Leaf Lettuce Yield Is More Sensitive to Low Daily Light Integral than Kale and Spinach

Kale, leaf lettuce, and spinach were grown for 28 days in growth chambers under daily light integrals (DLI) of 8, 10, 12, and 14 mol·m −2 ·d −1 . Fresh weight (FW), dry weight (DW), leaf area, and chlorophyll concentration were measured. Increasing DLI positively influenced lettuce FW; an increase from 1.27 g/plant to 4.33 g/plant was measured. DW for all species increased in a linear fashion under increasing DLI, with lettuce increasing 203%, kale 47%, and spinach 42% as DLI increased from 8 to 14 mol·m −2 ·d −1 . Leaf area response was species-dependent, with lettuce leaf area increasing under increasing DLI while kale leaf area decreased under higher DLI. Chlorophyll levels in kale leaves decreased from DLI of 8 to 12 mol·m −2 ·d −1 , and then increased to 14 mol·m −2 ·d −1 DLI. Chlorophyll content in kale leaves had a nonlinear response to DLI and the best fit was with a quadratic model. Growers wanting to add supplemental lighting can expect the greatest gains in lettuce yield compared with those of kale and spinach.