Effect of Temperature on Tipburn Development in Head Lettuce

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Abstract
Tipburn symptoms developed on the central leaves of mature detached field-grown heads of lettuce [Lactuca sativa var. capitata] exposed in growth chambers to 24-33.degree. C for 4-7 days. Tipburn severity also was enhanced by enclosing mature intact plants in the field with polyethylene-covered frames that raises head temperature about 6.degree. C above the ambient temperature outside the enclosures. Percentage of tipburned plants and disease severity increased directly with increase in time of exposure to tipburn-inducing temperatures. At all temperatures tested, about twice as much time was required to induce tipburn in 50% of the heads subjects to alternating high and low temperatures as compared with constant temperatures. Differences in vapor pressure deficits during temperature treatment did not affect tipburn development in harvested heads. Internal temperatures of mature heads in the field during sunny days usually was about 6.degree. C higher than ambient, which may be the reason why tipburn may occur at times when ambient temperatures do not exceed 24.degree. C, the minimum temperature for tipburn induction under laboratory conditions.

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