of the pharmacological potential for sustainability when irrigated with highly properties attributed to purslane in folk medicine may, saline drainage waters, there is a requirement that the in fact, be based on the ef®cacy of some of its con- species thrive in the presence of potentially toxic ions stituents which have now been identi®ed by modern such as selenium. Desirable traits for prospective halo- phytochemical methods. The shoot is a rich source
phytic species for the reuse system are (i) tolerance to of ! -3 fatty acids (Kumamoto et al., 1990; Omara-
highly saline, sodium- and sulfate- dominated drainage Alwala et al., 1991; Simopoulos and Salem,1986), -
water, (ii) accumulation of signi®cant amounts of Se in tocopherols, ascorbic acid, -carotene, and glutathione its tissues so as to limit Se accumulation in the drainage (Simopoulos et al., 1992). The seeds contain 21% pro- water, and (iii) production of a useful, economically- tein and 20% oil of which the major constituents are valuable crop. Several candidates have been proposed linoleic (46%) and linolenic (31%) acids (R Kleiman, to ®ll this niche in the drainage water reuse system, personal communication). e.g. Atriplex (Watson and O'Leary, 1993); Medicago, The unique nutritive qualities of purslane Trifolium, Leymus, Puccinellia (Parker et al., 1991); (Simopoulos et al., 1992) as well as its outstanding Festuca (Wu and Huang, 1991). tolerance to chloride salinity (Kumamoto et al., 1990) Plant response to salinity depends not only on the make this species a promising halophytic candidate osmotic potential of the substrate but also on the kinds for the drainage water reuse project. Consequently, we of ions that contribute to salinity. Furthermore, even evaluated the interactive effects of sulfate-dominated at isosmotic potentials, plant species differ in their salinity and elevated Se in the irrigation water on response to the composition of the salinizing medi- biomass production and mineral content in purslane um. Contrasting responses have been reported for grown in greenhouse sand cultures. two halophytic chenopods. Chenopodium rubrum L. grew better with chloride salinity than with the sulfate system (Warne et al., 1990), while Kochia scoparia Materials and methods (L.) Schrad., performed better with sulfate-dominated salinity (Curtin et al., 1993). At moderate salinities, Purslane seeds, obtained from Valley Seed Service1, sulfate-salinity was generally less growth-limiting to Fresno, CA in 1988, were increased at the US Salinity cereal crops than is chloride salinity (Boursier and Laboratory, Riverside, CA in 1988 and 1990. Seeds Lauchli,È 1990; Curtin et al., 1993; Khan et al., 1995). were planted in trays ®lled with vermiculite on 21 Dec In other species, growth in the chloride system led to 1994. Five-week old seedlings were transplanted into nutrient imbalances or chloride toxicity with subse- 18 sand tanks in a greenhouse. The tanks (1.2 by 0.6 by quent loss of yield (Mor and Manchanda, 1992). 0.5 m deep) contained washed sand having an average
Purslane (Portulaca oleracea L.), a common plant bulk density of 1.2 Mg m 3. At saturation, the sand had found throughout the United States, is a vigorous col- an average volumetric water content of 0.34 m3 m 3. onizer of disturbed, waste habitats. From the yield- Each tank contained ®ve rows with 10 seedlings per
response model of Maas and Hoffman (1977), purslane row. Plants were irrigated four times daily with a nutri- +
has been rated as salt tolerant with a threshold val- ent solution consisting of (in mol m 3): 5.0 Ca2 ,1.25