It is customary that growers apply high doses of nutrients to the soil in order to achieve high yields, with detrimental
consequences for the environment; but no information exists with regards to the crop response to high fertilization levels.
To evaluate the physiological response of lettuce (
Lactuca sativa
L.) to various root zone nutrient concentrations (expressed
as electrical conductivity, from 0.6 to 10 dS m
-1), a replicated experiment was conducted using increasing concentrations
of macronutrients applied to the root zone in an aeroponic system. Leaf photosynthesis and chlorophyll fluorescence
were measured using a portable infrared gas analyzer attached with a fluorometer. Leaf nutrient content was analyzed by
mass spectrometry and NO
3-N was determined by flow injection analysis. Leaf photosynthetic rates increased when the
solution concentration was raised from 0.6 to 4.8 dS m
-1, but further increases in solution concentration did not result in any
differences. The enhancement in photosynthetic rates was related to higher concentrations of N, P, Mg, and S in leaves. Leaf
K content was correlated with stomatal conductance. Maximum growth was achieved with solution concentrations between
1.2 and 4.8 dS m
-1 while at 10.0 dS m
-1 leaf production was reduced by 30%. It is concluded that at high concentration of
nutrients supplied in the root zone, yield reduces because of a combination of decreased stomatal conductance and leaf area.