Soil salinity and irrigated agriculture have coexisted since ancient times, and ever since the problem of salinity in agriculture has become a challenge. Soils are generally classified as saline when the electrical conductivity of the saturated paste extract (ECe) is 4 dS m-1 or more (which is equivalent to 40 mM NaCl) and generate an osmotic pressure of approximately -0.2 MPa. Based on this, plants differ greatly in their growth response to saline conditions and therefore classified as "glycophytes" or "halophytes" referring to their capacity to grow on highly saline environments (Munns and Tester 2008). Halophytes are remarkable plants which have the ability to complete their life cycle in a substrate rich in NaCl that normally found toxic to other species and destroy almost 99% of their population (Flowers and Colmer 2008). These are highly evolved and specialized organisms with well-adapted morphological, anatomical, and physiological characteristics allowing them to proliferate in the soils possessing high salt concentrations (Flowers et al. 1977; Flowers and Colmer 2008). Moreover, some halophytes consistently require a particular concentration of NaCl in the growth medium are referred as "obligate halophytes" or "true mangroves" and, apart from their growth in highly saline environment, some halophytes have capacity to grow on the soil devoid of salt are called as "facultative halophytes" or "mangrove associates." This presence or absence of substrate in the form of salt offers advantages for the halo-phytes in the competition with salt-sensitive plants (glycophytes) for the management of abiotic stress tolerance and utilization of these species for the improvement of crop yield.

In this regard, it is essential to understand the adverse effects of abiotic stresses and tolerance mechanisms developed by the halophytes and exploit such knowledge for the improvement of crop plants which can meet the demand of food, feed, fodder, and industrial raw material. The standard approach to this problem would be to increase the tolerance capacity of conventional crop plants, which otherwise are high yielders. An alternative strategy is to make use of halo-phytes that already have the required level of stress tolerance and are still productive at high external adverse conditions. Salinity is one of the major abiotic constraints, affecting almost every aspect of plant's physiology at both whole plant and cellular level through osmotic stress in an earlier phase and ionic stress at a later stage of

Table 2.1 List of halophytes used for saline agriculture in Pakistan and other countries (modified from Khan and Qaiser 2006)

Uses Plant species

Food A. hortensis, Aizoon canariense, Apium graveolens, Arundo donax, Atriplex halimus,

Avicennia marina, Cocos nucĂ­fera, Cynamorium coccinium, Echinochloa crusgalli, Glinus lotoides, Glossonema varians, H. stocksii, Haloxylon griffithii ssp griffithii, N. schoberi, Neurada procumbens, Nitraria retusa, Ochradenus baccatus, Oxystelma esculentum, P. sylvestris, Pedalium murex, Pentatropis nivalis, Pheonix dactylifera, Pisonia grandis, Polypogon monspeliensis, Portulaca oleracea, Rumex vesicarius, S. brachiata, S. persica, Salicornia bigellovi, Salvadora oleoides, Sesuvium portulacastrum, Solanum incanum, Suaeda fruticosa, Triglochin maritime, Zizyphus nummularia, Zygophyllum simplex Fodder A. griffithii, A. halimus, A. leucoclada, A. tatarica, Aegiceras corniculatus, Alhaji maurorum,

Anagallis arvensis, Artemisia scoparia, Arthrocnemum indicum, Atriplex canescens, Avicennia marina, B. glaucus, Beta vulgaris ssp maritma, Bienertia cycloptera, Bolboschoenus affinis, Caesalpinea bonduc, Camphorosma monspelictum, Carex divisa, Chloris virgata, Cressa cretica, Dalbergia sissoo, Glinus lotoides, Halocnemum strobila-ceum, Haloxylon stocksii, Lobularia maritime, Lolium multiflorum, Neurada procumbens, Orthochloa compressa, P. farcta, P. juliflora, Populus euphratica, Prosopis cineraria, Raphanus raphanistrum, Rhizophora mucronata, Salsola tragus, Seidlitzia florida, Seriphidium quettense, Suaeda fruticosa, T. repens, T. triquetra, Trianthema portulacastrum, Trifolium fragiferum, Vicia sativa, Zaleya pentandara, Zygophyllum simplex Forage A. littorali, A. macrostachys, Aeluropus lagopoide, Agrostis stolonifera, Aristida adscesho-

ines, Aristida mutabilis, Atriplex dimorphostegia, C. ciliaris, C. pennesittiformis, Cenchrus biflorus, Chloris gayana, Cynodon dactylon, D. aristatum, D. scindicum, Dactyloctenium aegyptium, Desmostachya bipinnata, Dichantheum annulatum, Diplachne fusca, E. crusgalli, E. japonica, E. superba, Echinochloa colona, Eleusine indica, Eragrostis curvula, Festuca rubra, Halocharis hispida, Halopyrum mucronatum, Haloxylon persicum, Lasiurus scindicus, Nitraria retusa.Oligomeris linifolia, P. minor, P. pratensis, Paspalumpasploides, Phalaris arundinacea, Poa bulbosa, S. helvolus, S. ioclados, S. kentrophyllus, S. tourneuxii, S. tremulus, S. virginicus, Sacchraum bengalense, Salvadora persica, Sporobolus coroman-delianus, Urochondra setulosa Ornamental Achillea millefolium, Alhaji maurorum, Ammi visnaga, Artemisia scoparia, Avicennia marina, Caesalpinea bonduc, Calotropis procera, Camphorosma monspelictum, Cassia italic, Centella asiatica, Ceriops tagal, Chenopodium ambrosoides, Corchorus depressus, Cressa cretica, Cynamorium coccinium, Erythrina herbacea, Evolvulus alsinoides, Glinus lotoides, Halogeton glomeratus, Imperata cylindrical, Inula brittanica, Ipomoea alba, L. gilsei, L. sinuatum, L. stocksii, Leptadenia pyrotechnica, Limonium axillare, Melhania denhamii, Microcephala lamellate, Neurada procumbens, olanum surrattense, Oligomeris linifolia, Oxystelma esculentum, P. oleracea, Pedalium murex, Pentatropis nivalis, Populus euphratica, Portulaca quadrifida, Psylliostachys spicata, Rumex vesicarius, S. quettense, Seriphidium brevifolium, Solanum incanum, Sonneratia caseolaris, Thespesia populneoides, Trianthema portulacastrum, Tribulus terrestris, Urginea indica,Verbena officinalis, Withania sominifera, Z. simplex, Zaleya pentandara, Zygophyllum propinquum Chemicals Aeluropus lagopoides, Ardisia solanacea, Calotropisprocera, Cenchrus ciliaris,

Clerodendrum inerme, Dalbergia sissoo, Euphorbia thymifolia, Ficus microcarpa, Halocnemum strobilaceum, Ipomoea pes-caprae, K.iranica, Knorringia sibirica subsp. Kochia indica, Mesembryanthemum crystallinum, N. schoberi, Nitraria retusa, Phyla nodiflora, Polypogon monspeliensis, Raphanus raphanistrum, S. taccada, Scaevola plumier, Sessuvium sessuvioides, T. passernioides, T. ramosissima, T. szovitsiana, T. tetragyna, Tamarix mascatensis, Thomsonii, Trianthema portulacastrum plant growth (Munns and Tester 2008) and leads to a series of morphological, physiological, biochemical, and molecular changes. In the past 2-3 decades, considerable progress has been made in the evaluation of halophytes to understand their survival mechanisms to be used as crop plants. In the present article, we document different aspects of halophytes, with an emphasis on mechanism of tolerance to salinity, drought and heavy metal tolerance, and their exploitation to manage the problems associated with the abiotic stresses as well as for environmental protection.

Halophytes respond to salt stress at cellular, tissue, and the whole plant level (Epstein 1980). In response to salt stress, the general physiology of halophytes has been reviewed occasionally (Flowers et al. 1977; Epstein 1980; Flowers 1985, 2004) and since then other reviews have examined their eco-physiology (Ball 1988; Rozema 1991; Breckel 2002), photosynthesis (Lovelock and Ball 2002), response to oxidative stress (Jitesh et al. 2006), and flooding tolerance (Colmer and Flowers 2008) . Therefore, studies on the halo-phytes can be instructive from three prospects: first, the mechanism by which halophytes survive and maintain productivity under abiotic constraints can be used to define a minimal set of adaptations required in tolerant germplasm. This knowledge can help to focus the efforts of plant breeders and molecular biologists working with conventional crop plants (Glenn and Brown 1999). Second, halophytes grown in an agronomic setting can be used to evaluate the overall feasibility of high-salinity agriculture, which depends on more than finding a source of tolerant germplasm (Glenn et al. 1997). Third, halophytes may become a potential source of new crops.

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