Responses of yield, fruit quality and water relations of sweet pepper in Mediterranean greenhouses to increasing salinity

Increasing salinization of groundwater used for irrigation is a current and growing problem for vegetable production in greenhouses in the Mediterranean Basin. Vegetable growers in this system require clear information of crop response to salinity, particularly of threshold values. For sweet pepper in Mediterranean greenhouses, this work (i) evaluated the effect of increasing salinity on various parameters of crop yield, fruit quality and plant water relations, and (ii) determined threshold and slope values for the Maas and Hoffman (M&H) model. Sweet pepper was grown with nutrient solutions (ns) with electrical conductivities (EC) of 2.5 (T1), 3.5 (T2), 4.5 (T3), 5.5 (T4), 6.5 (T5) and 7.5 (T6) dS m−1. There were substantial and statistically significant reductions in total and marketable yield, dry matter production (DMP) and average fruit weight, with increasing salinity. In treatment T4 (ns of 5.5 dS m−1), the respective reductions for these parameters, compared to T1 (ns of 2.5 dS m−1), were 37%, 37%, 47%, 52% and 30%. Applying the M&H model, soil solution EC threshold (ECsst) values for total and marketable yield were 4.8 and 5.0 dS m−1. The respective slope values were − 8.0 and − 12.0%/dS m−1. The more rapid decline in marketable yield was influenced by a large increase in the incidence of blossom end rot (BER) with increasing salinity. In treatment T1, 25% of discarded fruit had BER which increased to 70% in T6. Average fruit size was affected by salinity, but fruit number was not. Most fruit quality parameters were unaffected by increasing salinity. However, fruits were more red and more yellow. Leaf water potential (ΨW), leaf osmotic potential (ΨO), stomatal conductance (gS) and leaf turgor potential (ΨP) followed the Maas and Hoffman model, with ΨW, ΨO and gS declining and ΨP increasing after the EC threshold value. The ECsst value for ΨW was 5.0 dS m−1, and for ΨO, gS and ΨP was 5.6–5.8 dS m−1. Considering all data, an ECsst value of 5.0 dS m−1 is suggested for sweet pepper grown in Mediterranean greenhouses.