Long-term impact of biofortified iodine on tomato (Lycopersicon Esculentum) growth, yield, and quality

Iodine is found in soils as both inorganic and organic molecules. They include iodate (IO_3^-) and iodide. Iodine, a vitamin, is essential for cognitive and physical growth, according to Antonyak et al., (2018). As part of the thyroid hormone, iodine is essential for human health and metabolic functions (Sorrenti et al., 2021). The iodine cycle is slow and incomplete in some areas, causing soil and drinking water iodine loss (MacKeown et al., 2022). Agriculture on such soils may deplete iodine, resulting in insufficient iodine intake for humans and animals.
       
Biofortification has significantly reduced global iodine deficiency in recent decades. Adding iodine-containing salts or organic resources like seaweed to soils helps crops absorb and store this element. In biofortification, regularly consumed crops are fortified with iodine to avoid iodine deficiency (Lawson, 2015). Globally, Solanum lycopersicum L., or tomato, is a major vegetable crop. Its health advantages and economic importance stem from its value in fresh market consumption and processed product manufacture (Abdelgawad et al., 2019). Due to their nutritious value, tomatoes are called “protective foods”. The vegetable is adaptable and has a long history in Indian cuisine (Gayathiri et al., 2021). Tomato fruit quality is determined by appearance, size and flavour. Mazon et al., (2022) recommend total soluble solids (SS) and titrable acidity (TA) for fruit taste and quality evaluation. This vital vitamin boosts immunity lowers blood pressure and lowers cholesterol.
       
Tomato quality depends on soil fertility, namely potassium. This nutrient significantly impacts fruit quality and vital plant processes such as osmotic control, enzyme activation, photoassimilates transport, carbon dioxide assimilation and transpiration. A polysaccharide-rich fibrous material, chitin is found in prawns, lobster, crab and fungal exoskeletons and cell walls (Ali et al., 2022). Chitosan-iodate compounds enhance iodine absorption. Choosing fertiliser amounts for iodine biofortification is tricky. Krzepilko et al., (2019) state that plant element concentration is influenced by species, growth circumstances, fertiliser type, soil composition, moisture, pH and redox conditions.
       
The current work uses potassium iodate and iodine chitosan complexes to biofortify iodine. This biofortification approach will be tested on residual tomato plant and fruit growth, production and quality.

Two field experiments were conducted in the summer and kharif seasons of 2022 at Viraliyur village, located in the Thondamuthur block of Coimbatore district, Tamil Nadu, India (GPS coordinates: 10°.9'99.284"N; 76.7'82.652"E), to investigate the impact of residual iodine on growth, yield and quality. The studies were conducted utilizing a randomized block design with three replications and sixteen treatments in the Palaviduthi soil series. The hybrid tomato variety “Shivam” was utilized for the study. The plants were grown in clay loam soil with a neutral pH of 7.17. The soil had a low nitrogen content of 185.2 kg ha^-1, while phosphorus and potassium levels were medium, measuring 16.4 kg ha^-1 and 211.6 kg ha^-1, respectively. Additionally, the soil had a non-saline condition with an electrical conductivity of 0.45 dSm^-1. A random selection of five plants was made from the sample area and thereafter labeled to capture biometric observations at three distinct phases: the green, pink and red ripening harvest stages of tomato. The measurement of plant height and number of branches was recorded at four different time points: 15 days after treatment (DAT), 30 DAT, 45 DAT and 60 DAT. The data on dry matter production and fruit yield were also documented for the aforementioned plants. Typically, fruits were harvested on a bi-weekly basis. The quality criteria, namely ascorbic acid, titrable acidity and total soluble solids, were measured at various stages of harvest. Ascorbic acid was measured in tomato samples using titration. Soluble solids (SS) were measured in °Brix (±0.5) using a portable refractometer. To determine tomato titratable acidity (TA), 5 g of treated tomato fruit was homogenized with 50 mL of distilled water and filtered. The aliquot was titrated with 0.1 N NaOH using phenolphthalein (Perdones et al., 2016). The data were reported as a citric acid percentage using this formula:
The data obtained were subjected to one-way ANOVA. The programme IBM SPSS® Statistics, version 25 was used to run all statistical tests.

Effect of potassium iodate and iodine chitosan complex on growth, yield and quality of tomato
 
Growth parameters
 
Plant height
 
The results of this study indicated that the application of Chitosan-KIO₃ Complex-10 kg ha^-1 + FA-KIO₃-0.3% at 60 and 90 DAT resulted in superior plant development at all measurement periods in residual crops  (Fig 1). The rates of combined chitosan complex and KIO₃ foliar treatment were not significantly different. Combining chitosan and potassium iodate boosted tomato growth. El-Serafy (2020) found that chitosan boosts plant metabolic activity in leaves. Chitosan’s amino contents boost leaf nitrogen and the plant’s capacity to acquire nitrogen from the soil during breakdown. In contrast, potassium iodate boosted leaf development, which indirectly increased photosynthetic activity and tomato plant height (Houmani et al., 2022).
 

 
Number of branches
 
In residual tomato crops, the combination of Cs-KIO₃ and FA-KIO₃ treatments yielded the highest branch count, followed by SA-KIO₃ and FA-KIO₃. Nitrogen in chitosan aids protein, nucleic acid and protoplasm production (Fig 2). A unique entity stimulates cell division and meristematic activity to grow tissues and organs (Teklic et al., 2021). Potassium may have also helped growth early on, resulting in more branches.
 

 
Yield parameters
 
Dry matter accumulation is a key crop production indicator. Chitosan-KIO₃ Complex-10 kg ha^-1 + FA-KIO₃-0.3% at 60 and 90 DAT at 60 and 90 days after transplanting (DAT) increased residual crop dry matter and yield (Table 1). Cakmak et al., (2017) found that 18% potassium in KIO₃, which plant roots absorbed, increased plant height and branching. These may cause the crop’s peak dry matter output. Due to adequate nutrient levels that enhance photosynthetic activity, light absorption, dry matter synthesis, accumulation and partitioning, Cs-KIO₃ and FA-KIO₃ may have generated the maximum dry matter and yield. Krupa-MaIkiewicz and Fornal (2018) discovered that chitosan boosted plant fresh and dry weight. Adding potassium iodate to soil boosted fruit production, although not as much as chitosan. More leaves, which assist photosynthesis and create carbohydrates, may explain the rise in fruit yield (Charbonnier et al., 2017). Compare to other methods, foliar potassium iodate at 60 and 90 DAT lowers crop output. KIO₃ foliar spray may increase leaf and fruit iodine deposition without tissue absorption.
 

 
Quality parameters
 
Ascorbic acid
 
The main antioxidant in tomato fruit is ascorbic acid. The Chitosan-KIO₃ Complex at 10 kg ha^-1 and FA-KIO₃ at 0.3% produced the highest ascorbic acid levels in tomatoes during green, pink and red ripening (Table 2). Ascorbic acid in fruits has a key function in oxidation or respiration (Saleem et al., 2021). As the plant ripened from pink to crimson, fruit ascorbic acid levels decreased. The chitosan-KIO₃ complex and KIO₃ foliar spray showed a similar trend, although with a smaller drop. Chitosan reduces oxygen permeability in fruits during respiration, as observed by Krupa-Małkiewicz and Fornal (2018). Chitosan slows fruit ascorbic acid decomposition (Mageshen et al., 2022).
 

 
Total soluble solids
 
Total soluble solids (TSS) concentration is a major indicator of tomato fruit quality. Table 3 shows that the Chitosan-KIO₃ Complex-10 kg ha^-1 + FA-KIO₃-0.3% treatment had the highest concentration of total soluble solids at 60 and 90 DAT during tomato ripening stages (green, pink and red). The fruit’s soluble solid content increased when potassium iodate was used because potassium helps transport sucrose. The fruit’s total soluble solid content may grow due to cellulose and hemicellulose solubilization inside cell walls or water loss as suggested by Jiang et al., (2022). Treatments like potassium iodate applied to the soil, foliar application alone and soil and foliar treatments reduce total soluble solid content after the pink stage because fruits use sugar during respiration. However, chitosan alone and the chitosan iodate complex + FA-KIO³ treatments increase soluble solids. Chitosan reduces respiration, preserving fruit soluble solids. (Shah and Hashmi, 2020).
 

 
Titrable acidity
 
Potassium iodate and chitosan affected TA levels. Typically, tomato fruit titrable acidity decreases from green (0.59%) to red ripened (0.51%) in the leftover crop (Table 4). Titrable acidity decreases largely because fruits use it during respiration and metabolism. Ethylene production during fruit ripening lowers titrable acidity, according to Chauhan and Chauhan (2020). The combination of chitosan and potassium iodate reduced titrable acidity less than other treatments. Chitosan affects respiration rate, which is thought to be affected by tissue oxygen and carbon dioxide levels (Galus et al., 2021).
 

In the shivam hybrid of tomato, potassium helps move sugars and starch, maintains turgor pressure and regulates internal ionic balance, while chitosan boosts photosynthesis, food production and antioxidant activity. This has led to increased growth, yield (75.85 t ha^-1) and quality (ascorbic acid-3.56 mg 100 g^-1, titrable acidity-0.55% and total soluble solids-4.19%). Using a potassium iodate chitosan complex for biofortification improves fruit quality and yield more effectively. Chitosan’s durability and preservation properties make it ideal for biofortifying agricultural goods with iodine.

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All datasets generated or analyzed during this study are included in the manuscript.

This article does not contain any studies with human participants or animals performed by any of the authors.

The authors declare that there is no conflict of interest.