Macronutrient uptake in lettuce (Lactuca sativa L.): Influence of photosynthesis, solution concentration and time of supply

The absorption of N, P, K, Ca, Mg and S was studied in lettuce (Lactuca sativa L. cv. `Black Seeded Simpson') to generate information for the development of a mathematical model to predict the uptake of these nutrients under greenhouse conditions. The diurnal uptake patterns were studied in growth chamber experiments under three air temperatures (10, 25 and 35°C) at two light levels (600 or 1200 micromol m-2 s-1). The rates were analyzed at various plant ages in a greenhouse experiment. The analysis of the hourly uptake rates showed no marked differences between the hours of the day, mainly due to the high variability in the uptake rates among replicates. No differences were found in the absorption between daytime versus nighttime. However, increasing air temperature and light level drove higher total daily uptake rates. The uptake rates of N, P and K declined with plant age following a reduction in plant growth rate. When the uptake of each individual ion was compared to the whole-plant CO2 assimilation rate a significant curvilinear relation was found. The relationships of nutrient uptake with relative growth rate were different for each nutrient, where NO3-, H2PO4- and K+ showed a linear relation, whereas NH4+, Ca2+, Mg2+ and SO42- uptake was related in a curvilinear fashion with RGR. The response of plant growth and leaf photosynthesis to various concentrations of nutrients in the rootzone was also studied in aeroponic culture, under the hypothesis that high oxygen available to the roots would increase plant tolerance to high electrical conductivity (EC) levels. A significant interaction between leaf temperature and rootzone EC was found, where at temperatures below 20°C the rootzone nutrient concentration (and leaf tissue nutrient concentration) had no effect on single leaf photosynthetic rates but when the temperature is increased up to 40°, higher concentrations of nutrients stimulated higher photosynthetic rates. Plants grown in aeroponic culture with high concentration of nutrients (equivalent EC of 12.0 dS m-1) showed a ~30% reduction in plant growth compared to the best treatment (half strength Hoagland's solution, EC: 1.2 dS m-1) and no differences were found in leaf NO3- content. To study the response of plants to uneven supply of nutrients during a day cycle an experiment was conducted in aeroponics where two treatments differing in the nighttime solution concentration versus that applied during the daytime were compared to a third treatment with constant supply of nutrients. It was found that a high difference in the concentration between day and night reduced plant nutrient accumulation and root growth but not total plant growth. The concentration of leaf NO3- was reduced by ~75% when the supply of nutrients was higher during the night than during the day. The information collected in these experiments was compiled into a mathematical model that calculates nutrient demand from photosynthetic and plant growth rates. The model was able to accurately predict the demand for N, P and K in greenhouse conditions, as well as N in field conditions.