Iron in tomatoes
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Iron in Tomatoes: Mechanisms, Responses, and Biofortification
Iron Deficiency Responses in Tomato Plants
Physiological and Molecular Adaptations
Tomato plants (Solanum lycopersicum L.) exhibit a range of physiological and molecular responses to iron deficiency. These responses include the acidification of the rhizosphere, reduction of Fe3+ to Fe2+, and the transport of Fe2+ into cells. A genome-wide transcriptional analysis identified 97 differentially expressed transcripts in tomato roots under iron-deficient conditions, highlighting genes involved in iron uptake, root morphological modifications, and primary metabolic pathways such as glycolysis and the TCA cycle. Additionally, nitric oxide (NO) plays a crucial role in iron uptake by modulating the expression of iron uptake-related genes and regulating physiological and morphological responses in roots.
Root and Shoot Responses
Iron deficiency in tomatoes leads to visible symptoms such as chlorosis (yellowing) in the aerial parts of the plant and reduced growth and yield. Early responses include increased lateral root formation and subapical root swelling, followed by a decrease in chlorophyll content and an increase in ferric-chelate reductase activity in roots. Over time, iron deficiency negatively impacts overall vegetative performance, including plant height and biomass.
Iron Biofortification Strategies
Soil and Foliar Applications
Iron biofortification can significantly enhance the iron content in tomatoes. Field experiments have shown that both soil and foliar applications of iron, particularly in combination, can improve tomato yield, iron uptake, and iron use efficiency. The highest yield and iron uptake were observed with combined soil and foliar applications of iron sulfate (FeSO4).
Soilless Systems and Nutrient Solutions
In soilless systems, varying the concentration of iron in the nutrient solution, along with foliar applications, can maximize iron content in cherry tomatoes. The highest increase in fruit iron concentration was achieved with 2 mmol Fe L⁻¹ in the nutrient solution and 500 µmol Fe L⁻¹ foliar sprays, which also improved other mineral contents and quality traits of the tomatoes.
Enzyme Activity and Gene Expression
The application of citric acid (CA) and humic-like substances (HLS) can enhance iron nutrition in tomatoes grown in calcareous soils by modulating enzyme activities and gene expression related to iron uptake. These substances improve the activity of enzymes such as H+-ATPase and ferric reduction oxidase (FRO), and influence the expression of genes like LeFRO1 and LeHA1, which are crucial for iron absorption.
Impact of Iron Fertilization on Tomato Quality
Nutrient Uptake and Phytochemical Properties
Iron fertilization, whether through nano or chelated forms, significantly influences the marketable yield and phytochemical properties of tomatoes. Chelated iron, in particular, enhances the concentrations of vitamin C, β-carotene, total carotene, flavonoids, and polyphenols, thereby improving the antioxidant capacity of the tomatoes. The optimal rate for chelated iron application was found to be 20 mg/L, which provided the best balance of yield and nutritional quality.
Sulfur and Iron Metabolism
Iron deprivation in tomatoes also affects sulfur metabolism. Iron-deficient conditions upregulate sulfate transporter genes, enhancing the plant's ability to uptake and translocate sulfate. This adaptation helps meet the increased demand for sulfur-containing compounds like methionine, which are essential for coping with iron stress.
Conclusion
Tomato plants exhibit complex physiological and molecular responses to iron deficiency, which can be mitigated through various biofortification strategies. Soil and foliar applications of iron, optimized nutrient solutions in soilless systems, and the use of substances like citric acid and humic-like substances can significantly enhance iron uptake and improve the nutritional quality of tomatoes. Understanding these mechanisms and responses is crucial for developing effective agronomic practices to address iron deficiency in tomato cultivation.
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