Comparative effect of salinity on growth, grain yield, water use efficiency, δ¹³C and δ¹⁵N of landraces and improved durum wheat varieties

Highlights

• Genotypic performance of durum wheat to salinity was studied under field conditions.

• Yield and WUE was higher in modern lines than landraces regardless salinity level.

• Modern lines exhibited shorter cycle and lower kernel δ¹³C and N than landraces.

• Kernel δ¹³C was negatively correlated with grain yield and WUE across genotypes.

Abstract

Supplemental irrigation with low-quality water will be paramount in Mediterranean agriculture in the future, where durum wheat is a major crop. Breeding for salinity tolerance may contribute towards improving resilience to irrigation with brackish water. However, identification of appropriate phenotyping traits remains a bottleneck in breeding. A set of 25 genotypes, including 19 landraces and 6 improved varieties most cultivated in Tunisia, were grown in the field and irrigated with brackish water (6, 13 and 18 dSm⁻¹). Improved genotypes exhibited higher grain yield (GY) and water use efficiency at the crop level (WUE_yield or ‘water productivity’), shorter days to flowering (DTF), lower N concentration (N) and carbon isotope composition (δ¹³C) in mature kernels and lower nitrogen isotope composition (δ¹⁵N) in the flag leaf compared with landraces. GY was negatively correlated with DTF and the δ¹³C and N of mature kernels and was positively correlated with the δ¹⁵N of the flag leaf. Moreover, δ¹³C of mature kernels was negatively correlated with WUE_yield. The results highlight the importance of shorter phenology together with photosynthetic resilience to salt-induced water stress (lower δ¹³C) and nitrogen metabolism (higher N and δ¹⁵N) for assessing genotypic performance to salinity.

Introduction

Durum wheat is one of the most cultivated herbaceous crops in the southern and eastern Mediterranean basin (www.fao.org/statistics/yearbook). These environments are characterized by ‘terminal stress’ in the sense that drought develops during the last part of the crop cycle. One of the ways of increasing productivity in these semiarid environments is irrigation; however, this may expose soils to progressive salinization as a consequence of inappropriate irrigation practices [1], [2]. At the same time, competition for water resources among different social and economic sectors is growing, with agriculture being progressively forced to use lower quality water [3], and this may compromise yield and critically expose soils to progressive salinization [4]. In arid and semi-arid regions, water and soil salinity are among the main factors limiting plant productivity. Tunisia is a Mediterranean country burdened by this salinity problem. It is estimated that saline soils cover over 1800,000 ha [5], representing 11.6% of the total surface of the country. In Tunisia, most durum wheat is commonly grown on marginal soils under rainfed conditions [6]. While supplemental irrigation may be a method to increase yield in durum wheat it might also expose the crop to additional salinity. In that context, selecting more salt tolerant genotypes is a way of improving durum wheat performance in the Mediterranean and other dry areas [2].

The use of stable isotope variation in plant research has grown steadily during the past two decades. This trend will continue as researchers realize that stable isotopes can serve as time-integrated indicators of how plants interact with and respond to their abiotic and biotic environments [7]. In that context, analysis of the natural abundances of the stable isotopes of carbon (¹²C, ¹³C) and nitrogen (¹⁴N, ¹⁵N) in plants is of potential interest for studies on salinity resilience [8], [9], [10].

The stable carbon isotope fractionation (δ¹³C) by plant matter (frequently expressed as discrimination from the surrounding air, Δ¹³C) integrates over time the ratio of intercellular to atmospheric CO₂ concentration and thus the balance between the net photosynthetic assimilation and the stomatal conductance (i.e. the intrinsic water use efficiency) in C₃ species such as wheat [11], [12], [13]. Conditions inducing stomatal closure, such as water stress and salinity, restrict the CO₂ supply to carboxylation sites, which then increases δ¹³C (or decreases Δ¹³C) and the intrinsic water use efficiency of the plant [9], [12], [14], [15]. Moreover, genotypic variability for δ¹³C in wheat under drought and salinity has also been reported, with resilient genotypes exhibiting lower δ¹³C in kernels (or other organs developed during the last part of the crop cycle) and thus lower intrinsic water use efficiency [8], [16], [17], [18], [19]. However, the concept of water use efficiency is diverse [20] which implies that time integrated intrinsic water use efficiency not necessarily parallels the water use efficiency at the crop level (or ‘water productivity’) formulated as the ratio of total crop biomass or grain yield per unit of water used (evapotranspired).

Natural variation in the plant N isotope signature (commonly expressed as a composition, δ¹⁵N) as response to water stress and salinity has been reported. Moreover, δ¹⁵N has been proposed for genotypic screening under drought [21], [22] or salinity [8], [23] because it is linked to N metabolism [10]. The fractionation of nitrogen (N) occurs during N uptake, assimilation, recycling and redistribution within the plant [24], [25]. A change in the environmental conditions that impact on metabolism can cause a substantial change in the isotopic content of metabolites [26], [27]. However, reported environmental and genotypic effects are diverse, including increases and decreases in plant δ¹⁵N as response to growing conditions or related with genotypic resilience [10], [27].

A previous study on durum wheat under field conditions in Tunisia has shown the value of carbon and nitrogen isotope compositions in assessing the genotypic performance of durum wheat under different water regimes [28]. However, to the best of our knowledge there are few studies evaluating the genotypic tolerance and the related physiological traits of field-grown durum wheat to salinity. In this study we compared the response of a set of Tunisian landraces and several of the modern (i.e. improved) durum wheat genotypes most cultivated in this country to different levels of salinity imposed by irrigating with brackish water under field conditions. Main objective was to determine which physiological traits are potentially useful as a phenotypic indicator to assess genotypic tolerance to irrigation with different levels of salinity. Second objective was to determine what differences exist between landraces and modern varieties in adaptation to salinity. To that end crop yield and water use efficiency at the crop level (WUE_yield or ‘water productivity’, formulated as the ratio between grain yield and water evapotranspired) were assessed together with some agronomical yield components, the δ¹³C and the δ¹⁵N and nitrogen content of flag leaves and grains, the chlorophyll content of the flag leaves and the number of days from sowing to anthesis. This study analysed samples collected from a recent field work, where genotypic variability in grain yield and it agronomical components have been evaluated [29].