Single Cell Biology

Tomato Production and Associated Stress: a Case of African Climate

Oluwatosin Ayobami Ogunsola^* and Grace Ayomide Ogunsola Department of Crop, Soil and Pest Management,Federal University of Technology Akure, Nigeria
^*Correspondence: Oluwatosin Ayobami Ogunsola, Department of Crop, Soil and Pest Management,Federal University of Technology Akure, Nigeria, Email:

Received: 27-Oct-2021 Published: 17-Sep-2021

Introduction

Tomato (Solanum lycopersicum L.) is a popular and economically important crop plants around the world. The second largest vegetable both in terms of production and consumption [1]. It ranks 7th in worldwide production after wheat, rice, maize, potatoes, soybeans and cassava, reaching a worldwide production of around 170 million tonnes on a cultivated area of almost 5.2 million hectares in 2018 [2]. It has gained popularity in recent times with the discovery of health benefits inherent in the valuable compound, lycopene, which possesses anti-oxidative and anticancer properties [3, 4]. Lycopene in tomatoes enhance fertility by improving the quality and swimming speed of sperm whilst reducing the number of abnormal sperm in men [5, 6]. Alongside other nutrients, tomato fruit contains beta-carotene, flavonoids, vitamin C and phenolic compounds, which offer many health benefits for the consumers [7]. Despite all the numerous benefits from the crop, many challenges are making its production unprofitable in most developing countries especially those in Africa. The challenges faced by producers can either be in production, post-harvest, marketing or a combination of any of them. Global tomato production increased during the 1920s as a result of breakthroughs in technologies that made mechanized processing possible [8]. With increasing knowledge in benefits derived from genetic modification of tomatoes, more desirable parameters have been selected for varietal improvement to enhance the crop for human consumption thus creating a continued increase in tomato production and consumption [4].

Crops grown in open fields encounter multiple unfavorable conditions for optimal plant growth and yield, of both abiotic and biotic origin. According to [9], some biotic and abiotic factors have been attributed to low yields and the increased cost of production. The low diversity among commercial tomato varieties has been identified as one of the major factors that predispose the crop to biotic and abiotic constraints [10]. Poor soil fertility [11], extreme heat and insufficient or unpredictable water supply [12], which have hitherto become prevalent in the wake of a changing climate [13]. Biotic stress creates additional losses, with newly emerging plant diseases and pests causing decreased yields and serious reductions in produce quality. In West Africa, tomato growers are faced with a number of disease and pest problems. Soil-borne diseases include bacterial wilt caused by Ralstonia solanacearum and Fusarium wilt caused by Fusarium oxysporum f. sp. lycopersici; foliar diseases include bacteria spot caused by Xanthomonas spp., late blight caused by Phytophthora infestans and Septoria blight caused by Septoria lycopersici; and mosaic viruses, such as Tomato mosaic virus and Cucumber mosaic virus, can cause yield loss and produce damage. Pests include worms, such as the cotton bollworm (Helicoverpa armigera), nematodes, mining insects, thrips, various aphid species, and mites, e.g., spider mites (Tetranychus urticae). Recently, outbreaks of the tomato leaf miner ((Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)) have caused substantial damage to tomato crops in some West Africa countries [14 – 16]. Therefore, this substantiate the need to review the production output of tomato in the African production zones with a view to visualize the effect of associated stress and potential remedy backed by scientific research.

Tomato Production Output Index

Tomato is produced in temperate, subtropical and tropical areas around the world [17] and it is the second horticultural crop produced in terms of yield in the world [1]. Numbers from 2014 showed China, India, United States, Turkey and Egypt as the countries with the largest production area [1, 18]. There has been a steady increase in the production output (Table 1) in China, India, USA, and Turkey whereas the reverse is obtainable in Egypt, where there is a steady decline in the production output over the period under review (2014 – 2018). Although there is a steady increase in the global cumulative production output year-on-year however, the variations in the production output across the producing countries over the period signals the presence of attendant challenges associated with production – prominent amongst others are biotic and abiotic stress gaining ground with the changing climate. The outbreak of tomato Ebola and Tuta absoluta in Nigeria [16] could have contributed to the significant drop in production output in 2016.

Table 1: 15 highest tomato producers globally (Tonnes) FAOSTAT 2019

There exist a direct variation between the production output and hectares cultivated across the African regions however, there is a sharp contrast to this in West Africa (Nigeria) where there exist a shortfall in the production output as against the hectares cultivated (Table 2). Some of this shortfall could have been as a result of post-harvest losses however, the magnitude of shortfall during the pest infestation (2016) suggest more of biotic stress associated with the changes in atmospheric conditions experienced within the region. Consequently, the trend is maintained till 2018 hence making a case for the influence of biotic and abiotic stress on tomato production in the region. Farmer plays an important role by having impact on the quality of the produced tomato. The production is highly affected by the degree of the farmers’ knowledge and on the support by regional and national institutions. Lack of knowledge and support are known to increase harvest losses [19]. Regional efforts to increase tomato production by smallholders in Africa has shown positive results, e.g. when novel leaf curl disease resistant tomatoes seeds were released in West Africa [20]. In addition, regional efforts in the horticultural production chain is known to benefit women, due to the fact that women represent 80% of the workforce in this production chain, and increasing yield and quality of the produce results in more decision power for women within their communities [21].

Table 2: Tomato production in Africa over a five-year period (2014 - 2018) FAOSTAT 2019

Advances in Adaptation to Biotic and Abiotic Stress

Abiotic stress involves chilling, high temperature, osmotic shock, drought, salinity, water logging, wounding, exposure to ozone, toxic ions, excessive light and UV-B irradiation [22]. Unfortunately, abiotic stresses are complex in their nature but are controlled mostly by genetic and environmental factors that impede crop plant breeding strategies [23]. Abiotic stress can be identified by symptoms on leaves but most commonly observed symptoms are associated with certain diseases or nutrient deficiencies and requires keen observation of the stem or root to diagnose accurately. Abiotic stress in tomato plant may not be easily visible due to secondary colonizers such as pest and diseases albeit, it’s advisable to consider multiple problems that might have reduced its productivity. An understanding of tomato plant physiology, genetic makeup of the variety, nutrient, weather and soil conditions helps to overcome combinatorial stress from biotic and abiotic conditions [24]. Reactive oxygen species (ROS) are a common signal that activate plant response to abiotic stress [25]. Plants are susceptible to ROS when their production exceeds the capacity of the plant to scavenge them. Tomato varieties that are naturally tolerant to drought and heat stress are identified to have ability to respond positively to oxidative damage and regulation of ROS that causes molecular damage and cell death. Production of ROS causes impairments in DNA, lipids and protein which eventually leads to cell death and progressive aging of the plant [26, 27].

A major determinant of biotic and abiotic stress in tomato crop production is low diversity among commercial tomato varieties and negative interactions at the phenotypical levels [10, 28]. Consequently, in order to survive biotic and abiotic stresses in their environment, plants have evolved complicated mechanisms to identify external signals thereby producing optimal responses through plant proteins and phytohormones [25]. Proteins are difunctional in the formation of new plant phenotypes by regulating physiological characteristics to adapt to changes in the environment and also in maintaining cellular homeostasis [29]. Phytohormones such as salicylic acid (SA), jasmonic acid (JA), ethylene (ET), and abscisic acid (ABA) primarily regulate plants’ responses against biotic and abiotic stresses via synergic and antagonistic actions [30, 31]. Abscisic acid is a defense related phytohormone that promotes abiotic stress tolerance and suppresses signaling of the biotic stress-related hormone salicyclic acid. It signals in root-pathogen interactions where abiotic stress is encountered most directly [32]. ABA levels in tomato roots have been found to increase rapidly after salt stress exposure and during the onset of predisposition, and then decline to near pre-stress levels [32]. Indications for stress regulatory crosstalk can be found at the phenotypic level, and are evident as well at the gene expression level [28]. Recently, the transcriptome of Arabidopsis subjected to combinations of various abiotic and biotic stressors was analyzed [33 – 35]. In plants, polyamines not only play a role in abiotic and biotic stress, but also in many other physiological processes (organogenesis, embryogenesis, floral initiation and development, leaf senescence, fruit development and ripening) [36]. Recent studies have revealed that polyamine signaling is involved in direct interactions with different metabolic pathways and entangled hormonal cross-talks (e.g., abscisic acid involved in the regulation of abiotic stress responses) [36].

Breeding programmes have been used to achieve cultivars that are resilient to both biotic and abiotic stress in order to survive and reproduce genotypes that are tolerant to heat with locally adapted ascesions that are susceptible to these stresses can be bred to develop resistant varieties [37]. Recently, the introduction of transgenic technology has made transferrable traits of desirable genes that are resistant to abiotic and biotic stress possible and research on improved abiotic stress tolerance are ongoing. An offshoot of such research have developed transgenic tomato plants which over-expressed cytosolic ascorbate peroxidase (cAPX gene) with enhanced tolerance to heat (40OC), chilling and salt stress showed enhanced resistance compared to wild types. [29] found intricate molecular mechanisms to be involved in biotic stress and that Posttranslational mechanisms (PTMs) are critical for rapid reprogramming of cells, defense signal transduction and attenuated response and are important means by which plants maintain cell homeostasis at all levels of the immune response. Proteins in plant cells are post-translationally modified by covalent addition of some chemical units or by changing the structures of the amino acids themselves. Also, tomato seedlings inoculated with Streptomyces thermocarboxydus strain BPSAC147 under greenhouse conditions had enhanced its resistance to abiotic stress and diseases. Tomato plants with a DNA containing Arabidopsis C repeat/dehydration-responsive element binding factor 1 (CBF1) cDNA and a nos terminator, driven by a cauliflower mosaic virus 35S promoter had been successfully transformed and had been more resistant to water deficit stress than wild type plants [38].

Conclusion

Global production output of tomato is on the increase with a view to meet the demand on consumption of the commodity. African regions are key contributors to the overall output albeit the attendant constraints associated with the growing zones are enormous and this has been further deepened in the wake of a changing climate. Tomato production per unit area in the African growing zones has been significantly low when compared with that obtainable from Asia, America and Europe. Genetic improvement of hybrid varieties should be crossbred with local landraces to confer superior resistant ability to stress as well as increased productivity per unit area. This should be accompanied with trainings on Global Good Agricultural Practices (Global G.A.P), inclusions on agricultural technological interventions and integrating soil management practices as well as building a structured value chain to cater for exigencies related to post-harvest losses.

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