For tomato growers, managing potassium (K) nutrition is critical for both yield and fruit quality. While most producers understand K’s importance, many don’t realize how significantly temperature influences a plant’s ability to uptake and utilize this essential nutrient. With increasing heat extremes, understanding this relationship becomes even more crucial for successful production.
The Temperature-Potassium Connection
Potassium Uptake
Research shows that potassium uptake in tomato plants is highly temperature dependent. Studies conducted by Tindall et al. (1990) found that K uptake peaks at a root zone temperature of approximately 80°F. Their research revealed:
- Optimal uptake range: K uptake performs best when root zone temperatures are between 70-80°F
- Cold and hot stress: Both excessively low (below 68°F) and high (above 86°F) root temperatures substantially reduce K uptake
- Quadratic response: K uptake increases as temperatures rise from cool to optimal, then decreases sharply as temperatures become too hot
This relationship helps explain why we sometimes observe quality issues such as yellow shoulder disorder (YSD) and uneven ripening, even when soil tests indicate adequate K levels. When roots get too hot, plants simply can’t access the K that’s present in the soil.
Fruit Exposure to Sunlight
Yellow shoulder disorder (YSD) presents as sectors of yellow or green tissue under the peel of tomato fruit, creating quality issues that significantly impact grading and processing efficiency. This condition results from a complex interplay of several factors. High temperatures inhibit or prevent the production of lycopene (the red pigment in tomatoes), with symptoms appearing first at the fruit shoulder where direct sunlight exposure is highest. Research has consistently demonstrated a strong relationship between potassium deficiency and YSD, with studies showing that soil K levels are negatively correlated with both the frequency and severity of the disorder. When plant tissue potassium levels drop from adequate (4-6%) to low (2-3%), yellow shoulder becomes more prevalent, particularly during periods of high temperature and intense sunlight. Rather than simply being a delayed ripening issue, ultrastructural analysis reveals that YSD affects cell enlargement early in fruit development, making it an altered developmental process. Growers can minimize YSD by selecting varieties with the uniform ripening trait, ensuring adequate foliage cover to protect fruits from excessive sunlight, and most critically, maintaining sufficient potassium levels in the soil. However, even with very high potassium applications, yellow shoulder may still occur to some degree during seasons with drought conditions and extremely warm, sunny weather.
How Heat Stress Impairs Potassium Uptake
Recent studies by Giri (2013) have identified the molecular mechanisms behind heat-induced K deficiency. When root zone temperatures exceed 95°F:
- Transport protein reduction: Heat stress decreases the concentration of KT1, the main potassium transporter protein in roots
- Energy deficit: High temperatures interfere with photosynthesis, reducing the energy available for active transport of K across cell membranes
- Slow recovery: After severe heat stress (above 104°F), K uptake systems take a week or more to recover fully
The research found that roots were more affected by heat than shoots, as indicated by a decrease in the root-to-shoot mass ratio. This explains why fruit quality issues often appear during hot weather, even when foliage looks healthy.
Implications for Tomato Production Practices
To maintain optimal K nutrition during hot weather, consider these management strategies:
- Temperature Management
- Use light-colored mulch in hot weather to reduce soil temperature
- Schedule irrigation for later in the day to help cool root zones and increase soil moisture content
- In high tunnels, ensure proper ventilation and consider using a 30% black shade cloth, that is applied over the tunnel, when temperatures exceed 90°F
- Promote deeper rooting where soil temperatures are more moderate. Assure that K levels are adequate
- Nutrition Program Adjustments
- Timing is critical: Begin monitoring tissue K levels at first flowering—tissue K should exceed 3% by dry weight
- Increase availability during heat: During hot periods, apply K through fertigation to enhance availability
- Maintain proper ratios: Keep potassium levels 2-3 times higher than soil nitrogen during fruiting. During fruiting, the high demand for K shifts the overall nutrient balance, impacting the availability of nutrients such as nitrogen. The prioritization of K uptake could potentially reduce the absorption and translocation of nitrogen
- Consider K form: Research shows that the particle size of potassium sulfate significantly affects its availability. Some large particles (several mm in diameter) may remain intact for years without dissolving
- Varietal Selection
- Select varieties with heat stress tolerance and tolerance to abiotic disorders such as uneven ripening, internal white tissue, or yellow shoulder.
In a 2019 study at the University of Delaware, researchers found significant differences in susceptibility to heat-induced quality issues (Johnson and Carvel, 2019). Varieties with a low incidence of white tissue under high temperatures included Jamestown, Primo Red, and Red Bounty, while Camaro, Mountain Merit, Mountain Fresh, Red Snapper, Marshall, and Myrtle showed high susceptibility.
Diagnosing Potassium-Related Quality Issues
Yellow shoulder disorder (YSD) and uneven ripening are the most common symptoms of inadequate K availability during fruit development. These disorders are characterized by:
- Areas of yellow or green tissue under the fruit peel and in walls of tomato fruit
- Uneven coloration during ripening
- Lower quality and value of harvested fruit
- Increased susceptibility during hot weather
Remember that visual symptoms alone don’t always indicate the timing or severity of K deficiency. Regular tissue testing throughout the season provides the best guidance for nutrient management.
Recovery from Heat Stress
The good news is that K uptake systems can recover after heat stress. Research indicates that when temperatures return to optimal levels, K uptake recovers more quickly than other nutrients, such as nitrogen and phosphorus. After severe heat stress, allow at least 7 days for full recovery of the K uptake system. Maintaining adequate soil moisture during recovery is essential, as K uptake depends on active transport through well-hydrated root systems.
Conclusion
Tomato growers can significantly improve fruit quality, even during challenging weather conditions, by understanding how temperature affects K uptake and implementing management strategies that optimize root zone conditions. Regular monitoring of both soil and tissue K levels, combined with temperature management and appropriate variety selection, provides the best defense against heat-induced quality problems.
Resources
Tindall, J.A.; Mills, H.A.; Radcliffe, D.E. 1990. The effect of root zone temperature on nutrient uptake of tomato. J. Plant Nutr. 1990, 13, 939–956. https://www.tandfonline.com/doi/abs/10.1080/01904169009364127
Francis, D. M., S.A. Barringer, and R.E. Whitmoyer. (2000). Ultrastructural Characterization of Yellow Shoulder Disorder in a Uniform Ripening Tomato Genotype. HortScience HortSci, 35(6), 1114-1117. Retrieved, from https://doi.org/10.21273/HORTSCI.35.6.1114
Giri, A. Effect of Acute Heat Stress on Nutrient Uptake by Tomato Plants. Master’s Thesis, University of Toledo, Toledo, OH, USA, December 2013. https://etd.ohiolink.edu/acprod/odb_etd/ws/send_file/send?accession=toledo1384445444&disposition=inline
Hochmuth, G.J. (1994). Efficiency Ranges for Nitrate-Nitrogen and Potassium for Vegetable Petiole Sap Quick Tests. HortTechnology horttech, 4(3), 218-222. Retrieved May 16, 2025, from https://doi.org/10.21273/HORTTECH.4.3.218
Johnson, G and A.V. Carvel. 2019. Tomato Variety Trial Results. Retrieved from https://www.udel.edu/content/dam/udelImages/canr/pdfs/extension/sustainable-agriculture/vegetable-trials/TomatoVarietyTrial2019.pdf
Maynard, L. 2021. Reducing Blossom End Rot and Yellow Shoulder/Internal White Tissue in Tomato. Vegetable Crops Hotline, Issue 688. Retrieved from https://vegcropshotline.org/article/reducing-blossom-end-rot-and-yellow-shoulder-internal-white-tissue-in-tomato/
Sideman, R.G., B. Hoskins, M. Hutton, H. Bryant and E. Sideman. 2020. Optimizing potassium application in organically-grown high tunnel tomato (Solanum lycopersicum) in the northeastern United States. Acta Hortic. 1296. ISHS 2020. DOI 10.17660/ActaHortic.2020.1296.137. https://www.ishs.org/ishs-article/1296_137
Trinklein, D. 2022. Tomato Fruit Disorders. Retrieved from https://ipm.missouri.edu/meg/2022/7/tomatoFruitDisorders-DT/