Various treatments were then applied, including growing with added nitrogen (120kg/ha), added calcium (80k/ha), and with different ratios of potassium to nitrogen. Lettuce heads were harvested at three different stages of maturity, trimmed, chopped into square pieces and stored in semi-permeable bags at 3 degsC.
Adding nitrogen to the soil did not affect the degree of discolouration, while adding calcium had a small beneficial effect in certain circumstances. There were no differences in discolouration between crops irrigated from overhead sprinklers and those irrigated by means of a drip system. The ratio of potassium to nitrogen in the soil did not affect discolouration either.
Discolouration was less evident when crops were transplanted as late as July but became more evident with increasing maturity of the heads at the time they were processed. Overall, agronomic procedures had little impact on tissue discolouration and the main influence was the maturity of the lettuce heads at harvest.
The Influence of Agronomic Factors on the Visual Quality of Field-grown, Minimally Processed Lettuce by Hilton, Clifford, Wurr and Burton (2009). Journal of Horticultural Science & Biotechnology 84 (2): 193-198. The authors' abstract of their manuscript can be seen in full at www.jhortscib.com.