Hearting and Mature Head Characteristics of Lettuce (Lactuca SativaL.) as Affected by Shading at Different Periods During Growth

Abstract

Comparisons of the growth and mature head characteristics of field grown lettuce cv Cobham Green were made between plants shaded from 50% emergence until the start of hearting, plants shaded from the latter period until maturity, and unshaded plants. Early shading lengthened the outer non-hearting leaves and the outer heart wrapper leaves by 12% but reduced the length of those leaves which, at the start of hearting, were destined to form the inner heart wrapper leaves and the first few heart ‘filler’ leaves. As a result, early shading decreased mature head weight but head density and volume were not significantly affected. For a sowing in May, late shading of plants increased head volume and decreased head density but head weight was unaffected. For a sowing in July late shading had no significant effect on head volume but head weight and density were significantly reduced. This reduction in weight and density was associated with an increase in the length/breadth ratio of the inner heart wrapper leaves and the heart filler leaves. The relationship between these responses to shading and the known effects of sowing date on head characteristics is discussed.