2.2. Morphological Responses of Forage Grasses to Water Stress
Soil water stress resulted in a lower growth rate in plant height of
U. brizantha cv. BRS Piatã,
P. glaucum cv. ADR 300 and
P. maximum cv. Mombaça compared to high soil water regime (
Table 2). The plant height of
P. maximum cv. Mombaça,
U. brizantha cv. BRS Piatã and
P. glaucum cv. ADR 300 was 22%, 28%, and 44% lower in plants grown under a low soil water (LSW) regime when compared to plants under a high soil water (HSW) regime. However, soil water regimes did not inhibit the height growth rate of other forage grasses.
Petter et al. [
6] reported that the growth rate of
U. brizantha cv. Xaraés,
U. ruziziensis cv. Comum and
P. glaucum cv. ADR 7010 was not negatively affected by plant exposure to water stress. However, Zuffo et al. [
25] showed that soil water stress inhibited the height growth rate of
U. brizantha cv. BRS Piatã and
P. glaucum cv. ADR 300. These results show that the adverse effects of water stress on the height growth rate of forage grasses are still inconsistent and depend on the grass development stage at which water stress occurs. Soil water stress imposed during the initial growth stage has a more significant negative impact on plant height than when imposed during the grass tillering stage [
16].
The highest plant height under different soil water regimes was observed for
P. glaucum cv. ADR 300; however, the plant height was similar to
P. maximum cv. Aruana under an LSW regime (
Table 2). These results are associated with the growth habits of forage grass cultivars. The
P. glaucum is more extensive and has an erect growth habit, while
U. brizantha and
P. maximum plants are smaller, have a more tufted growth habit, and have a greater number of tillers [
6,
26].
Water stress significantly reduced (
p < 0.05) the number of leaves of
U. ruziziensis cv. Comum,
P. maximum cv. Aruana,
P. maximum cv. Mombaça, and
P. Atratum cv. Pojuca, while the other forage grasses did not significantly reduce the number of leaves when grown under different soil water regimes (
Table 2). Under an HSW regime, the highest number of leaves was observed for
P. atratum cv. Pojuca, while under MSW and LSW regimes, the highest number of leaves was obtained in
U. ruziziensis cv. Comum and
P. atratum cv. Pojuca plants. Petter et al. [
6] also reported a higher number of leaves in
U. ruziziensis plants exposed to soil water stress. These results indicate that even the plants of
U. ruziziensis have a significant reduction in the number of leaves when subjected to soil water stress; this species can maintain a high number of leaves under LSW regimes. The lower emergence of new leaves under water stress conditions has been considered a plant strategy to reduce the transpiration rate and increase water use efficiency [
15]. However, this water stress tolerance strategy is conditioned on the specific response of the genotype [
9]. The lower number of leaves has been a response of forage grasses to ensure their survival to relatively long periods of low soil water availability [
8].
Leaf area was significantly (
p < 0.05) smaller under an LSW regime for all forage grass cultivars except for
P. glaucum cv. ADR 300 and
P. atratum cv. Pojuca (
Table 2). Under an LSW regime, the leaf area reduction of forage grasses ranged from 45% to 62% compared to plants under an HSW regime. Leaf area reduction in three forage grass cultivars (
U. brizantha cv. BRS Piatã,
U. brizantha cv. Marandu,
P. glaucum cv. ADR 300) exposed to water stress conditions were also reported by Zuffo et al. [
25]. The reduction in leaf area has been reported as a typical response of forage grasses when exposed to soil water stress [
8,
9,
10,
11,
12]. One of the first processes affected in response to decreased soil water availability is cell expansion, a highly dependent process of turgidity in plants. However, with the advancement of soil water stress, other physiological processes are negatively affected, with direct effects on the photoassimilates accumulated by the forage grasses, reduction in the carbon assimilation rate, and relative growth rate [
8,
9]. As a result of these effects, there is a reduction in leaf area and biomass production. The reduction in leaf area occurs as a defense reaction of plants to water stress, reducing the transpiration rate and, consequently, water loss to the atmosphere [
15].
Soil water regimes did not alter the root dry matter accumulation of
U. brizantha cv. Xaraés,
P. glaucum cv. ADR 300 and
P. atratum cv. Pojuca, while the root dry matter accumulation of the other forage grasses was significantly lower when grown under water stress, especially under an LSW regime (
Table 3). Under an HSW regime, root dry matter was higher for
U. brizantha cv. BRS Piatã,
U. brizantha cv. Marandu,
U. ruziziensis cv. Comum,
P. maximum cv. Aruana,
P. maximum cv. Mombasa and
P. maximum cv. Tanzania. However, when forage grass cultivars were grown under an LSW regime, there were no significant differences in root dry matter production (
Table 3). The lower production of root dry matter of forage grasses under water stress conditions was also reported by Petter et al. [
6] and Fariaszewska et al. [
9]. Under soil water stress, plants reveal mechanisms to combat cellular tissue dehydration. The decrease in soil water availability causes an increase in the synthesis of abscisic acid (ABA) and stress proteins, which protect cell membranes and participate in osmoregulation [
8]. The increase in the concentration of ABA in the cells reduces the transpiration rate by closing the stomata and results in greater water use efficiency [
9]. In addition, abscisic acid inhibits shoot growth but simultaneously stimulates root growth and development, which essentially helps to overcome stress [
27]. However, this stimulus in root growth caused by the higher concentration of ABA under water stress conditions has been commonly reported under field conditions [
8,
12]. In pot experiments, as in this study, the growth of the plant root system was limited by the soil volume in the pot.
Total dry matter accumulation and root volume were significantly lower (
p < 0.05) under water stress conditions for all forage grass cultivars (
Table 3). Under HSW regime,
P. maximum cv. Mombaça plants have greater total dry matter accumulation and greater root volume than other forage grasses. Under soil water stress, plants of
U. ruziziensis cv. Comum,
P. maximum cv. Aruana,
P. maximum cv. Mombaça, and
P. maximum cv. Tanzânia had a greater total dry matter production, except for
P. maximum cv. Aruana under an MSW regime (
Table 3). Under water stress conditions, there was no significant difference in root volume value between forage grass cultivars, except for
P. glaucum cv. ADR 300 and
P. atratum cv. Pojuca under MSW, which had lower root volume than other forage grasses. The lower total dry matter production and root volume of plants exposed to soil water stress is a consequence of plant adaptation mechanisms to avoid excessive water loss [
11,
12], as well as the adverse effects of water stress on plant physiological metabolism, especially on the photosynthetic activity of the plants [
9]. Under water stress conditions, the rate of photosynthesis decreases, which is related to a decrease in rubisco activity, a reduction in stomatal conductance, and reduced availability of CO
2 [
8].
Plants of
U. ruziziensis vc. Comum has a higher tiller emission rate, while plants of
U. ruziziensis cv. Comum,
P. maximum cv. Aruana,
P. maximum cv. Mombaça, and
P. maximum cv. Tanzânia had a higher production of shoot dry matter (
Table 4). Tillers are very important to understanding forage grass growth and regrowth. Tillers are new grass shoots made up of successive segments called phytomers. The tillering rate of grass species is controlled by the emergence rate of phytomers, genetic characteristics, and plant density in the field [
5]. Thus, shoot dry matter production results from accumulated phytomers per stem and the stem density per area [
11,
16].
The LSW regime inhibited the tiller emission of forage grasses (
Table 4). Shoot dry matter production was drastically reduced when grasses were exposed to water stress conditions. The lower tillering rate and shoot dry matter accumulation of
U. brizantha cv. BRS Piatã and P. glaucum cv. ADR 300 under water stress conditions was also reported by Zuffo et al. [
25]. When water stress occurs at the initial stage of grass development, the reduction in stomatal conductance and photosynthesis rate results in lower plant tillering potential and lower shoot dry matter production [
9], which results in significant forage production losses [
8]. According to Fonseca and Martuscello [
5], the main morphological characteristics that directly affect the forage production potential are the number of tillers, the number of leaves, and leaf size.
2.3. Interrelationship between Morphological Traits and Forage Grass Cultivars
Canonical correlation analysis was used to verify the contribution of each dependent variable measured in the tropical forage grasses as affected by soil water regimes (
Figure 1). For scores to be represented in a two-dimensional graph, the percentage of retained variance must be higher than 80% [
28]. In this study, variances accumulated in the two main canonical variables were 94.2%, 88.2%, and 82.9%, respectively, for each graph (
Figure 1A–C), allowing an accurate interpretation.
Under the HSW regime, an angle (between vectors) less than 90° indicates a positive correlation between the dependent variable plant height (PH) with the
P. glaucum cv. ADR 300 (T1); number of leaves (NL) with
P. atratum cv. Pojuca (T8); and the number of tillers, leaf area, shoot dry matter, root dry matter, total dry matter, and root volume with
P. maximum cv. Tanzania (T2),
U. ruziziensis cv. Comum (T3),
P. maximum cv. Aruana (T4),
P. maximum cv. Mombaça (T5),
U. brizantha cv. Marandu (T6),
U. brizantha cv. BRS Piatã (T7), and
U. brizantha cv. Xaraés (T9) (
Figure 1A).
Under the MSW regime, there was a positive correlation between plant height and
P. glaucum cv. ADR 300 (T1); the number of leaves and tillers with
U. ruziziensis cv. Comum plants (T3) and
P. atratum cv. Pojuca (T8); and leaf area, root volume and shoot, root and total dry matter with
P. maximum cv. Tanzânia (T2),
P. maximum cv. Aruana (T4),
P. maximum cv. Mombaça (T5),
U. brizantha cv. BRS Piatã (T7) and
U. brizantha cv. Xaraés (T9) (
Figure 1B).
Under the LSW regime, there was a positive correlation between the number of leaves and tillers with
U. ruziziensis cv. Comum (T3) and
P. atratum cv. Pojuca (T8); plant height and root dry matter with
U. brizantha cv. Marandu (T6),
U. brizantha cv. BRS Piatã (T7) and
U. brizantha cv. Xaraés (T9); and leaf area, root volume, shoot, and total dry matter with
P. maximum cv. Tanzânia (T2),
P. maximum cv. Aruana (T4) and
P. maximum cv. Mombaça (T5) (
Figure 1C).
The greater or lesser negative impact of soil water stress on the growth and development of forage grasses is determined by the genetic traits of the genotype’s tolerance when exposed to water stress conditions. Each forage grass has distinct morphological characteristics that can be modified by the pasture’s production environment and technical management. However, this modification of the morphological traits of grasses is limited by the phenotypic plasticity of the genotype [
12]. Therefore, the cultivation environment of forage grasses results in distinct gradual, reversible changes in the morphogenic and structural characteristics of the plants [
29].
The results shown in
Figure 1 indicate that under non-stressful or stressful conditions, forage grasses of the species
P. maximum have a greater capacity to produce leaf area, root volume, shoot, and total dry matter. Therefore, when the farmer aims at greater forage production, cultivars Aruana, Mombaça, and Tanzânia of
P. maximum are excellent option for cattle feeding. Under the LSW regime, grasses of the species
U. brizantha have a greater capacity to produce root dry matter; however, the highest root volume under LSW was observed for
P. maximum cultivars.
U. ruziziensis plants have a higher tillering potential under non-stressful and stressful conditions. Therefore, it can be seen that each forage grass cultivar has its intrinsic characteristics and directs the accumulation of photoassimilates to different drains, either for the growth of the stem, leaves, roots, or tillers.
The pattern of dry matter allocation among different plant organs can change throughout the plant development stages, especially when exposed to stressful environmental conditions. However, this pattern of photoassimilate allocation is essential to optimize crop growth and development under stressful conditions. This is because the pattern of photoassimilate allocation can affect plants’ competitive and adaptive capacity and their responses to the stresses imposed by the cultivation environment [
29]. The pattern of dry matter allocation in forage grasses is directly related to the optimization of capturing the scarcest resources from the cultivation environment. Under non-stressful conditions, grasses can allocate more photoassimilates to leaves to increase plants’ light energy uptake and photosynthetic rate and increase forage production. On the other hand, grasses can allocate more photoassimilates to the roots under water stress conditions to improve water and nutrient uptake when soil water availability is low or limited [
12].
2.4. Water Stress Tolerance Indices
The highest shoot biomass production under HSW regime (Y
P) was obtained for
P. maximum cv. Aruana,
P. maximum cv. Mombaça, and
P. maximum cv. Tanzânia (
Table 5). Under the MSW regime, the nine forage grass cultivars were grouped into the same group based on shoot biomass production (Y
S) and TOL, YSI, DI, YI, k
2STI, SSPI, and ATI indices. These results indicate that these tolerance indices did not effectively differentiate the water stress tolerance levels of forage grass cultivars exposed to moderate water stress conditions. Menezes et al. [
18] also reported that the TOL and YSI indices did not differentiate water-stress-tolerant grain sorghum genotypes adequately. On the other hand, the MP, STI, GMP, and HM indices classified the forage grass cultivars into two tolerance groups, and the plants of
U. ruziziensis cv. Comum,
P. maximum cv. Aruana,
P. maximum cv. Mombaça, and
P. maximum cv. Tanzânia belonged to the group with the highest values of water stress tolerance indices (
Table 5).
Under LSW regime, the TOL, YSI, and SSPI indices were not efficient in differentiating the water stress tolerance level of the nine forage grass cultivars (
Table 5). On the other hand, the MP, GMP, and k
2STI indices separated the forage grass cultivars into three tolerance groups. In contrast, the DI, STI, YI, and HM indices divided the forage grass cultivars into four tolerance groups (
Table 5). These results indicate that these tolerance indices were the most sensitive to differentiate forage grass cultivars regarding water stress tolerance levels. Naghavi et al. [
20] showed that the STI, YI, SSPI, k
1STI, and k
2STI indices were the most suitable to identify water-stress-tolerant maize genotypes. Cabral et al. [
19] reported that the MP, STI, GMP, and HM tolerance indices are the most appropriate to identify water-stress-tolerant soybean cultivars. Sánchez-Reinoso et al. [
24] reported that only the SSPI index effectively identified water-stress-tolerant bean genotypes.
2.5. Interrelationship between Biomass Production and Water Stress Tolerance Indices
A network diagram was constructed based on the shoot biomass production of forage grasses exposed to non-stressful and stressful conditions and on all stress tolerance indices and their respective correlations (
Figure 2). The correlation network diagram shows the interactions between all the water stress tolerance indices with the shoot biomass production of the grasses. Positive and highly significant correlations were detected between water stress tolerance indices and shoot biomass production of forage plants grown under MSW (
Figure 2A) or LSW (
Figure 2B) regimes.
Under MSW regime, positive and significant correlations were detected between shoot biomass production under nonstressful conditions (Y
P) with all stress tolerance indices; ATI with TOL, SSPI, k
1STI, MP, GMP; k
1STI with all tolerance tolerances except YSI. Negative and significant correlations were detected between shoot biomass production under stressful conditions (Y
S) with SSPI, TOL, and ATI; between YSI and TOL, SSPI, and ATI; between TOL and MP, YI, STI, HM; between k
2STI and SSPI; and between ATI and k
2STI and YI (
Figure 2A).
Under LSW regime, positive and significant correlations were detected between shoot biomass production under nonstressful (Y
P) and stressful conditions (Y
S) with all water stress tolerance indices. Negative and significant correlations were detected between the YSI with the SSPI and TOL indices (
Figure 2B).
Discrimination of the water index tolerance level of the nine forage grass cultivars based on only one criterion or tolerance index can be contradictory (
Table 5). Therefore, forage grasses should be differentiated and separated into different water stress tolerance levels based on all tolerance indices [
20]. The ranking method has been used to classify crop genotypes into different water stress tolerance levels [
21]. The ranking score of the nine forage grass cultivars in each of the 12 water stress tolerance indices under MSW and LSW regimes is shown in
Table 6.
2.6. Ranking
Considering all water stress tolerance indices, the cultivars
P. maximum cv. Mombaça and
P. maximum cv. Tanzânia had the highest stress tolerance indices (
Table 5) and the best-ranking scores under the MSW regime (
Table 6). Therefore, these two forage grass cultivars were identified as tolerant to moderate water stress. The cultivars
U. brizantha cv. Marandu, U. ruziziensis cv. Comum, and
P. maximum cv. Aruana were identified as moderately tolerant to moderate water stress, while
P. glaucum cv. ADR 300 and
P. atratum cv. Pojuca was the most sensitive cultivar to moderate water stress (
Table 6).
Under LSW regime, the cultivars
P. maximum cv. Aruana and
P. maximum cv. Mombaça were classified as water-stress-tolerant, whereas the cultivars
U. ruziziensis cv. Comum and
P. maximum cv. Tanzânia were classified as moderately tolerant to severe water stress (
Table 6). These results suggest that the cultivation of
P. maximum cultivars ‘Aruana’, ‘Mombaça’, and ‘Tanzânia’ is an excellent option for feeding cattle during the dry season in Brazil since these forage grass cultivars were identified as tolerant to water stress and have a high capacity for forage production in low soil water availability conditions. Indeed, using tolerant cultivars in areas subject to water stress is the best solution to face the predicted climate changes in the coming years and decades.
Fonseca and Martuscello [
5] reported that forage grasses belonging to the species
P. maximum have a high potential for forage yield response to stressful environmental conditions, confirming the results of this study. Some studies with wheat [
30], canola [
31], and sugarcane [
10] suggest that the degree of water stress tolerance is related to the ability of plants to uptake and accumulate mineral nutrients when exposed to low soil water availability conditions. In general, water stress tolerance is the characteristic of a plant species or cultivar to adapt to growing environments with low water availability. Under these stressful conditions, the extension of the root system has been a fundamental morphological characteristic to improve the ability of plants to extract water and nutrients from the soil [
12,
16].
Plants of
P. atratum cv. Pojuca were identified as sensitive to water stress under MSW and LSW regimes (
Table 6). Therefore, the cultivation of this forage cultivar during the dry season in tropical regions of Brazil should not be recommended due to its low capacity to produce shoot biomass in soil water stress conditions.