Effect of air and root-zone temperatures on growth and quality of indoor hydroponic lettuce
Type of DegreeMaster's Thesis
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An experiment was conducted in an environmental control chamber to determine the effects of air and root-zone temperature on growth of hydroponic lettuce (Lactuca sativa L. ‘Rex’). For each replication, lettuce seeds (Lactuca sativa L. ‘Rex’) were sown and grew in the greenhouse for two weeks. The seedlings were transferred to environmental control chamber. There are six replications of lettuce cultivation. Three replications are under the condition set to 20°C air temperature and 65% relative humidity, and another three replications were under 24 °C air temperature and 70% relative humidity. Each replication had five nutrient solution temperature treatment (20, 22, 24, 26, and 28 ℃) and a control treatment which the plant root-zone was not artificially manipulated. Full spectrum LED lights were used to supply 15 mol m-2 d-1 of photosynthetically active radiation with a photoperiod of 18 hr d-1. During each replication, lettuces were harvested after 30 DAT (days after transplant), and head fresh weight (HFW), size index (SI), root dry weight (RDW) and root length were measured. Lettuce samples grown under 24 °C air temperature was also used to measure total phenolic content. Air temperature did not influence lettuce HFW, height, or root length. Root zone temperature (RZT) influenced lettuce HFW, which reached a maximum of 167 g at Actual root zone temperature (RZTA) 25.6 °C. Lettuce plants grown at RZTT of 22 °C, 24 °C, and 26 °C had 11%, 15%, and 11% higher HFW, respectively, than those grown under the control treatment. Lettuce SI was influenced by both TA and RZT. Plants grown at 24 °C TA having a 7% greater SI compared to those grown at 20 °C TA and SI displayed a quadratic relationship with RZTT. Lettuce head shape would change with different RZTs, and lettuce grow at RZTT 28 °C produced flatter head. The root system changed with increasing root zone temperature, from long, cylindrical shape to short, clustered shape. Since 25.6 °C produced the greatest HFW and relatively large-size head and 24 °C TA produced larger lettuce head, a 24°/25°C air/nutrient solution condition was recommended for butterhead lettuce production. In addtion, lettuce grown at 24 °C air temperature with different root-zone temperature treatments showed no significant difference on total phenolic content.