Introduction

The study of arid and semi-arid zones is an important issue at present, because in this type of regions there are adverse conditions for the production and survival of existing forest species, as a consequence of the prolonged severe drought (^{García et al., 2005}).

In these places, water is one of the most important factors that influence the distribution, growth and development of vegetation and is essential for all vital processes such as photosynthesis and respiration. It works as a means for the absorption of nutrients and metabolism of plants. The plant cell requires 85 to 90 % water to maintain vital dynamics and carry out enzymatic processes in a plant cell. A decrease in the water content below this level reduces the metabolic activity of the cells and the growth of the plants (^{Maiti et al., 2016}); during this condition, the cells remain smaller and the leaves have less development which, consequently, affects the photosynthetically active leaf area (^{Parra et al., 1999}). Furthermore, under such conditions the hydrolytic activity of the enzymes increases considerably, the transport of the ions decreases and the respiration, in general, increases (^{Salisbury and Ross, 2000}).

Under these conditions, the plants have developed different responses and adaptations that allow them to survive in conditions of constant water deficit (^{Nilsen and Orcutt, 1996}). Many of these are related to a greater ability to absorb water or more efficient use of this resource. Some plants have adjusted processes such as the C4 metabolism and the acid metabolism of Crassulaceae or CAM, which allow them to adapt to more arid environments (^{Black and Osmond, 2003}; Lüttge, 2004) and successfully face long periods of water scarcity and the colonization of adverse environments.

When the water deficit develops slowly, plants can present acclimation responses that have effects on growth, such as decreased leaf expansion and increased root growth (^{Potters et al., 2007}; ^{Shao et al., 2008}). Another mechanism of resistance at the physiological level is the closure of stomata, structures responsible for the highest proportion of water loss in plants (^{Taiz and Zeiger, 2006}). This response is mediated by the abscisic acid hormone (ABA) (^{Abe et al., 1997}; ^{Kang et al., 2002}).

Since water stress is the most limiting factor in the northeastern region of Mexico, the present work focused on studying how the seasonal water potentials of native tree species are related to the availability of soil water and the components of evaporative demand (temperature, relative humidity and precipitation). The study of native species in this region provides the opportunity to investigate, from an ecophysiological perspective, the response of shrub species to changes in the availability of resources, in this case, the moisture content of the soil, to better understand how ecosystem can sustain the productivity of biomass. Therefore, as an approach to understand how water potentials of plant xylem relate to environmental conditions, an investigation was conducted on four species of native plants to describe adaptive responses to the scarcity of water resources. The application of the results can be translated into a better management of each species, with a greater probability of success in future programs of plant production and reforestation with native species tolerant to stress due to the water deficit.

Materials and Methods

Study site

This study was carried out in the Experimental Research Station of the Facultad de Ciencias Forestales de la Universidad Autónoma de Nuevo León (School of Forest Sciences of the Autonomous University of Nuevo Leon) (24°47' N; 99°32' W, 350 masl) in Linares municipality, Nuevo León state, Mexico (Figure 1). Climate is subtropical and semi-arid with a warm summer. The average monthly temperature varies from 14.7 °C in January to 22.3 °C in August. The average annual rainfall is around 800 mm (^{González et al., 2004}). Soils are predominantly dark gray, vertisoles of clay-silt texture, with montmorillonite, which expands and contracts remarkably as the moisture content of the soil varies (^{González et al., 2004}).

Figure 1 Location of the study site 

Results and Disscusion

Air temperature, relative humidity and precipitation recorded 15 days before each sampling date during the experimental period are shown in Table 1.

The cumulative rainfall recorded during the study period was 340 mm. The maximum precipitation peak occurred on June 2^nd with 152 mm and the minimum precipitation was recorded on April 4^th with 0.4 mm. The seasonal moisture content of the soil, at different depths of the soil layer, showed a typical response with rainfall events. Significant differences were detected (P<0.022 and P<0.026) in soil moisture content between the soil layers for the sampling dates of March 22^nd and April 18^th, respectively (Figure 2). During the driest period, May 2, the soil moisture content was around 0.10 kg kg^-1, while, during the wettest period, June 2, the soil moisture content varied from 0.21 kg kg ^-1 (depth 0-10 cm) to 0.16 kg kg^-1 (40-50 cm depth) and coincides with the peak of highest rainfall. The gravimetric moisture content of the soil at the depth of 0-10 cm was more sensitive to rainfall events than the deeper layers. However, since there was great variability in rainfall and air temperature, the water content of the soil could not be controlled and maintained and, therefore, soil moisture may not be available for absorption in deep roots due to at the low rate of infiltration.

Contenido gravimétrico de humedad del suelo = Gravimetric moisture content of the soil; Fecha de muestreo = Samplig date; Precipitación = Precipitation

The P value is shown on each sampling date within the graph. Each value represents the mean ± standard error (n = 4). The accumulate precipitation (mm) is included for 15 days before each sampling date.

Figure 2 Seasonal pattern in the gravimetric moisture content of the soil at five depths. 

The responses of the water content of the soil in the present work agrees with other studies that have demonstrated the seasonal variability of substrate moisture in arid and Mediterranean grassland ecosystems (^{Anderson et al., 2001}; ^{Bussotti et al., 2002}; ^{González et al., 2000}; ^{López-Hernández et al., 2010}; ^{González-Rodríguez et al., 2016}). The erratic and dry environmental conditions that were observed during most of the experimental period could be related to frequent seasons of extreme environmental conditions such as droughts, typical of the northeastern region of Mexico (^{González et al., 2000}, ²⁰⁰⁴).

The seasonal variation of the water potential of the xylem at 06:00 h and 14:00 h in the four plant species is shown in Figure 3. At 06:00 h, the values were significantly different (P<0.05) among the four species on all sampling dates (Figure 3a). In the wettest period (June 2^nd), the highest in the water potential at the same time (-0.67 MPa and -0.76 MPa) were registered in C. pallida and C. boissieri, respectively, which are higher than -0.96 MPa observed in P. laevigata and -0.93 MPa in C. hookeri.

Potencial hídrico = Water potential; Tiempo (fecha de muestreo) = Time (sampling date).

The value of P is shown on each sampling date. Each value represents the mean ± standard error (n = 5).

Figure 3 Water potential of xylem at 06:00 h and 14:00 h in four species of native plants. 

On the contrary, in the driest period (May 2^nd), when the shrub species faced severe water stress, the most outstanding value (-1.52 MPa) in the potential at 06:00 h was registered in P. laevigata, while the lowest one (-2.92 MPa) was detected in C. hookeri, which suggests that the latter species is more sensitive to the lack of water.

It can be inferred that the recovery of the values of initiation of response in the water potential at 06:00 h in the four shrub species for the sampling dates from June 2 to June 30 is mainly related to the events of precipitation (Table 1).

In regard to the same potential at 14:00 h., significant differences (P<0.05) were identified among the native species in seven of ten sampling dates (Figure 3b). In the wettest (June 2^nd), when the water potential at the time indicated was the highest and when the water content of the soil in the layer 0-50 cm was higher than 0.20 kg kg^-1 (20 %) (Figure 2), C. pallida recorded the highest value (-1.07 MPa), while C. hookeri the lowest (-1.78 MPa); on the other hand, on the driest sampling date (May 2^nd), when the average water content in the soil was 0.10 kg kg^-1 (10 %), the values in the water potential at 2:00 p.m. varied from -1.76 MPa (C. pallida) to -3.10 MPa (C. hookeri). Similar results have been documented by ^{González and Cantú (2001)}; ^{López-Hernández et al. (2010)}; ^{González et al., 2011}; ^{González-Rodríguez et al. (2011)}; and ^{González et al. (2016)}; ^{Adhikari and White (2014)} who studied the adaptation of native shrubs to drought stress.

For the sampling dates between March 2 and May 17^th, when the average moisture content in the soil was approximately 0.12 kg kg^-1, an unusual response was found in the water potential at 14:00 h in particular, in C. hookeri and C. pallida, for which the water potential registered in the afternoon was higher than the morning one. In fact, the imbalance between the moisture content of the soil (or water potential of the soil) before sunrise and the water potential of the plant tissue (leaf and xylem) has been recorded in various mesophytic and xerophytic plants (^{Donovan, 2003}).

Among the possible factors responsible for this imbalance is the hydraulic resistance to the continuous soil-plant (^{Myers and Neales, 1984}), the smallest size of the plant (^{Brown and Archer, 1990}); the heterogeneity of soil moisture (^{Ourcival et al., 1994}); the reduction of the water potential of the cells due to nocturnal growth (^{Boyer, 1995}); the low osmotic potential (^{Berger et al., 1996}), open stomata and transpiration at night (^{Donovan et al., 2003}, ^{Bucci et al., 2004}, ^{Howard et al., 2009}). In addition, it is assumed that night transpiration and hydraulic lifting benefit the absorption of plant nutrients under drought conditions (^{Matzner and Richards, 1996}; ^{Sellin, 1999}; ^{Donovan et al., 2003}).

These observations are consistent with the results obtained in the present work, but in the context of the determinations of the water potential of the plant since the aforementioned processes were not carried out in this work.

With the exception of C. hookeri, in the depths 0-10 cm (0.705) and 10-20 cm (0.717), the water potential at 06:00 h of all the shrub species studied did not show a significant correlation (P>0.05) with the moisture content of the soil for each depth of the soil. On the other hand, the water potential values at 06:00 h correlated significantly and positively with those at 2:00 p.m. the values fluctuated from 0.867 in C. pallida, to 0.900 in C. boissieri. Regarding precipitation, all species showed a positive and significant relationship (Table 2); the values ranged from 0.697 (C. pallida) to 0.894 (C. hookeri).

During the course of the study and according to the water potential data, the four plant species faced mild to severe drought periods; P. laevigata and C. pallida showed higher water potential values at 06:00 h as well as at 2:00 p.m. compared with the other species, indicating an isohydric water regulation and probably a better adaptation to severe conditions. of drought (^{Breshears et al., 2009}). It seems that P. laevigata and C. pallida are examples of shrub species that have adapted to the low water availability of the soil, since they tend to maintain a humidification of the tissues, while the adaptation of C. hookeri and C. boissieri in dry periods it seems to depend on the strategies that allow them to cope with internal desiccation and, consequently, have lower values in water potentials (^{Gebrekirstos et al., 2006}).

The study suggests that the first two species can serve as a model to evaluate drought adaptation strategies in plant species with high water potential, while the latter would function as an adequate model to study the adaptation of the plant to drought with a low potential. In addition, P. laevigata is the species that under conditions of stress due to drought tends to achieve and maintain significantly greater water potential.

Despite their different capacities to withstand a wider range of water availability, all species investigated are considered suitable candidates for the restoration and continued conservation of drought-prone sites. Therefore, a combination of P. laevigata and C. pallida with other drought-tolerant native species can be recommended for reforestation, in order to prevent soil erosion and maintain, in a sustainable manner, the vegetation cover of the scrub ecosystems.

Based on the uncertain development of climatic variables and the range of responses to water stress of the species, maintaining the current diversity of Tamaulipan thorn scrub may be difficult in the future. In fact, the growth of the plant and the survival in the field depend, to a large degree, of a complex interaction of numerous environmental factors and internal mechanisms of the plant. Therefore, the assessment of the variability of the water status of the xylem in native shrubs of the northeastern scrublands of Mexico adapted to drought conditions represents an important step in the evaluation of some ecophysiological characteristics of plants for water stress studies.

Conclusions

The implications of this study showed that the species P. laevigata and C. pallida recorded the highest values of the water potential of the xylem at 06:00 a.m. and at 2:00 p.m., under conditions of water stress, so these species considered as drought tolerant; on the other hand, C. hookeri and C. boissieri, which recorded lower, lower values, would be susceptible to water deficit, and therefore, will be in a situation of physiological disadvantage in conditions of water scarcity.

The gradual decrease in the xylem water potential at the hours defined for the four shrub species gave a remarkable response to trends in soil water content and precipitation. The temperature of the air negatively influenced the values of the water potential of the xylem in all species. In contrast, the water potential values of the xylem increased as the water content of the soil and the relative humidity increased.

With this kind of studies it is possible to identify and know the patterns of absorption and water requirements among shrub species and understand the processes of plant succession. Based on this, drought tolerant species can be selected, which can be used in the productive recovery of arid and semi-arid ecosystems, in such a way that reforestation with them generates favorable conditions for the natural recovery of the ecosystems of this kind.