Introduction

Printer-friendly version

Environmental salt is a legacy of global geology and vascular plants have responded during their evolutionary history by producing tolerance mechanisms across a wide range of taxa. Convergent evolution of life forms such as mangroves and dryland halophytes has resulted in devices for salt exclusion by roots, vascular compartmentation of tissue solutes and leaf excretion of excess salt that rely on similar mechanisms across widely separated taxonomic groups. Mechanisms for exclusion and compartmentation are also present in non-halophytic crop plants, although less highly developed, and these cultivated species together with their wild relatives show considerable genetic variation in salt tolerance. Such variation can be exploited for genetic improvement of commercially significant species or cultivars, and is especially effective when based on known and heritable mechanisms of salt tolerance. Development of salt-excluding rootstocks in horticulture is a good example.

Geologic time scales of marine and terrestrial salinisation permitted a parallel evolution of salt-tolerant variants of both unicellular and more complex vascular plants. Humankind has accelerated salinisation, especially over the past century, so that development of salt-tolerant plants now becomes a matter of urgency for both food production and land reclamation based on revegetation. An understanding of species variation in salt tolerance aids both development and field implementation.

Extensive land clearing followed by cropping or grazing can result in secondary salinisation of drylands, while irrigation agriculture frequently results in soil degradation due to saline irrigation water and/or inadequate drainage. Salt-tolerant cultivars and salt-excluding rootstocks allow cropping in those circumstances, and are of enormous economic significance worldwide. Indeed, stable cropping rotations can be based on such germplasm in regions where salt and hydrologic cycles permit.

Meanwhile, with expansion of cropping areas worldwide, general availability of easily accessible arable land must diminish so that utilisation and even reclamation of salt-affected land becomes an expedient. Thanks to soil science, amelioration is technically feasible but is nevertheless protracted and costly, taking at least tens and more commonly hundreds of years to achieve. Salt-tolerant plants will be prominent in either process, and their adaptive features that were once confined to nature now find expression in managed landscapes. Those features are discussed here.

»