Cultivar and Calcium Management to Minimize Lettuce Tipburn in Greenhouse Hydroponics

Tipburn injury is an important and serious abiotic disorder most likely associated with calcium deficiency. Tipburn can severely reduce the quality and marketing value of hydroponic lettuce (Lactuca sativa L.). This research was designed to screen eleven lettuce cultivars (Aerostar, Annapolis, Bibb, Coastal Star, Dragoon, Fenberg, Green Forest, Holon, Outredgeous, Parris Island, and Sparx) for tipburn resistance. The influence of increasing calcium levels (187, 280 and 373 mg.L^-1) in reducing tipburn injury was further investigated. Lettuce seedlings were grown in greenhouse conditions under the nutrient film technique hydroponics (NFT) for multiple growing cycles (2015-2016). Lettuce tipburn was graded in three categories following U.S. standards (injury, damage, serious damage). Significant differences in tipburn injury were found between cultivars. Annapolis, Fenberg, Green Forest, Outredgeous and Sparx cultivars were more resistance to tipburn as compared to Aerostar, Coastal Star, Holon, and Parris Island. Bibb had the highest rate to tipburn. Increased calcium concentration from 187 (recommended by growers) to 280 mg.L^-1 reduced tipburn damage in Parris Island by 20% and Bibb by 60% in two out of three cycles. However, Dragoon, Holon and Parris Island showed inconsistent results. Higher level of calcium (373 mg.L^-1) eliminated the grade of serious damage in Fenberg and Bibb, in one out of three cycles, but had no effect in increasing tipburn resistance in Aerostar, Dragoon and Holon. While higher than the recommend calcium concentration reduced tipburn in some cultivars, screening for genetic resistance to tipburn appears to be the best strategy as shown in this lettuce study.