Genetic diversity of plants and its exploitation for stress environments

Full text: Water deficiency and elevated temperatures may be the most important of numerous environment stresses that tend to constrain plant growth and survival. We are addressing moisture stress and its ramifications on the macro and micro levels, and are examining means of increasing agricultural productivity in arid and semi-arid environments. Although water covers more of the surface of the globe than does land, it is mainly saline. Of all surface waters 97% lies in the oceans, 2% is held in ice caps and only 1% is fresh, originating from precipitation and snowmelt and present in lakes, rivers and biological systems. Of the 1% fresh water, a fraction is harnessed for agricultural, industrial and domestic purposes, with agriculture consuming 70%. Fresh water is a scarce commodity that is distributed unevenly geographically and seasonally, leaving vast areas arid and semi-arid. The worst affected are, perhaps, areas located in the northern and southern sub-tropics. In addition to natural factors, exacerbating forces of human origin are affecting large areas, with global implications. Plants still provide the most economical and convenient means of harnessing solar energy. Over millennia they have evolved and adapted to varied environmental conditions; they grow on mountains, in the plains, in deserts, submerged in water, and even in the sea. From the photosynthetic capture of photons to the storage of chemical energy, the reactions take place with varied efficiency according to the species and the prevailing environmental conditions. Similarly, in terms of uptake and utilization of water and nutrients, plants possess vast variability, most of which is still grossly under-utilized. Saline groundwater is present in a number of arid countries. This water could be utilized to help specific plant species, not only to survive but to thrive to produce biomass that could be used as food, forage, fuel, manure, or as industrial feedstock (biosaline agriculture). There is no doubt that halophytes and other salt-tolerant varieties of some species could be grown using saline water. However, the extent of sustainability would depend on controlling salt build-up in the surface of the irrigated soil and by having estimates of the extent of the groundwater reservoir, its dynamics, and qualitative and quantitative changes over time. Nuclear techniques offer a means of obtaining useful, rather critical, information on these issues. The IAEA has initiated a Model Project to examine the feasibility and extent of sustainability of biosaline agriculture in eight countries in North Africa, West Asia and South Asia. The project has four main activities/thrusts: To introduce salt-tolerant plant species on a demonstration site of at least 10 ha and to select those showing comparative advantage under the prevailing socio-economic conditions; To achieve good irrigation management with the help of the neutron moisture gauge (NMG) and supplementary techniques to ensure that salt does not accumulate in the soil surface; To regularly monitor (by chemical and isotopic means) the groundwater of the area to estimate quality, quantity, and sources of recharge to ascertain the extent of sustainability of the agricultural activity; To pass the technology on to the end-user. Locally available and exotic salt-tolerant perennial and annual plant species, most of them nitrogen fixing, have been introduced on 10-ha sites in arid wastelands of Morocco, Tunisia, Egypt, Syria, Iran, and Pakistan. Some species of trees, bushes, and grasses known to be salt tolerant, Acacia, Prosopis, Casuarina, Tamarix, Eucalyptus, Olea, pistacchio, date palm, Atriplex, Kochia, Hordeum, Leptochloa, Sporobolus, Sesbania, Brassica, cactus, and some others, have been successfully grown using groundwater of medium to high salinity. The saline irrigation is being managed using the NMG and regular analyses of the soil. The groundwater(s) in a radius of a several kilometers around each site is monitored for chemical and isotopic (mainly isotopes of H, O, and C) characteristics to obtain information on the quality, quantity, and possible sources of its recharge. The project aimed at utilizing saline water to help at least some of the selected species not merely to survive but to thrive. It also aimed at using these plants as the initiator and sustainer of processes that ultimately improve the environment. In at least one country, the participating scientists have clearly shown beneficial effects of plant growth on the semi-arid environment in general and on the soil in particular; the surface salinity has decreased, soil structure has improved, and fertility increased. Experience gained on the successful cultivation of some of the introduced species has already been passed on to end users for economic benefit in two countries. With growing pressure on limited water resources, the cultivation of plant species that are tolerant of drought and saline conditions on one hand and the adoption of moisture-conserving techniques on the other, will be critically important for the improvement of degraded soils and for their return to agricultural productivity. Non-destructive isotopic techniques, particularly those based on stable isotopes, need to be developed for convenient selection for tolerance of drought and salinity, for superior water-use efficiency (i.e. better utilizers of solar energy), and for superior nitrogen-fixing capability. Results indicate that existing genetic variability in plants can be fruitfully exploited not only to grow them in stress environments, but for amelioration of those environments. However, developments in nuclear and molecular techniques are needed to tailor plants for specific situations. (author)