Rise of nightly air humidity as a measure for tipburn prevention in hydroponic cultivation of butterhead lettuce

The physiological disorder tipburn remains a constant threat for growers of butterhead lettuce in greenhouses. In standard soil cultivation, tipburn is controlled reasonably well by using traditional growing methods, combined with the selection of tolerant cultivars. However, when applying novel growing techniques such as hydroponics, current practices to prevent tipburn show to be no longer sufficient. Therefore, a research project was started up, aiming at the development of a more rational tipburn prevention strategy. In the first year, tipburn occurrence was studied in successive soil-grown lettuce crops at two locations. Out of several environmental factors, which might induce tipburn according to literature, low nightly air humidity (RH.) appeared to be the most important one in commercial growing conditions. Several successive experiments with hydroponic lettuce (NFT) confirmed that maintaining RHn at a high level is an efficient measure to decrease tipburn occurrence. Whereas 60 to 100% of the control plants was affected by tipburn, maintaining RHn above 95% markedly reduced tipburn injury to 3 to 50%, respectively. This effect was associated with an improved Ca transport to the leaf margins (1 cm zone) of young lettuce leaves and values higher than 2-3 g Ca kg(-1) DM were obtained. The most efficient technique to decrease tipburn occurrence by raising RHn was the combination of water spraying underneath the gullies with nebulization above the plants. Second best techniques were the mere spraying of water under the gullies and covering the plants with plastic foil