Growth Response of ‘Grand Rapids’ Lettuce and ‘First Lady’ Marigold to Increased Far-red and Infrared Radiation under Controlled Environments1

Lettuce ( Lactuca sativa L. cv. Grand Rapids) and marigold ( Tagetes erecta L. cv. First Lady) plants were grown in controlled-environment chambers at equal levels of photosynthetically active radiation but with varying amounts of far-red and infrared radiation. Levels of FR and IR were obtained by varying the incandescent level in each chamber to provide 10, 18, 25, or 31% of the total input wattage for lettuce and 17 or 30% for marigold. In lettuce, increasing INC level from 10 or 18% to 31% of the total input wattage markedly increased stem elongation, node number, and fresh and dry weight of tops. In marigold, increasing INC level from 17 to 30% significantly increased the fresh and dry weight of tops, but had no effect on stem length or node number. Mean relative growth rate was unaffected by INC level in either species.