Variation in elemental content of ‘grand rapids’ lettuce grown under four levels of far‐red and infrared radiation in controlled environments

Lettuce (Lactuca sativa L. ‘Grand Rapids') plants were grown in four separate controlled‐environnent chambers under equal levels (325 μEm‐2s ‐1) of photosynthetically active radiation (PAR) but with various amounts of infrared (IR) radiation, obtained by varying the wattage of incandescent (INC) lamps in each chamber from 25 to 100 W to provide 10 to 31% of the total input wattage. Plants were grown for 28 days under a specified set of base‐line environmental conditions. Sequential harvests were made at 7, 14, 21, and 28 days. Plant tissue was collected at 28 days and analyzed for 25 elements by arc emission spectrography. These included 10 essential elements (P, K, Ca, Mg, Fe, Mn, B, Zn, Cu, Mo) and 15 non‐essential elements (Al, Si, Ti, Sr, Ba, Na, Pb, V, Li, Sn, Co, Ni, Cr, Ag, and Cd). Increasing the amount of INC wattage in the growth chamber from 10% of the total input wattage to 18 or 25% decreased the Mn content and increased the P, B, Pb, and Na contents of lettuce leaf tissue; a further increase in INC wattage from 18 or 25% to 31% increased the Mn content and decreased the P, Mo, and B contents based on Duncan's multiple range test. Except for P, Mn, and Ko, spectral quality treatment made no significant contribution to the overall variance based on the F test. These findings are in contrast to the growth responses of lettuce plants which increased markedly with increased levels of FR and IR radiation, and suggest that this species can adapt to a wide range to FR and IF levels without being affected in its mineral composition. Examination of variance components indicated that most of the variation was contributed by plant‐to‐plant variation and/or analytical error.