GROWTH, NUTRIENT-UPTAKE AND TIPBURN SEVERITY OF HYDROPONIC LETTUCE IN RESPONSE TO ELECTRICAL-CONDUCTIVITY AND K-CA RATIO IN SOLUTION

A commercial hydroponic nutrient solution formulation was developed to reflect the high K:N uptake of a range of vegetable crops (Huett and Dettmann, Aust. J. Agric. Res. 1992, 43, 1653-65). This standard formulation had a K:N ratio of 1.7:1.0 and a K:Ca ratio of 1.25:1.00 (equivalent weight basis). Head lettuce cv. Coolguard and cv. Fame and non-heading cv. Red Mignonette were grown in recirculating culture to maturity with the standard formulation to examine the effect of electrical conductivity (EC) (0.4-3.6 dS m(-1)) and, for the latter two cultivars, nutrient formulation K:Ca ratio (from 1.00:3.50 to 3.50:1.00) on growth, nutrient uptake and tipburn severity. Plants grown at an EC of 0.4 dS m(-1) were N and K deficient, while recently matured leaves and youngest leaves contained the highest Ca concentrations which decreased with increasing EC. When cv. Coolguard was grown at an EC of 1.0 dS m(-1) N and K deficiency was eliminated and leaves contained the highest Ca concentration. Maximum fresh weight of leaf and head was recorded at 1.6 dS -1 and the decline in nutrient solution N and K concentrations over the growth period was 13 and 42% respectively. An increase in the K:Ca ratio of the formulation increased (P < 0.05) leaf by 13% and leaf+head fresh weight at maturity by 10% for cv. Fame whereas, for cv. Red Mignonette, a reduction in the K:Ca ratio increased (P < 0.05) leaf fresh weight by 29 degrees compared with the standard formulation. A reduction in the K:Ca ratio increased (P < 0.05) the Ca concentration in the youngest leaves of cv. Fame and of cv. Red Mignonette at 1.6 and 3.6 dS m(-1). Leaf K concentrations were generally reduced (P < 0.05) by low EC and low K:Ca ratio. Hot conditions led to tipburn developing in cv. Red Mignonette at the 2 week harvest. The number of leaves plant(-1) with tipburn at maturity (week 3) was reduced (P < 0.01) from 23.1 to 4.4 as EC was reduced from 3.6 to 0.4 dS m(-1) and was reduced (P < 0.05) from 15.2 to 12.3 as the nutrient formulation K:Ca ratio was reduced from 3.5:1.0 to 1.25:1.00. Over the last week of the growth period, the number of leaves with tipburn remained stable at an EC of 0.4 dS m(-1) whereas, at 3.6 dS m(-1), the number of leaves with tipburn increased by 253%. Tipburn developed in young leaves of cv. Red Mignonette which had a Ca concentration range from 1.7 to 3.2 g kg(-1) and was generally absent from recently matured leaves which had a Ca concentration of 11.0 g kg(-1). Young leaves of the tolerant cv. Fame had a Concentration of 5.9 g kg(-1)