Salinity impairs photosynthetic capacity and enhances carotenoid-related gene expression and biosynthesis in tomato (Solanum lycopersicum L. cv. Micro-Tom).

Andrés Leiva-Ampuero,Claudia Stange,Paulo Canessa,Andrea Vega,Marjorie Reyes-Díaz,José Tomás Matus,Camila Huidobro,Claudio Inostroza-Blancheteau,Mario Agurto,Gustavo Hoppe

doi:10.7717/peerj.9742

Abstract

Carotenoids are essential components of the photosynthetic antenna and reaction center complexes, being also responsible for antioxidant defense, coloration, and many other functions in multiple plant tissues. In tomato, salinity negatively affects the development of vegetative organs and productivity, but according to previous studies it might also increase fruit color and taste, improving its quality, which is a current agricultural challenge. The fruit quality parameters that are increased by salinity are cultivar-specific and include carotenoid, sugar, and organic acid contents. However, the relationship between vegetative and reproductive organs and response to salinity is still poorly understood. Considering this, Solanum lycopersicum cv. Micro-Tom plants were grown in the absence of salt supplementation as well as with increasing concentrations of NaCl for 14 weeks, evaluating plant performance from vegetative to reproductive stages. In response to salinity, plants showed a significant reduction in net photosynthesis, stomatal conductance, PSII quantum yield, and electron transport rate, in addition to an increase in non-photochemical quenching. In line with these responses the number of tomato clusters decreased, and smaller fruits with higher soluble solids content were obtained. Mature-green fruits also displayed a salt-dependent higher induction in the expression of PSY1, PDS, ZDS, and LYCB, key genes of the carotenoid biosynthesis pathway, in correlation with increased lycopene, lutein, β-carotene, and violaxanthin levels. These results suggest a key relationship between photosynthetic plant response and yield, involving impaired photosynthetic capacity, increased carotenoid-related gene expression, and carotenoid biosynthesis.

Highlights

In plants, salt stress can be considered as an environmental restriction consisting of an osmotic or water-deficit and a salt-specific or ion-excess effect
Photosynthesis is reduced under salt stress in this species, displaying, for example, a reduction of maximum efficiency of PSII photochemistry and alterations in the size and number of active reaction centers of the photosynthetic apparatus, as well as the quantum yield of primary photochemistry and electron transport (Moles et al, 2016; Singh, Singh & Prasad, 2016)
Salinity negatively affects the normal development of vegetative tissue in tomato plants, it has been reported that salt stress improves fruit quality by increasing sugar and organic acids contents, while increasing the carotenoid levels, thereby improving the nutritional value (Adams & Ho, 1989; Adams, 1991; De Pascale et al, 2001; Wu, Buck & Kubota, 2004; Wu & Kubota, 2008; Borghesi et al, 2011; Massaretto et al, 2018)

Summary

Introduction

Salt stress can be considered as an environmental restriction consisting of an osmotic or water-deficit and a salt-specific or ion-excess effect. In both cases, salt stress interferes with the plant’s nutrient balance, negatively affecting growth and development. Tomato plants exposed to high levels of salt (i.e., 70 mM NaCl) display a decrease in seed germination and an alteration in the absorption of water by roots (Cuartero & Fernández-Muñoz, 1999), as well as a reduction in shoot growth, stomatal density, leaf number and area (Li et al, 1999; Romero-Aranda, Soria & Cuartero, 2001). Studies in tomato landraces indicated that salinity improve fruit quality attributes, including flavonoids and sugar (Moles et al, 2019)

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