Drip fertigation enhances the growth of hydroponic lettuce (Lactuca sativa) using polyester fiber substrate

Abstract The nutrients that are supplied in hydroponic culture using certain substrates such as polyester fiber substrate (PFS) are sometimes unable to reach the root surfaces, but few studies have investigated on the effective fertigation methods. Therefore, in the present study we compared the growth of lettuce (Lactuca sativa ‘Sunny lettuce’) plants that were cultivated hydroponically using PFS or rockwool with nutrient solution using drip fertigation and bottom fertigation. We found that drip fertigation, which provides a continual supply of water and nutrients by circulating the nutrient solution into the medium from the bottom of the container, resulted in a higher plant growth rate than bottom fertigation in all three repetitions, for example, with average shoot fresh weights of 59.0 g for the control (bottom fertigation), 41.6 g for the control with air supply, and 112.7 g for continuous drip fertigation 35-days after treatment. Furthermore, the water absorption rate per plant per day was higher with the drip fertigation treatment compared with the control treatment through the entire period across the three replications. The net photosynthetic rate was always higher in the drip fertigation treatment in all light intensities than other two treatments and was positively correlated with stomatal conductance. In addition, the stomatal densities were also higher with the drip fertigation treatment (drip fertigation, 85.7 mm−2; control, 58.2 mm−2; control with air supply, 46.6 mm−2), suggesting that the increased photosynthetic rate with the drip fertigation treatment was caused by an increase in stomatal density. Together, these findings indicate that the provision of water and nutrients to plants by drip fertigation increases the stomatal density resulting in a higher photosynthetic rate.