The use of legume and grass cover crops induced changes in ion accumulation, growth and physiological performance of young olive trees irrigated with high-salinity water

Highlights

• The effects of different cover crops (legume and grass) on olive trees performance irrigated with salinity water (120 mM) were assessed.

• When intercropped with legume, root: leaf sodium and leaf nitrogen of olive trees increased significantly.

• The Legume cover crop induced a increased shoot growth and improve the physiological performance of olive trees.

Abstract

The impact of intercropping with two different forage crops (legume and grass) on the shoot growth, ion accumulation and some physiological traits of 3-old year olive trees irrigated by high-salinity water (120 mM NaCl) was assessed under greenhouse conditions. Olive plants were subjected to one of the following treatments: TC (control); TG – olive plants intercropped with grass (Oats: Avena sativa L) and TL – olive trees intercropped with legume (Berseem Clover: Trifolium alexandrinum L). Results showed that sodium root:leaf ratio and nitrogen concentration in olive leaves were significantly increased in TL treatment. Olive shoot length and maximum quantum yield of photosystem II (Fv/Fm) were significantly improved by using Berseem Clover as a cover crop. Negative correlation was observed between the total chlorophyll content and the leaf Na⁺ concentration. On the other hand, polyphenol content was significantly increased in olive leaves when oats were used as cover crops, in relation to the high N-deficiency observed in olive leaves of TG treatment.

Introduction

Salt affects one third of the worlds irrigated surface, especially in arid, semiarid and coastal regions (Munns, 1993; Tabatabaei, 2006). Saline areas occupy 1 billion ha and are increasing worldwide (Munns, 1993). Salinity problems in crop production will become worse in areas with rapidly growing human population and limited water resources, which force growers to use poor quality water for irrigation. Soil salinity is a serious threat limiting crop production since it adversely reduces the overall productivity of the ecosystem (Munns, 2002).

In Tunisia, nearly 100 000 ha of irrigated lands were deeply affected by salinization and 75% of the soils were in the range of medium to highly sensitive to salinity. Irrigated lands by saline water were mostly occupied by forage crops resistant to salinity. Forages produced by irrigation with saline water provide additional income sources for farmers in marginal lands (Stenhouse and Kijne, 2006). In Tunisia in areas irrigated with saline water (4–7 g/l) the most commonly used forage species were Berseem and Oats. Agarwal et al. (2010) reported that most salt-tolerant genotypes of Berseem (Trifolium alexandrinum L.) were sometimes used in salinized areas where they showed significant reductions in biomass productions only at salinity levels of 70 and 140 mM NaCl. According to Murty et al. (1984), Oats (Avena Sativa L.) are considered to be a moderately salt-tolerant species compared with other cereal or forage crops. More recently NSW Department of Primary Industries (2017) found that oats show significant reduction in biomass when salinity exceeds 40 mM.

In a large Tunisian agricultural area, olive is the most cultivated fruit crop, with approximately 70 million trees covering 1600 thousand hectares of land. Traditionally, olive tree has been grown under rainfed conditions, since it is a crop well adapted to the semiarid and arid Mediterranean climates, and able to overcome periods of intense drought, while still producing a reasonable yield. However, the surface of irrigated olive orchards has increased considerably during the last years, motivated by the improvement in the yield when irrigated, and also by the spectacular increase of olive oil prices. But, increasing irrigated areas is very difficult for the olive industry, due to water scarcity and the increased competition with non agricultural uses (Fereres et al., 2003). Olive trees (Olea europaea L.) that are broadly cultivated in the Mediterranean region have an intermediate tolerance to salinity stress. In fact, the exposure to high sodium for a prolonged period of time severely reduces the yield (Gucci and Tattini, 2010; Fernández, 2014). Tolerance to NaCl in olive trees is mostly related to the salt exclusion mechanism at the root level, which prevents Na⁺ accumulation in leaf tissues, as well as to the ability of the olive tree to maintain an adequate K⁺/Na⁺ ratio (Chartzoulakis, 2005; Kchaou et al., 2010). When olive is irrigated with water containing NaCl, growth reduction and onset of damage are more correlated to Na⁺ than Cl⁻ accumulation in leaves (Kchaou et al., 2010; Bader et al., 2015). As a result, the intercropping of olive trees with salt-tolerant forages may be a promising solution to increase olive productivity when irrigated with saline water.

The aim of this work was to investigate the effect of intercropping olive trees with two forage crops (grass: Oats, and legume: Berseem Clover) on sodium and nitrogen accumulation in olive roots and leaves of Chemlali olive cultivar, irrigated with high salinity water that is frequently used in the arid region of Tunisia. Furthermore, we evaluated changes in some plant physiological (Fv/Fm, chlorophyll content, total phenols) and growth (shoot length) traits induced by the cover crops. To our knowledge, this is the first report that addressed the above issues, testing the possibility to manage ecologically salinity and increasing saline water productivity.