Water shortage is the major constraint affecting cereal production in the Mediterranean Basin. The climate of this region is characterised by erratic and unpredictable precipitations [1]. Although drought may occur at any stage of cereal development in these areas, climatic frequency studies have identified two major periods when drought is most likely to occur [1,2]. The first occurs in autumn during the period from sowing to tillering. The second major drought period coincides with the grain-filling phase. Impacts of terminal water stress on cereals have been thoroughly investigated, while studies of early season drought are lacking. An early season drought may affect considerably yields through the limitation of tiller survival rate and number of kernels produced [1,3].

Selection of tolerant cultivars has been considered as an economic and efficient means to improve drought tolerance [4,5]. A better understanding of mechanisms of adaptation to water deficit and maintain of growth, development and productivity during stress periods would help the drought-tolerance breeding [4]. Nevertheless, drought tolerance is a complex trait resulting from the contribution of numerous factors. Among the several putative characters, water status parameters [6,7], carbon isotope discrimination [8,9], roots characters [3,10,11] are interesting traits for drought-tolerance evaluation.

The development of extensive root system contributes to differences between cereal cultivars for drought tolerance [5,12]. Genotypic variability has been found in many species for root characters [13–16], and their interests for drought tolerance improvement discussed [4,5,12]. Several works have studied root development mostly in wheat, maize, sorghum and oat. The scarce root development studies in barley have been done on adventitious roots under water-deficit conditions [10,11]. Moreover, these studies were performed at flowering stage. An appreciable genotypic diversity for root growth at early stages was reported in barley under favourable conditions or under other abiotic stress [14,17,18]. However, these reports cannot reflect the plant behaviour under harsh conditions prevailing in the Mediterranean Basin. There is limited insight into morphological traits of seminal roots in barley under drought, although this species is mainly cultivated without irrigation in arid and semi-arid regions. The comparison between adventitious and seminal roots in wheat showed that the two root types perceive and respond differently to drought stress and demonstrated the extreme sensitivity of seminal roots to water stress [19,20]. Other studies showed that damages caused to wheat seminal roots and seedlings depend more on the timing and severity of drought than on its duration [19–21]. Indeed, these works emphasised that severe drought, even during short periods, affects long-term growth of cereals plants. In addition, early water stress seems to damage the number of tillers and the number of grains per ear [1,19,20], which strongly reduces yield production in cereals [5,22].

The most studied traits were volume, length and number of seminal roots as well as the root-to-shoot dry matters' ratio. It was reported that the number of seminal roots is slightly or not affected by water deficit [19,20,23,24]. Other studies reported that this trait could not be used to discriminate cereal genotypes [13]. In contrast, volume and length of both adventitious and seminal roots were considered as efficient characters to evaluate genotype response to water stress [10,13,16,18–21,23,24]. All these works were performed on total root system. Limited studies have considered the response of adventitious roots at different soil layers. Similar investigations on seminal roots are lacking.

In the absence of any information on morphological traits of seminal roots in barley under water-deficit conditions, the major objective of this study is to examine the differences in some morphological traits of seminal roots at different soil layers among barley cultivars in response to the variation in the intensity of water deficit.

2.1 Plant material

2.2 Experimental design and water conditions

2.3 Measurements

2.4 Statistical analyses

The profiles of soil water content for 10-cm depth increments in each water treatment are presented in Fig. 1. Differences in soil humidity were detected at the topsoil layer (0–20 cm). The reduction in soil water content in the upper soil layer between the control (100% of field capacity (FC)) and severe water deficit (25% FC) was significant. Major differences between water treatments were observed in the deepest layer. These differences are more pronounced between 100 and 25% FC treatments. The soil water content under severe stress was significantly lower than under the three other water treatments.

Genotypes differed significantly in all measured traits (Table 2). Detailed genotypic values are presented in Tables 3 and 4. There were significant differences between studied water treatments. The genotype by water treatment interaction was also significant for all characters (Table 2).

Measured traits were significantly affected by water treatments. Mean seminal root length value was near 65 cm at 100% FC. This value decreased slightly with the increase of water stress intensity (Fig. 2a). Moreover, no significant difference between 100 and 75% FC treatments was observed. In contrast, when water deficit went below 50% FC, seminal root length decreased strongly and reached a value of 19.5 cm at 25% FC (Fig. 2a). In fact, the seminal root length decreased more than three times from 100% to 25% FC. Seminal root volume was studied in three soil layers (0–20 cm, 20–40 cm and beyond 40 cm) for each water treatment. At the topsoil layer (0–20 cm), there were no significant differences between water treatments for seminal root volume (Fig. 2b). In the second soil layer (20–40 cm), no significant differences were found between favourable conditions and both low and moderate water deficit. However, the value of root volume in this soil layer fell in completely below 50% FC (Fig. 2b). In the third soil layer (beyond 40 cm), the root volume decreased slightly under low water deficit and strongly when water became scarcer (Table 4). Indeed, roots did not grow deeper than 40 cm. Under both 100% FC and low water deficit conditions, the root volume was distributed over the three studied soil layers. Moderate water deficit induces a decrease of root volume in the deeper layer and an increase of values of this trait in the upper layer. At 25% FC, 90% of roots were confined in the topsoil layer and did not grow in the deepest layer (Fig. 2c).

In contrast to the former traits, root to shoot dry matters ratio (RDM/SDM) increased when soil water decreased (Table 3). This trait increased nearly four times from favourable to severe water conditions.

A large genotypic variability was noticed for all measured traits. For both root length and volume, globally and as described before, there was a slight increase of trait values followed by a strong one below 50% FC treatment (Tables 3 and 4). Nevertheless, the studied genotypes did not present similar behaviour in response to water deficit. The genotypic values of measured traits are presented in Tables 3 and 4. For roots length, genotypes 3, 2 and 6 showed the highest values, whereas genotype 5 exhibited the shortest seminal roots under favourable conditions. Inversely, while genotype 5 presented the longest roots, genotypes 7, 2 and 6 have the shortest ones under severe water stress. The root length of genotype 5 did not vary from favourable to moderate stress, but decreased under moderate stress conditions (i.e., 50% FC), and decreased more than twice from favourable to severe water conditions. The difference observed for this trait on genotypes 7, 2 and 6 was more marked. The root length under 100% FC was 4.6 (genotype 7) and 3.9 (genotypes 2 and 6) higher than under 25% FC. In contrast to the majority of genotypes where a strong decrease of values was noted below 50% FC, genotype 6 (see Table 3) seems to respond early to water deficit. This line which presents the highest value at 75% FC showed the lowest root length at 50% FC (Table 3). For RDM/SDM, the lowest values were observed under favourable conditions and genotypes were slightly different. Genotypic differences were more marked, especially under severe water deficit. The RDM/SDM was 4.63, 4.40 and 4.22 higher under severe water stress than under favourable conditions for genotypes 6, 4 and 2, respectively. The lowest difference between the two water treatments was found for genotype 7.

Significant differences were observed between studied water treatments (Figs. 2 and 3). Similar differences were already reported for several species on seminal and on adventitious roots [10,11,26–29]. Most of these works have also found significant interaction between water treatment and genotype, which mirrored differential responses patterns to increasing water stress. These results are confirmed in our study (Table 2).

A large genotypic variation was observed here for all studied root traits (Tables 3 and 4). These results confirm the broad variability found for morphological traits in seminal [20,23,30] and adventitious roots [15,28,31,32]. All these studies were done in wheat, maize or sunflower. The few studies performed in barley on adventitious roots under water stress [10,11] or on seminal roots under favourable conditions [33,34] also observed genotypic variability. Our results emphasised the existence of appreciable genotypic differences in seminal roots traits in barley grown under water-deficit conditions (Tables 3 and 4).

Seminal roots' length was affected by the decrease in soil water moisture. This decrease is slight under low and moderate water stress. In contrast, the root length is strongly affected under severe water-stress conditions (Fig. 2a). Several studies have already reported the effect of water stress on adventitious roots length [3,27,29,35,36]. The few studies performed on seminal roots also noticed the decrease of root length under water stress [18–20]. However, all these studies have been done on wheat or oat. The scarce works on barley were realised on adventitious roots [10,11]. Seminal roots were also studied under acid and acid/aluminium stress [17] or under favourable conditions in barley [33] or in comparison with other cereals [13]. This latter work emphasised the superiority of barley compared to other cereals in seminal root expression.

Root volume was strongly affected by water stress (Fig. 2b). Similar trends were observed on adventitious roots of wheat [27–29,36] and of barley [10,11]. Most of these studies have also noticed that root volume was not significantly affected in topsoil layer, but strongly decreased in deeper layers. These results were confirmed here. Nevertheless, the severity of stress has an additional impact on this trait, which was not studied in the cited works. Our results showed that low and moderate water treatments affect more slightly root volume in top and middle soil layer than expected (Fig. 2b). In contrast, under severe water stress, there were no roots in the deepest soil layer (Table 4). Indeed, more than 90% of roots were confined in the topsoil layer confirming previous report [28]. This latter study showed that 81% of roots of wheat genotypes cultivated under water stressed conditions were distributed in the 0–30-cm topsoil layer. A significant decline in root volume under early-season drought in deep soil layer was observed in durum wheat [3]. However, the decline observed in this latter work is quite small, whereas our results showed a strong decrease of root volume in the deepest layer. The difference was probably due to the induced water stress between the two works, suggesting that root growth depends on the intensity of water stress. The soil structure and composition could also contribute to the difference observed between the two studies.

Several works have studied the effect of stress duration on adventitious roots [10,27,37]. Other studies demonstrated that the seedling stage is more important than the duration of the stress when water stress occurs [19]. The root growth is dependent on both stress severity and timing [18,19,21]. The extreme sensitivity of seminal roots to water deficit was probably caused by the absence of cuticle that could protect against water evaporation [38]. Due to the death of existing apices and no initiation of new lateral roots [18,26] under severe water stress, seminal roots cease to grow [39]. Significant differences were found between water treatments for RDM/SDM (Table 2). An increase in values corresponding to this trait from favourable conditions to severe water deficit was found (Table 3). These results were expected and confirm those reported for several other species [10,11,27,29,36,37]. This ratio was higher under drought conditions than under well-watered conditions, due to reduced shoot growth rather than root growth [3,18], as presented in Fig. 3b. Under severe water deficit, both root and shoot dry mass production are reduced, as described in previous reports [18,39].

A differential genotypic response was observed here. Nevertheless, these differences are not related to the geographic origin. In fact, studied genotypes have been released under harsh Mediterranean conditions. Mid-East genotypes have been reported to respond differently from those of North Africa for water status parameters [40], carbon isotope discrimination [9], gas exchange [41], and root characteristics [10,29]. It could be interesting to study seminal root traits in a large collection of genotypes from different geographic regions.

In conclusion, an appreciable genotypic difference for seminal roots traits was observed in this study. A differential impact of water stress on roots was noted. Seminal root characters were affected by water stress and the impact of water deficit on these traits is related to the intensity of stress. It seems that the severe water stress causes the death of apices, which limits root growth. Anatomical approaches of seminal roots under severe water deficit may help to understand the changes induced by water deficit or operated by plants in response to this stress.