The effects of phosphorus supply limitation on photosynthesis, biomass production, nutritional quality, and mineral nutrition in lettuce grown in a recirculating nutrient solution

In this study, we investigated the impact of phosphorus (P) supply levels (0.8, 1.3 and 1.8 mM) in two green leaf lettuce (Lactuca sativa L.) types, namely ‘Romaine’ (cv. Nader), and ‘Lollo’ (cv. Bionda) grown in a recirculating nutrient solution. The biomass yield of the Romaine type was reduced by 15% at the lowest P supply (0.8 mM) and plants showed reduced photosynthetic function (i.e., net photosynthesis, stomatal conductance, quantum yield and electron flow) and increased root/shoot ratio. On the other hand, the Lollo type maintained photosynthetic rates and biomass accumulation in all cases and proved less sensitive to low P levels in the NS. At the lowest P supply, the P concentration in the recirculating solution declined to 0.1 mM and showed a decline in leaf P and Ca concentrations and an increase in total sugar and nitrate content, depending on the cultivar. Also, lowering P supply boosted P-use efficiency (kg biomass kg−1 P supply) in both types of lettuce. Nutrient to water uptake ratios of macro nutrients N, P, K, Ca and Mg were determined as follows: (i) 16.0, 1.3, 9.1, 3.1 and 0.9 mmol L−1, respectively for Romaine, and (ii) 16.4, 1.3, 9.1, 3.3 and 0.9 mmol L−1, respectively for Lollo. The current experimental results suggest that, reducing the P supply to lower levels than those currently recommended for lettuce crops grown in closed hydroponics considerably improves the P use efficiency in Mediterranean greenhouses without compromising yield.