Tipburn of romaine lettuce is a serious quality defect, which causes significant economic loss to lettuce growers in the Salinas Valley of California. This defect is particularly problematic for the producers of fresh-cut salad mixes because the presence of even a few affected leaves in a consumer product is unacceptable. Tipburn is generally recognized as a localized calcium (Ca) deficiency that induces collapse and necrosis of the margins of actively expanding leaves (Collier and Tibbitts, 1982). Under field conditions, tipburn development is often related to factors other than soil Ca supply. Tipburn occurs primarily in the final weeks before harvest, when crop growth rate is most rapid. Environmental conditions such as high light intensity (Tibbitts and Rao, 1968) or warm temperatures (Rao, 1966) that encourage rapid growth also promote tipburn. Because Ca moves primarily by transpirational mass flow in the xylem (Clarkson, 1984), inner leaves that are enclosed within the developing head are particularly prone to the disorder because of their low transpiration rate.
Effective tipburn prevention measures have proven elusive. Cultivars vary considerably in tipburn susceptibility (Collier and Tibbitts, 1982; Ryder and Waycott, 1998), but no commercially acceptable romaine cultivar has been developed that is reliably tipburn-resistant. Under controlled conditions, Ca application to soil, foliage, or in nutrient solution has reduced tipburn (Ashkar and Ries, 1971; Sonneveld and van den Ende, 1975; Thibodeau and Minotti, 1969). Frantz et al. (2004) and Goto and Takakura (1992) reduced tipburn by blowing air onto the developing leaves, thereby increasing transpiration. However, under field conditions, success in reducing tipburn has been limited. Misaghi and Matyac (1981) found that soil Ca application at rates as high as 160 kg·ha−1 reduced tipburn in only one of six fields. Bangerth (1979) and Collier and Tibbitts (1982) concluded that in soils normally used for lettuce production, soil Ca supply was not a significant factor in tipburn development.
Despite the generally poor success reported for soil Ca application, fertigation of products such as calcium nitrate or calcium thiosulfate remains a common practice in the California lettuce industry. Ca fertigation is done without regard for soil test Ca level on the belief that in the high pH, alkaline soils typical of this region, the common soil test procedure (ammonium acetate extraction; Thomas, 1982) does not provide an accurate estimate of soil Ca availability. This study was undertaken to determine whether soil Ca availability played any substantive role in tipburn development in romaine lettuce under representative field conditions and whether Ca fertigation could improve lettuce Ca uptake and decrease tipburn severity.
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