Open-cycle drip vs closed-cycle subirrigation: Effects on growth and yield of greenhouse soilless green bean.

Closed-cycle management of soilless cultivation is one of the promising strategies to prevent pollution, while increasing the efficiency of water and fertilizer use. However, in order to encourage the adoption of closed-cycle cultivation even in countries characterized by a low-tech greenhouse industry, effective easy-management systems need to be developed. A greenhouse experiment was carried out to determine the influence of two irrigation methods (drip irrigation open-cycle and subirrigation closed-cycle) and two nutrient solution (NS) concentrations (100 and 80% of the macro-element concentration normally used for soilless cultivation) on substrate electrical conductivity (EC), plant growth, gas exchange, yield, fruit quality, and water-use efficiency of green bean (Phaseolus vulgaris L., cv. Saporro). Subirrigated plants showed higher substrate EC and higher Na+ and Cl- concentrations, especially in the upper layer of the growing medium, while moisture distribution throughout the substrate profile showed an opposite trend and was more uniform in drip irrigation. The total fresh and dry biomass of the drip-irrigated plants was higher than in those grown in subirrigation, while whole plant dry matter percentage was lower. Differences in terms of plant height, number of leaves, leaf area, root growth, chlorophyll content, net photosynthesis, stomatal conductance and transpiration rate were recorded, with higher values in drip-irrigated plants. With closed-cycle subirrigation, green bean total and marketable yield (598 and 574 g?plant-1, respectively) were on average 33% lower than for drip irrigation, but fruit quality was higher in terms of "extra class" number of pods (93.6 vs 88.3%) and dry matter percentages. On average, both total and marketable yield were 9% higher with NS 80%, with a 13% higher water consumption, while slight or no effects were observed on net photosynthesis, stomatal conductance and shoot growth. The highest WUE (11.2 kg.m-3) was obtained with subirrigation, where recirculating NS was never discharged, compared to drip irrigation (8.5 kg.m-3). The green bean response to closed-cycle subirrigation showed a general decrease in productive performance. Detrimental effects were reduced by reducing the NS concentration. Lower yield could be compensated by savings of water and fertilizers and lower environmental impact.