Growth, nutrient content and tipburn in lettuce (Lactuca sativa L. 'Frillice') in response to light intensity and aerial environment

Abstract

The incidence of tipburn in the vegetable cultivation industry has been a major challenge to the Norwegian growers particularly to the growers of Frillice lettuce (Lactuca sativa L. ‘Frillice’). In periods, Norwegian growers can have up to 20% loss due to tipburn in greenhouse production of ‘frillice’. This thesis aim is to investigate how climate factors, including light, relative air humidity and CO2 are influencing tipburn occurrence and severity in lettuce (Lactuca stiva L. ‘Frillice’) and the role of nutrient especially calcium in this disorder to further broaden the understand to already existing studies done. It focuses on the role played by climate and to identify better cultivation practise that can help to control the tipburn incidence. Common assumptions by a number of researchers has attributed this condition to various abiotic stress factors including high light intensity, elevated relative air humidity and temperature as well as nutrient deficiency like calcium (Ca). The Ca deficiency is coupled with transpiration limitation as a major contributor to this physiological disorder. Tipburn in lettuce has been described as a necrotic condition occurring in the outer margins. To better understand this condition, several manipulations of climate condition were tested in controlled growth chambers to assess the level to which each factor would influence tipburn occurrence in ‘Frillice’ lettuce. Two lamp types: white LEDs (light emitting diodes) and HPS (High pressure sodium) were used and effects of light intensity, from low (150µmolm-2s-1) to high (300µmolm-2s-1) and light quality (additional far-red), elevated relative air humidity (RH) at night and elevated CO2 was investigated. Day temperature of 18℃- and 20℃-night temperature was kept constant throughout all experiments. Elevated RH during night (90% compared with 70%) and elevated CO2 (400ppm – 1000ppm) were tested. Nutrient analyses were conducted for plants exposed to 150µmolm-2s-1 on both source (outer) and sink leaves (inner) it was measured N, C, Ca, Mg, and K to assess if a relationship exist between nutrients, especially Ca, and tipburn. An analysis of antioxidant capacity (FRAP) was also performed on source and sink leaves and roots to assess the level of antioxidants in plants grown at high RH during night to verify if there exist any relationship between FRAP and tipburn occurrence. High light intensity generally increased the severity of tipburn under all climate conditions. In general, white LED increased outer tipburn under moderate light compared with HPS but under high light intensity HPS induced more tipburn than white LED. Additional far-red light did not strongly influence on the incidence of tipburn. Treatments with elevated RH during night resulted in the strongest reduction in severity under both moderate and high light conditions, with HPS and LED as light source. Elevated CO2 reduced tipburn severity in both LED and HPS, but the effect was strongest in HPS. Lower calcium content was found in the sink leaves compared with source leaves in all experiments, but no correlation was seen between calcium and tipburn. Antioxidant capacity measured in leaves and roots from the experiment with elevated RH during nigh did not show any correlation with tipburn incidence and is not a good indicator for tipburn