Effect of temperature on composition of hydroponic lettuce

Temperature changes the composition of plants through its effects on metabolism and nutrient uptake. Hydroponic lettuce growing in naturally sunlit greenhouses was subjected to two temperature treatments. Minimum temperatures were 20 or 10 degrees C, and ventilation temperatures were 26 and 16 degrees C, for warm and cool treatments, respectively. Tissue composition data for three plantings were subjected to analysis of variance with fixed effects of warm or cool temperature, high or low daily light integral, and am or pm harvest time. Plant size was included as a covariate. Warm compared to cool temperature increased relative growth rate by 85% for small plants of 3 to 10 g fresh weight (FW), compared to 30% for medium plants of 40 to 89 g FW. Warm temperature increased specific leaf area for plants grown under a low light integral of 4.5 mol m(-2) d(-1), compared to a high light integral of 15 to 17 mol m(-2) d(-1). Dry matter content was 20% greater for plants grown under cool compared to warm conditions. Total reduced nitrogen was 7% less under cool temperature. The concentration of nitrate was 40% lower, while the concentration of sugars was about 50% higher, under cool compared to warm temperature. Cool temperature increased the tissue concentrations of malic acid and potassium, but had no effect on phosphorus.