Hydroponic greenhouse vegetable production is growing rapidly (Walters et al., 2020) with the increased interest in consumption of fresh, high-quality, local food year-round. Among differe nt hydroponic systems, leafy green production using NFT is the most common practice (Walters et al., 2020). With this system, nutrient solution is recirculated continuously, replenishing the nutrients and dissolved oxygen during the production cycle. Commercial hydroponic fertilizer formulations consist of all the macro- and micronutrients required for plant growth. Nutritional recommendations for hydroponic crops are given based on EC levels and acidity or basicity (pH) of the nutrient solution (Brechner et al., 2013). EC indicates the total ion concentration in the solution and not individual nutrients. However, EC levels can be measured rapidly with inexpensive equipment, making it easier for growers to manage their nutrient supply. As plants use nutrients, EC levels are reduced, or as water evaporates from the solution, EC levels increase. To maintain a desired EC level, the solution is adjusted periodically with the addition of fertilizer mixtures or dilution with water to maintain a constant EC level throughout the production period. Regardless of how much nutrients are added to the nutrient solution with proper EC management, pH level can affect macro- and micronutrient availability in the solution (De Rijck and Schrevens, 1999; Whipker et al., 2001). Solution pH can affect the solubility of some nutrients, which determines those nutrients available for plant uptake. Solution pH adjustments are conducted by adding acids or bases to the solution (Brechner et al., 2013). Solution EC and pH management are therefore key for successful nutrient management in a hydroponic system.
Climatic conditions such as temperature and humidity vary within a greenhouse during the growing seasons. Recommended day- and nighttime temperature set points for hydroponic lettuce in a greenhouse is 24 °C day and 19 °C night (Brechner et al., 2013). Based on our greenhouse temperature records and as reported by others (Watson et al., 2019), although the greenhouse air temperature is 24 °C or less from fall to spring, summer air temperature can reach 32 °C or more even with cooling systems running at full capacity. Therefore, the temperature difference between summer months and the rest of the production season (fall to spring) can be 20 °C or more in northern U.S. greenhouses. Lower yields as well as physiological disorders such as tipburn often result from growing lettuce under such conditions (Frantz et al., 2004). Researchers have found the general trend of lowering EC to improve lettuce growth in a high-temperature environment because high transpiration rates can lead to a buildup of EC (Samarakoon et al., 2006). A reduction in EC levels from 1.8 to 1.0 mS·cm–1 (Abou-Hadid et al., 1995) and from 3.5 to 1.5 mS·cm–1 (Serio et al., 2000) increased the fresh weight of lettuce in soilless culture.
Tipburn in lettuce is characterized by necrotic tips and margins on younger leaves, and can render crops unsalable. Tipburn is a common issue in hydroponically grown lettuce in NFT during the summer and under supplemental lighting during other seasons, as observed in commercial greenhouses and previous investigations (Samarakoon et al., 2019). Inner leaf tipburn is caused by an inadequate Ca supply to the younger leaf tissues (Barta and Tibbitts, 2000) because Ca cannot be mobilized from older tissues via phloem (White and Broadley, 2003). Climatic conditions that lead to faster growth rates such as high temperature and high light levels can induce this disorder. Although Ca is abundantly available in the nutrient solution, high humidity can reduce transpiration rates and affect water flow through the xylem, and therefore limit Ca uptake (Gilliham et al., 2011). Vertical airflow fans to circulate air and increase transpiration around the meristem can eliminate tipburn (Frantz et al., 2004). The aim of our study was to use a nutrient management approach to alleviate tipburn.
Both EC and pH are important parameters in nutrient management. Based on previous investigations with lettuce (Samarakoon et al., 2019), yields were the greatest at EC levels of 1.8 mS·cm–1, and yield reduction was associated with low (0.8–1.2 mS·cm–1) and high (2.4 mS·cm–1) EC levels. In ‘Green Butter’ lettuce, tipburn was associated with a greater yield resulting from an adequate fertilizer supply at an EC of 1.8 and 2.4 mS·cm–1 (Samarakoon et al., 2019). The association of growth rate and tipburn occurrence in lettuce has been noted previously (Cox et al., 1976) and occurs because the Ca supply cannot compensate for the demand from the fast-growing younger tissues in the plant (White and Broadley, 2003). An important tradeoff for using EC management as a method to control tipburn is that yields might also be reduced. In our study, therefore, we investigated an EC range closer to the recommended level. On the other hand, current recommendations are to use a pH of 5.6 to 6.0 (Brechner et al., 2013). The availability of Ca in a solution can vary with changes in pH (Whipker et al., 2001) and can influence the availability for uptake.
In addition to root-zone nutrient management, direct foliar nutrient application can be used to increase Ca content in the leaf tissues (Samarakoon et al., 2017). High humidity can reduce transpiration and limit Ca uptake from the root-zone nutrient solution even if a high concentration of Ca is available (Kuo et al., 1981). Foliar application might allow nutrients to be absorbed directly into leaves. CaCl2 has been used as a foliar spray to provide Ca as a nutrient source (Samarakoon et al., 2017) and disease control (Samarakoon et al., 2016; Starkey and Pedersen, 1997).
Hydroponic solution EC and pH, as well as Ca uptake are all related and affect lettuce yield and tipburn occurrence. Therefore, we investigated the yield and degree of tipburn of three lettuce cultivars (Red Butter, Green Butter, and Red Oakleaf) of the Salanova® series in response to varying EC, pH, and foliar Ca application under summer greenhouse growing conditions.
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