GROWTH, YIELD, AND MINERAL CONTENT OF BUTTERHEAD LETTUCE (LACTUCA SATIVA VAR. CAPITATA) GROWN IN NFT

To improve greenhouse lettuce yield and quality and avoid enviromental pollution it is necessary to optimize plant nutrition. Plants uptake nutrients at different ratios during the growing period. Therefore, total N, K+, Mg2+, and Ca2+ accumulation and NO3-, NH4+, K+, Mg2+, and Ca2+ uptake were evaluated during greenhouse growth of two cultivars of butterhead lettuce (‘Mindoro’ and ‘Tibet’) in a discontinous closed loop NFT sstem. Growth rate and water consumption were also measured. In the last 20 days of the growing cycle, lettuce plants produced 70% of the shoot dry weight and 60% of the leaf area obtained in the enitre growing period. From 30 DAT to harvest, the shoot dry matter content decreased from 7.0 to 5.1 g·100 g-1 fw in 'Mindoro', and from 7.0 to 6.0 g·100 g-1 fw in 'Tibet'. From 30 to 50 DAT, both cultivars had increasing content of leaf total N up to 4.3 and 4.6 g·100 g-1 dw in 'Mindoro' and 'Tibet', respectively, when RGR was still high. Thereafter, and up to harvest the total N concentration decreased more markedly in ‘Mindoro’ (14%) than in 'Tibet' (6%) while simultaneous dry biomass production was peaking. Similar variations with time were found for K+, Ca2+, and Mg2+ leaf contents. 'Mindoro' had higher N use efficiency (NUE) than 'Tibet'. Leaf NO3 content was moderate (1,622 mg·kg-1 fw) and 80% of total N was in reduced form. During the whole growing cycle, 5.5 L/plant of water was used with a water use efficiency (WUE) of 12.4 dw g·L-1