Tomato and salinity
Introduction
Tomato is a widely distributed annual vegetable crop which is consumed fresh, cooked or after processing: by canning, making into juice, pulp, paste, or as a variety of sauces. The tomato crop is adapted to a wide variety of climates ranging from the tropics to within a few degrees of the Arctic Circle. However, in spite of its broad adaptation, production is concentrated in a few warm and rather dry areas: more than 30% of world production comes from countries around the Mediterranean sea and about 20% from California (FAO, 1995). These areas are also those where the highest yields are reached.
Natural soil-forming processes in warm and dry regions frequently produce saline and gypsiferous soils with low agricultural potential. Also in these areas, most crops (including tomato) must be grown under irrigation. Inadequate irrigation management leads to salinisation of water resources and soils and this secondary salinisation affects 20% of irrigated land worldwide (Ghassemi et al., 1995). This leads to a net loss of irrigated land to agriculture and estimates of this net loss vary widely – the highest figure being some 107 ha annually (Szabolcs, 1994). Hence, in the areas with an optimal climate for tomato, salinity is a serious constraint, not only for planting new lands with this crop but also for maintaining high productivity on those currently under irrigation. So, important, but difficult, aims are to cultivate or increase tomato yields in areas with salt-affected soils, and/or simply to be able to irrigate with waters that are not currently used because of their high salinity. The tomato could act as a model crop for saline land recovery and use of poor-quality water as there is a wealth of knowledge of the physiology and genetics of this species.
In the first part of this review we describe some effects of salinity on characteristics that affect tomato fruit production, and in the second part we will review the cultural techniques applied to alleviate the deleterious effects of salt. Special attention will be paid to the possibilities of future development of cultivars tolerant to salinity.
Section snippets
Germination
Tomato crops may be directly seeded into their final cropping positions or transplanted, the seedlings being raised under protected conditions. Both the substrates and the water employed for the latter do not usually have salinity problems, so the study of effects of salt on germination is only relevant to the case of direct sowing where poor germination and emergence would jeopardise the economical viability of the crop.
Germination is characterised by three phases. The first, imbibition, takes
How to ameliorate deleterious effects of salt on tomato plants
As stated above, salinity affects root, shoot, flowering, fruiting and fruit quality. For cropping tomatoes in salinised soils or with saline water the application of a battery of strategies each contributing to a small extent to enable the tomato plant to better withstand the deleterious effects of salt may be more successful than searching for a hypothetical single strategy with a strong effect. If many small improvements prove additive in their effect, tomato production would be achieved in
Concluding remarks
In most environmental conditions in which it is cultivated, the tomato begins to lose yield when irrigated with water whose EC is above 2–3 dS m−1: when compared to fresh water irrigation, 50% yield reduction occurs with moderately saline water of ≅9 dS m−1. A commercial tomato crop is not profitable when yield reductions between 10 and 15% are reached. To crop tomatoes profitably at salinities of about 9 dS m−1 seems, nowadays, far from realistic. Such a goal would be achieved only with the
Acknowledgements
We are indebted to Professor T.J. Flowers for his valuable critical review of the manuscript. This work was partially supported by project AGF95-0037 of CICYT and contract 93AVI008 of the European Union.
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