Elsevier

Scientia Horticulturae

Volume 78, Issues 1–4, 30 November 1998, Pages 83-125
Scientia Horticulturae

Tomato and salinity

https://doi.org/10.1016/S0304-4238(98)00191-5Get rights and content

Abstract

The effects of salinity on tomato plant growth and fruit production, the cultural techniques which can be applied to alleviate the deleterious effects of salt, and the possibilities of breeding salt-tolerant tomatoes are reviewed. Salinity reduces tomato seed germination and lengthens the time needed for germination to such an extent that the establishment of a competitive crop by direct seeding would be difficult in soils where the electrical conductivity (EC) of a saturated extract was equal to or above 8 dS m−1. Priming seeds primed with 1 M NaCl for 36 h seems advisable to establish a crop by direct sowing in saline soils, and seedling conditioning, either by exposure to moderately saline water exposure or by withholding watering until seedlings wilt for 20–24 h, can be recommended for crops that are to be established by transplanting. Yields are reduced when plants are grown with a nutrient solution of 2.5 dS m−1 or higher and above 3.0 dS m−1 an increase of 1 dS m−1 results in a yield reduction of about 9–10%. At low ECs, yield reduction is caused mainly by reduction in the average fruit weight, whilst the declining number of fruits explains the main portion of yield reduction at high ECs. Since the smaller the fruit, the less important the reduction in fruit weight caused by salt, small size tomatoes are recommended to be grown at moderate salinity. Short cycle crops, in which only 4–6 trusses are harvested, are also recommended – especially since upper inflorescences are particularly sensitive to salt. Root growth, which slows when salinity reaches 4–6 dS m−1, appears to be less affected by salt than shoot growth. Salinity raises Na+ concentration in roots and leaves of tomato plants. A higher Na+ concentration in the leaves lowers the osmotic potential and promotes water uptake, but it is the ability to regulate Na+ in older leaves while maintaining a low Na+ concentration in young leaves which seems to be related to salinity tolerance. Ca2+ and K+ concentrations in roots of salinised tomato plants change little under salinity whilst they are greatly reduced in leaves; those plants taking up more Ca2+ and K+ from the salinised medium will have lower Na+/K+ and Na+/Ca2+ ratios and an equilibrium of nutrients more similar to the non-salinised plants. Increasing Ca2+ and K+ concentrations in the nutrient solution is, consequently, advisable. Root NO3 concentration is maintained for longer periods after salinisation or under higher salinity levels than leaf NO3 concentration. Salinity enhances tomato fruit taste by increasing both sugars and acids, fruit shelf life and firmness are unchanged or slightly lowered, but the incidence of blossom end rot is much higher. Breeding of tomato cultivars tolerant to moderate salinity will only occur after pyramiding in a single genotype several characteristics such as greater root volume, higher efficiency in water absorption and dry matter formation per unit of water absorbed, higher selectivity in absorption of nutrients, and higher capability to accumulate toxic ions in vacuoles and old leaves.

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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