Shade cloths and polyethylene covers have opposite effects on tipburn development in greenhouse grown lettuce

Spring-summer production of lettuce under greenhouses is highly affected by Tipburn development, a physiological disorder attributed to localized calcium (Ca) deficiency. As Ca is distributed along the plant tissues together with the water flux, Tipburn development is related to environmental conditions. Until now, no growing condition that avoids this disorder, without an associated yield penalty, has been clearly identified. In this work we used shade cloth (SC) or polyethylene (PE) covers in a greenhouse to modify the growing environment of pot-grown lettuce and to examine the effect of these conditions on growth and Tipburn severity. Two levels of Ca in the soil were evaluated. PE generated a good environment for lettuce growth reflected in high dry mass. However, high Tipburn development occurred despite Ca concentration being higher than in leaves of control lettuces. SC generated a similar environment to control conditions but with a lower light intensity. Under this environment thy mass production was lower than in control plants and plants produced thin and elongated leaves. Under SC no Tipburn symptoms were registered irrespective of soil Ca level and leaf Ca content was higher than in control plants. Ca fertilization had no effect on Tipburn development in either of these treatments. These results reinforce the idea that Tipburn is a physiological disorder related to environmental conditions rather than Ca deficiencies. Results highlight that 1- the use of Polyethylene as SC increased Ca content in lettuce leaves in respect to control plants but both treatments had opposite Tipburn incidence; 2- SC significantly reduced dry weight of plants and improved Ca distribution towards leaf margins; and, 3- Ca fertilization had no effect on Tipburn development in any of the treatments. Overall these results reinforce the idea that Tipburn is a physiological disorder related to environmental conditions.