RESULTS AND DISCUSSION

Table of Contents:

The seasonal pattern in gravimetric soil water content at different soil layers as well as the cumulative rainfall registered during the experimental period is shown in Figure 7.1. Cumulative rainfall registered during the experimental period was about 340 mm. Major precipitation peak occurred in June, 2 with 152 mm and minimum rainfall was registered on April, 4 and May, 2 with 0.4 and 2.0 nun, respectively. Seasonal soil water content, at different soil-layer depths, showed a typical response with the onset rainfall events. There were soil water content differences (p < .05) among soil layers in two (March, 22 and April, 18) out of the ten sampling dates (Fig. 7.1). During the driest period, April, 4, the soil water content was around 0.10 kg kg"1 soil, while during the wettest period, soil water content ranged from 0.218 (depth 0-10 cm) to 0.16 kg kg'1 soil (depth 40-50 cm) and coincides with major precipitation peak. The soil moisture content in the soil layer depth 0-10 cm was more responsive to rainfall event than deeper layers.

The seasonal variation in predawn and midday xylem water potential in the four plant species is shown in Figure 7.2. Predawn xylem water potential values were significantly differed (p < .05) among plant species in all sampling dates. At the wettest period (02-Jun), water potential ranged from -0.67 MPa (C. pallida) to -0.94 MPa (C. hookeri and C. boissieri). In contrast, on the driest period (02-May), significantly higher water potential values varied from -1.52 MPa (P. laevigata) to -2.92 MPa (C. hookeri). With respect to the midday xylem water potential, significant differences (p < .05) among native plant were observed in seven sampling dates out of ten. During the wettest sampling date (June, 2), higher midday water potential was recorded in C. pallida (-1.07 MPa), while a minimum value was observed in C. hookeri (-1.78 MPa). In contrast, during the driest period (May 2), midday water potential values ranged from -1.76 MPa (C. pallida) to -3.10 MPa (C. hookeri). Predawn water potential values were highly and positively correlated with midday ones (r values varied from 0.900, in C. boissieri, to 0.867 in

C. pallida) and rainfall (r values ranged from 0.894 in C. hookeri to 0.697 in C. pallida). Correlation analysis between soil water content at different soil layers with predawn water potential values was weak. During the study and accordingly with water potential data, native plant species faced mild to severe drought periods, being the species P laevigata and C. pallida the ones that achieved higher predawn and midday water potential values. Thus, these species could be considered as drought-tolerant species while C. hookeri and C. boissieri showed lower water potentials and could be in a physiological disadvantage under soil water stress. The study suggests that the first two species may serve as a model to evaluate the strategies of adaptation to drought at high tissue water potential while the latter may serve as an adequate model to study plant adaptation to drought at low tissue water potential. Another explanation for this physiological response is that, perhaps, P laeigata and C. pallida seem to tolerate drought using their deeper rooting system, while C. hookeri and C. boissieri use other physiological or morphological strategies to overcome water stress. Similar findings have been reported by Lopez-Hemandez et al. (2010) studying the adaptation of native shrubs to drought stress in northeastern Mexico. The capacity for osmoregulation among native shrubs and trees that grow in the northeastern region of Mexico has suggested a range value between —1.11 and -2.65 MPa (Gonzalez and Silva, 2001). The results of the present study have indicated that the response of a shrub species to evade drought stress is related to their water and osmotic potentials and to the response of interacting to environmental variables; soil water content, rainfall or evaporative demand components. Also, the ability of native species to cope with drought stress depends on the pattern of water uptake and the extent to control water loss through the transpirational flux. In a humid environment, the rate of transpiration is lower than that in hot summer days which control directly the speed of transpiration and movement of water from the roots upwards to the leaves. This is the driving force for the growth and development of a plant. Optimum moisture is essential for plant growth. Lack of soil moisture causes drought and reduce the growth. In a forest, species susceptible to drought, thereby causing the decrease in growth. Some species are resistant/tolerant to drought due to the presence of morpho- anatomical and physiological mechanisms. Krober et al. (2015) analyzed leaf morphology of 40 evergreen and deciduous broadleaved subtropical tree species and relationships to functional ecophysiological traits. The authors asked whether the ecophysiological parameters such as stomatal conductance and xylem cavitation vulnerability could be predicted from microscopy leaf.

The implications of this study suggest that the species respond differently to drought through the employment of different strategies and there is scope for forest and range management practices in the selection of drought-tolerant species for planting and reforestation of drought-prone areas.

Seasonal pattern in gravimetric soil water content at five soil depths

FIGURE 7.1 Seasonal pattern in gravimetric soil water content at five soil depths. Values are means±standard errors (n = 4). Cumulative rainfall (mm) for a 15-day period prior to each sampling date is shown. Within the graph, at each sampling date, the asterisk and ns denote significant (*,p < .05) or not significant (p > .05) differences among soil layers, respectively, in soil water content according to the Kruskal-Wallis test.

Since water availability is the most limiting factor controlling tree growth, survival, and distribution in diy climates (Newton and Goodin, 1989), the great diversity of native shrubs in this region reflects the plasticity among these species to cope with a harsh environment. Therefore, shrub and tree plants have evolved key morphological and physiological traits suited for adaptation to environmental constraints, especially in drought-prone regions. The strategies involve early leaf abscission, limited leaf area, an extensive and deeper root system, epidermal wax accumulation associated with the reduction of water loss by stomatal closure, and accumulation of organic and inorganic solutes (Newton et al., 1991). Shrubs and trees growing in this region under adverse environmental conditions have to seasonally adjust their morpho-physiological traits to cope successfully with changes in soil water availability (Bucci et al., 2008).

Seasonal predawn and midday xylem water potential in four native plant species

FIGURE 7.2 Seasonal predawn and midday xylem water potential in four native plant species. Values are means = standard errors (n = 5). At each sampling date, the double, and simple asterisks and ns denote significant (**,p < .01; *,p < .05) or not significant (p > .05) differences among plant species, respectively, based on the Kruskal-Wallis test.

KEYWORDS

  • trees
  • shrubs
  • water potential
  • drought resistance
  • physiological adaptation

REFERENCES

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