Paspalum dilatatum, known as Dallis grass, is a C4 forage grass native to South America. Although this species is spread all over the tropical, subtropical, and temperate areas of the world [1,2], it is a vigorous, persistent, and highly palatable species that withstands heavy grazing. This species is conspicuous or even co-dominant in most plant communities of the Flooding Pampas (Argentina). Different than other subregions of the Pampas, the Flooding Pampas remain covered mainly by temperate native grasslands, mainly due to the simultaneous occurrence of water and salt stress [3]. The area is characterized as a wet one, suffering commonly water logging conditions in winter and early spring, and even flooding, while soils show a low salt concentration. However, water and salt stresses are concurrent during most summers when drought occurs together with sudden increases in topsoil salinity. After late autumn rainfalls, water stress diminishes and salinity returns to its low current level, showing clearly the temporal variation of salinity in nature [3].

Water and salt stress impose an adverse impact on plant biomass production. They are usually related. Water stress has been identified as the single most biomass-reducing factor for the plants [4,5]. It is an almost universal plant stress, either permanent or transient, and periods of drought may occur frequently, even in regions characterized by high annual rainfall [4]. The present global warming situation increased drought occurrence around the globe [4]. Drought conditions affect the growth of plants by affecting photosynthetic and biochemical processes which result in a substantial decline in crop productivity [4,6]. Salt stress is usually localized from arid to humid areas, salt stress has a profound effect on both natural vegetation and crops, in certain conditions, salt stress may resemble water stress. Salts cause osmotic or water-deficit effects, in addition to salt-specific effects. The water deficit mediated by salts occurs because the presence of salts reduces the water uptake of plants which leads to a decrease in growth rate, accompanied by various metabolic changes similar to those induced by water stress [7]. The specific effects of salts are attributed to the intake of excess Na⁺ and Cl⁻ ions [4,8]. Recent studies have revealed that the response of plants to a combination of water and salt stresses is unique and cannot be directly extrapolated from the response of plants to each of the different stresses applied individually [9,10].

Water and salt stresses can differentially affect the mineral nutrition of plants by reducing nutrient uptake by the roots and transport from the roots to the shoots, by restricting transpiration rates and impairing active transport, and membrane permeability [7,10]. Salinity stress also induces nutrient deficiency or causes nutrient imbalance due to the competition among them, such as potassium (K), calcium (Ca), and sodium (Na) [11]. Several studies have shown that P. dilatatum is tolerant of both floods and drought [12–14]. Cook et al. [15] indicated that this species is remarkably tolerant of drought because of its thick rhizomes. On the other hand, from the pioneering work of Gauch et al. [16] P. dilatatum is known as a species moderately resistant to salinity [17,18]; According to Lair [18], upper limits of this species survival is around 8–12 d·Sm⁻¹. Other species of the genera, like Paspalum vaginatum Swartz, are highly tolerant to salts [19,20]. According to Chavarria et al. [21], Paspalum vaginatum reduces its performance by 62% on average, at Electrical conductivity (EC) as high as 45 d·Sm⁻¹. It is remarkable that Paspalum vaginatum is also found in the Flooding Pampas grasslands, but is much less conspicuous than P. dilatatum [8]. While most studies were performed using a continuous salt supply, crops respond differently when coping with continuous salinity vs. peaks or pulses of salts [11].

Our objective was to determine the effects of drought and salinity on biomass production of the green and dry components and chemical composition of Paspalum dilatatum plantlets, mimicking the stress conditions in the form of pulses of water deficit and salt excess. We hypothesize that the simultaneous negative effect of water stress and salt stress on the biomass production of P. dilatatum increases with the magnitude of salt stress.

The effects of the different treatments on total aerial biomass and its components and on root biomass are shown in Fig. 1 and in Table 1.

Fig. 1 shows that all plant components significantly decreased when subjected to both stresses while Table 2 shows the statistical significance of the found results. The extreme stress treatment (SS2/WS1) reduced to a half total biomass when compared to control. In all cases the whole aerial biomass was lower than the root biomass: the aerial biomass varied from 67% compared to root biomass in the check treatment to 37% in the treatment SS2/WS1.The proportion of dry rest to green components of the plants was also affected by the treatments. In all cases but in one, the dry materials were lower, from 18% to 83%, than the green components. The exception was treatment subjected to both water plus strong saline stress (SS2/WS1), in which the green components account 88% of the dry rest. Using the RWC as an indicator of water deficit, the significant effects of water and salt stress were verified on P. dilatatum (p < 0.001 and p < 0.02, respectively) (Fig. 2).

Sodium, K, Ca and Mg concentration in plant components are shown in Table 2.

The N concentration in the different components of the plants showed no significant differences due to the imposed treatments. The green leaves and non-leaf green material contained to 0.85–1.39 mg N·kg⁻¹, dry material to 0.21–0.60 mg N·kg⁻¹ and roots to 0.81–1.10 mg N·kg⁻¹. P concentration increased significantly in green leaves, non-leaf green material and dry material, as stress, especially salinity, increased. Conversely, P concentration in roots did not show significant differences in any of the treatments (Fig. 3).

Potassium, Ca and Mg concentrations were not largely affected by the imposed treatments in green leaves, non-leaf green material and dry material, as compared with control (Table 2). Conversely, Na concentration increased significantly in these plant components as salinity stress increased. (Table 2). Consequently, the K/Na ratio in all aerial plant components (Fig. 4) as well as the Ca/Na (Fig. 5) and Mg/Na ratios (data not shown) decreased (p < 0.001). Sodium concentration did not increase in roots and as a consequent the Na/K and Ca/Mg ratios in roots were not affected by the treatments.

The effects of water stress on plants depend on its absolute value and temporal variation [25]. In present case plant components significantly decreased when subjected to both stresses, but different than reported [5] the whole aerial biomass was more affected than the root biomass.

In the present experiment the RWC measured at the end of the stress period corresponded to a level equivalent to initial wilting stage [19]. The RWC was more critical on salt stressed treatments, which agree with the fresh/dry weight ratio which diminishes under water limitation [19]. These results are similar to that found by Lee et al. [20] with different ecotypes of the salt tolerant Paspalum vaginatum, although when subjected to a salinity increase. results varied among ecotypes. The RWC decreased under water stress although somewhat less than with salt stress, in agreement with the moderately tolerance to drought of P. dilatatum [5,15]. Focusing on salt stress effects, the results show that P. dilatatum subjected to salt pulses, even including water stress, can tolerate more salinity than is shown by classical experiments using continuous salinity regime and without water stress [16–18]. Water stress is normally a coexisting factor with salt stress, however, the effect of the superposition of both on P. dilatatum cannot be quantified in this work. Anyway, different results show that due to plants subjected to pulses of salinity accumulate fewer ions in their tissues they can tolerate higher salinities than those subjected to continuous salinity [11].

Results showing that leaf N concentration did not vary due to the imposed treatments, but P concentration increased significantly confirm that the interactions between N and P nutrition and water and salt stress are complex. It depends on the magnitude of the stress and the stress tolerance of plants [26]. Yadav et al. [8] indicated that the water stress results in N absorption limitations. Salt stress (as well as other stresses) may reduce N accumulation [7,26] but in our experiment, the plant N concentrations were not affected by the treatments. The N concentration in green leaves was similar than that obtained previously in P. dilatatum from the Flooding Pampas by Ayala Torales et al. [27] and Vivanco et al. [28]. However, our foliar N concentration was lower than that found by Andrew et al. [29] in other latitudes and also lower than that recorded for Paspalum vaginatum [19]. The saline stress may decrease, increase or not affect P accumulation in plants [7,26]. In our case, the P concentration in plant tissues increased with the treatments but, this fact could be related to lower biomass production, as usually established: the lower the biomass, the higher the P concentration [30]. Phosphorus concentration in leaves was similar as those obtained by Vivanco et al. [28] and again lower than those found by Andrew et al. [29].

In general terms non-halophytic plants tend to absorb Na in excess in saline soils, causing a relative and even absolute decrease in K and Ca concentration. These processes are related to the K-Na antagonism [26]. The Ca/Na ratio, which is related to plant salt tolerance, is also affected [8,31]. In the present case, the Na concentration increased, except in the roots, while that of the rest of the cations was not substantially modified (Table 2), which caused the K/Na, Ca/Na and Mg/Na ratios to decrease in all components. aerials (Figs. 4 and 5). The highly salt tolerant Paspalum vaginatum can maintain a high concentration of K in its shoots and can limit the movement of Na toward shoots. Besides, this species accumulated more Na and Ca in its dead leaves than in the green leaves. The maintenance of a high K/Na ratio allow P. vaginatum to avoid the cation damage caused by salt stress [19], different than the non-salt tolerant P. scrobiculatum, which is unable to regulate the transfer of Na from the green leaves [32].

This suggested that one way by which P. vaginatum tolerates salinity may be by a mechanism of reducing Na transport to leaves and moving excess of cations from young leaves to older and possibly dying leaves [32]. In our experiments, the combined effects of water and salt stress on cationic concentrations was complex because there were large differences in its concentration between the different plant components. Most results in present experiment (Table 2) showed that P. dilatatum when subjected to salinity stress did not show the same behavior than the salt tolerant P. vaginatum, with which coexist in the grasslands. Different from that species, our results show that P. dilatatum does not accumulate Na and Ca in dead leaves.

We found that the representation of natural conditions in a controlled experiment provides some different results as compared with similar experiments but where is not considered the variability of salinity and the water stress. The comparison with other species of the same genus allowed us to advance with the way this species deals with salinity, but this is a further field to study. From a more applied point of view, the present data contribute to better knowledge of the species for a more rational management of natural grasslands.

P. dilatatum biomass production in the different plant components, green leaves, non-leaf green material, dry material, and roots, decreased when subjected to individual stress and decreased even more when both stresses were applied simultaneously. However, under the conditions of the present experiment, the hypothesis was not validated: the lower and the higher saline stress affected biomass in a globally similar way when subjected to simultaneous water stress. When salts were applied in the form of a pulse, mimicking the nature, P. dilatatum tolerated higher salinity than that found by other authors using non-realistic permanent salt levels. Besides this fact, the plant chemical composition related to the accumulation of cations in plant tissues allows us to infer that the Na accumulation of P. dilatatum behaves in an intermediate range, compared to salt-tolerant or non-salt tolerant species of the Paspalum genus.