Calcium localization and tipburn development in lettuce leaves during early enlargement

Tissue concentrations of Ca, Mg, and K were determined across immature leav es of lettuce (Lactuca sativa L. 'Buttercrunch') at different stages of enl argement using electron microprobe x-ray analysis. The analysis was with a wavelength dispersive spectrometer to permit detection of low concentration s of Ca. Patterns of mineral accumulation in immature leaves that were expo sed were compared to patterns of accumulation in leaves that were enclosed within a dec-eloping head. The leaves developing without enclosure were fre e to transpire and developed normally whereas leaves developing with enclos ure were restricted in transpiration and developed an injury that was chara cteristic of Ca deficiency. In the exposed leaves, Ca concentrations increa sed from an average of 1.0 to 2.1 mg.g(-1) dry weight (DW) as the leaves en larged from 5 to 30 mm in length. In the enclosed leaves, Ca concentrations decreased from 1.0 to 0.7 mg.g(-1) DW as the leaves enlarged from 5 to 30 mm in length. At the tips of these enclosed leaves a larger decrease was fo und, from 0.9 to 0.3 mg.g(-1) DW during enlargement. Necrotic injury first became apparent in this tip area when the concentration was approximate to 0.4 mg.g(-1) DW. Magnesium concentrations across the exposed leaves were si milar to concentrations across the enclosed leaves, and did not change with enlargement. Magnesium concentrations averaged 3.5. mg.g(-1) DW in both en closed and exposed leaves during enlargement from 5 to 30 mm, In both expos ed and enclosed leaves, IC concentrations increased during enlargement from 40 to approximate to 60 mg.g(-1) DW. Potassium concentrations were highest toward the leaf apex and upper margin where injury symptoms occurred, and this may have enhanced injury development. This research documents the crit ical low levels of Ca (0.2 to 0.4 mg.g(-1) DW) that can occur in enclosed l eaves of plants and which apparently leads to the marginal apex necrosis of developing Leaves seen frequently on lettuce and other crops.