Effect of Hydroponic System Type on Growth and Nutrient Uptake of Lettuce (Lactuca sativa ‘Rex’) Irrigated with Aquaculture Effluent

Abstract

Aquaponics (AP) combines the use of traditional hydroponic (HP) practices with aquaculture effluent which is used to irrigate and provide necessary nutrients to the plants. Current methods of HP production include Nutrient Film Technique (NFT), deep water culture (DWC) and substrate culture (SC). Currently there are few papers detailing differences in production based on system type. Thus, there is a need for research to help bolster and expand the knowledge base for aquaponic lettuce production. The objective of the first experiment was to assess the effect of HP system type on the growth and nutrient uptake potential of lettuce irrigated with sole aquaculture effluent. It was hypothesized that a system type utilizing a substrate could possibly allow for increased foliar nutrients when compared to other HP systems in low stocking conditions. Three different hydroponic system types (NFT, DWC and SC) were arranged in a randomized complete block design. The trial lasted a total of 45 days from start of germination to harvest. Analyses of foliar quality, nutritional content, yield, and water nutritional content were conducted at the end of the growth cycle at harvest. Results from foliar analyses showed SC had higher uptake of nitrogen, magnesium and copper when compared to DWC and NFT systems. Substrate culture and NFT reported higher fresh head weight than DWC in the first trial, while SC reported the highest fresh weights in contrast to the other system types in the second trial. The objective of the second experiment was to assess the effect of HP system type on the growth and nutrient uptake potential of lettuce irrigated with aquaculture effluent supplemented with fertilizer. It was hypothesized that a system type utilizing a substrate could possibly allow for increased plant foliar quality when compared to other HP system types at recommended nutrient levels in irrigation solution. The three different hydroponic system types were arranged in a randomized complete block design. This trial lasted a total of 45 days from seeding to harvest. Analyses of foliar quality, nutritional content, yield, and water nutritional content were conducted at the end of the growth cycle at harvest. Fresh head weight was highest in SC and NFT systems and lowest in DWC. Leaf chlorophyll estimates were highest in SC and DWC systems and lowest in NFT. Stomatal conductance to water vapor (gsw), vapor pressure deficit (vpd), leaf apparent transpiration (E), photosystem II efficiency, and electron transport rate (ETR) were shown to be similar between all system types. Lettuce cultivated utilizing SC reported the highest foliar accumulation of magnesium and calcium. Plants grown in DWC systems reported increased foliar accumulation of potassium when compared to SC yet was similar to NFT levels. All system types were similar in foliar concentrations of nitrogen, phosphorous, sulfur, boron, iron and copper.