CRITICAL CHLOROPHYLL, TOTAL NITROGEN, AND NITRATE-NITROGEN IN LEAVES ASSOCIATED TO MAXIMUM LETTUCE YIELD

An experiment was conducted to establish chlorophyll, nitrate-nitrogen (NO3-N), and total nitrogen (N) critical concentrations in lettuce (L actuca sativa) leaves that are associated with maximum dry matter yiel d. Six N levels (0, 60, 120, 240, 480, and 960 mg kg(-1) soil) as ammo nium nitrate (NH4NO3) were applied in a randomized block design with f our replications. Two wrapper leaves were collected at the eighth-leaf stage and analyzed for their chlorophyll a + b and total N contents. Forty-five days after transplanting, plants were harvested, dried, and analyzed for their NO3-N and total N contents. Critical levels associ ated with maximum lettuce yield at the eighth-leaf stage were: 1,837 m g chlorophyll kg(-1) fresh weight and 4.27 g N 100 g(-1) dry matter, a nd at harvest: 6,361 mg NO3-N kg(-1) dry weight and 3.75 g N 100 g(-1) dry matter. In the soil, N levels associated to maximum plant dry wei ght were 34 mg NO3-N kg(-1) and 390 mg N kg(-1).