Water use and crop performance of two wild rocket genotypes under salinity conditions

In literature, the parameters of salinity tolerance of the main cultivated species are known, but are missing for many minor species such as wild rocket, whose cultivation in many areas of southern Italy affected by salinity is growing. Therefore, a research has been carried out i) to evaluate the response to salinity in water use, water use efficiency, yield characteristics and morfological features, and ii) identify the salinity tolerance parameters of two genotypes of wild rocket: Diplotaxis tenuifolia (L.) DC and D. muralis (L.) DC. The study was carried out in the spring of 2007 and 2008 in Policoro (MT), southern Italy, under unheated plastic greenhouse conditions. Wild rocket was sown in plastic pots containing 20 dm3 of soil. For each genotype, six soil salinity levels were compared, obtained by accurately mixing before sowing the soil with 0.0, 0.5, 1.0, 2.0, 3.5 and 5.5 g dm-3 of NaCl + CaCl2 1:1 (on a weight basis). Irrigation was performed with fresh water having electrical conductivity of 0.5 dS m-1. In each year, 3 harvests were performed; water use and the main production and plant growth parameters were recorded. D. tenuifolia provided a yield 47.3% higher than D. muralis. By rising salinity, progressive decline in marketable yield and growth of the leaves was recorded, while the dry matter content increased. The increase in salinity has led to the progressive reduction of water use in both genotypes. From moderate salinity values (about 5.5 dS m-1), the reduction in yield water use efficiency as a result of increased salinity has been observed. In addition, salinity reduced specific leaf area and increased leaf succulence. Both genotypes rank among moderately salt sensitive species, according to Maas and Hoffman's model (1977). However, D. tenuifolia, with a critical threshold of 1.98 dS m-1 and a slope of 6.61% m dS-1, showed a slightly higher tolerance than D. muralis (threshold 1.34 dS m-1 and slope 7.25% m dS-1). Reduction in yield due to salinity occurred mainly for the decrease in leaf size and, secondly, number of leaves.