Effects of relative humidity and root temperature on calcium concentration and tipburn development in lettuce.

Growth chamber studies were undertaken with a tipburn-sensitive cultivar of romaine lettuce (Lactuca sativa L. cv. Lobjoits Green Cos) grown under a photosynthetic photon flux density of 320 micromoles s-1 m-2 for 16 hours; light and dark temperatures were 26.0 degrees and 12.5 degrees C, respectively. As the relative humidity (RH) during the light period was decreased from 74% to 51%, growth was retarded, Ca concentration increased, and the onset of tipburn delayed. Decreasing RH during the dark period from 95% to 90% reduced growth and resulted in lower Ca concentrations and earlier tipburn development. Further decreases from 90% to 65% caused no additional change in growth or tipburn response. Root temperatures of 23.5 degrees, compared with 15.0 degrees, slightly increased Ca concentration but induced earlier tipburn development. Ca concentrations were increased and tipburn delayed by humidity conditions which provided large diurnal fluctuations in water potential in the plant and which encouraged root pressure flow during the dark period. Elevated root temperatures did not provide expected increases in Ca accumulation in young leaves.