Plant–Water Relations in Native Shrubs and Trees, Northeastern Mexico

HUMBERTO GONZALEZ RODRIGUEZ',

HAYDEE ALEJANDRA DUENAS TIJERINA, ISRAEL CANTU SILVA, and R. K. MAITI

Facultad de Ciencias Forestales (School of Forest Sciences), Universidad Autonoma de Nuevo Leon, Linares, NL 67770, Mexico

'Corresponding author. E-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

ABSTRACT

Since plant internal water potentials (¥) are a consequence of the soil-plant atmospheric continuum, measuring the actual internal plant water potential and soil water status directly in the field gives us reliable information about the water stress plants may suffer. Hence, it is important to measure water potential to identify the plant’s capacity and strategy to cope with drought. According to the above mentioned, this chapter deals with the physiological adaptation of native trees and shrubs to drought stress in the semiarid ecosystems of northeastern Mexico. The present study has the objective to relate the soil water content with xylem water potential in native woody species such as Condalia hookeri (Rhamnaceae), Cordia boissieri (Boraginaceae), Prosopis laevigata (Fabaceae), and Celtis pallida. Seasonal xylem predawn (06:00 hrs) and midday (12:00 hrs) water potentials were determined at 15 day-intervals. Maximum and minimum seasonal predawn values ranged from -0.67 MPa (C. pallida) to -2.92 MPa (C. hookeri). At midday, xylem water potential varied from -1.07 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, 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. In conclusion, the study shows that the ability of species to control its water status may depend on the response capacity to absorb water and to control the loss of water during the day.

INTRODUCTION

Water is one of the most important factors influencing the distribution, growth, and development of vegetation and is essential for all vital processes such as photosynthesis and respiration. It forms the essential constituent of cell sap and cell vacuoles. It works as a medium for the absorption of plant nutrients and plant metabolism. Plant cell requires about 90% of water to maintain its vital activity. A substantial decrease in water in the cell causes plasmolysis, thereby, inhibiting all metabolic activities. Water potential plays an important role to maintain a pressure gradient in the plant cells required for the transport of water from one to another. Water absorbed by roots is transported upwards through xylem vessels up to the leaves and other organs to help in metabolic activities and the excess of water is lost through transpiration via the stomatae, thereby, maintaining a water balance in the plant cells. Deficiency of water causes a lowering in plant water potential, excess of water due to flooding affects respiration and plant growth. The sequential process of water absorption, its translocation, and loss by the transpirational flux is discussed in brief below. The deficiency of water called water stress affects the growth and development of the plant. Each species requires an optimum amount of water below which the growth of the species is reduced. The species adapted to drought is called drought resistant. Drought resistant plants have several morphological, anatomical, and biochemical mechanisms of resistance. The density of trichomes, leaf surface with a waxy coating, thick cuticle, compact palisade cells, and few biochemical components, such as proline, sugars, and ABA are related to drought resistance. In the semiarid tropics, several species are tolerant to drought, others not so much. We need to identify them in a forest ecosystem.

7.1.1 PLANT-WATER RELATIONS AND FOREST PRODUCTIVITY

The growth and development of a plant are highly dependent on the availability of soil moisture, soil nutrients and water content in the plant cell. The plant cell requires 85-89% water to maintain the dynamic vital activity and enzymatic processes in a plant cell. A decrease in the water content in the cell below this level reduces the metabolic activity of plant cells and plant growth. Therefore, there is a great necessity to maintain water balance in between plant cells starting from the roots to the leaves. Water content in the cell maintains water potential in the cell and the hydrostatic balance among cells. From the roots up to the leaves, there is a gradual decrease in water content maintaining water potential in the cells which force the movement of water from the root cells up to the leaves leading to the loss of water through transpiration. Plant cells need to maintain this hydrostatic balance for maintaining the plant growth. Water is absorbed by roots and move from the peripheral root cells to the interior of the cells, then reach endodermis and finally to the xylem vessels in the roots owing to the gradients of water potential starting from the peripheral root cells to xylem cells in the roots. Water once entering the xylem vessels is retained in the narrow vessels owing to adhesion force between water molecules. Xylem vessels form capillary tubes connected from the roots upwards to the stems pump water from the roots upwards by root pressures. The loss of water through transpiration creates a vacuum pressure in the leaf mesophyll due to which water in the xylem is under negative pressure and creates cohesive forces in xylem vessels between water molecules to maintain the water columns intact. Thus, roots absorb water from soils owing to the difference of water potential between soil and cells. There is always a gradient of water potential from peripheral cortical cells to the interior cells. Once reaching the xylem vessels water moves up by the suction force called ascent of sap. The adjacent phloem tissue is under positive pressure that is maintained osmotically with assimilated sugars and dissolved minerals. Variability in soil and atmospheric conditions influence the interaction between the pressures and structural properties determine the tissue resistivity against embolism formation under high negative pressures in xylem tissue that threaten the integrity of xylem transport.

7.1.2 PLANT-WATER RELATIONS STUDIES IN NORTHEASTERN MEXICO

The climate in northeastern Mexico is characterized by the alternation between favorable and unfavorable periods of soil water content which affects plant growth and development throughout the year. Plants differ widely in their capacity to cope with drought (Stienen et al., 1989). Adaptations exist to explain these differences and can be conveniently referenced to the capacity to maintain water status (water potential and/or relative water content, RWC). Plants under such conditions regulate their water status using several strategies, namely, osmotic adjustment, stomatal aperture, turgor maintenance, root distribution, and leaf canopy properties (Rhizopoulou et al., 1997). The main type of vegetation in northeastern Mexico, known as the Tamaulipan thorn scrub, is distinguished by a wide range of taxonomic groups exhibiting differences in growth patterns, leaf life spans, textures, growth dynamics, and phenological development (Reid et al., 1990; McMurtry et al., 1996). This semi-arid shrub-land, which covers about 200,000 km2 including southern Texas and northeastern Mexico, is characterized by an average annual precipitation of 805 mm and a yearly potential evapotranspiration of about 2200 mm. Vegetation is utilized as forage for livestock and wildlife, fuel-wood, timber for construction, traditional medicine, fencing, charcoal, agroforestiy, and reforestation practices in disturbed sites (Reid et al., 1990). Since water stress is the most limiting factor in this region, a work was focused to study how seasonal leaf water potentials of native tree species are related to soil water availability and evaporative demand components (Gonzalez- Rodriguez et al., 2009). The study of native species in this region provides an opportunity to investigate, from an ecophysiological perspective, the response of shrub species to changes in resource availability, in this case, soil moisture content, to gain a better understanding of how such an ecosystem may sustain biomass productivity. Thus, as an approach to understand how seasonal plant xylem water potentials are related to environmental conditions, a study was conducted in four native plant species to describe the adaptive responses.

MATERIAL AND METHODS

7.2.1 STUDY SITE

This research was carried out at the Experimental Research Station of Universidad Autonoma de Nuevo Leon (24°47' N; 99°32' W; 350 m amsl) in

Linares municipality, Nuevo Leon, Mexico. The climate is subtropical and semi-arid with a warm summer. Mean monthly air temperature ranges from 14.7°C in January to 22.3°C in August. Average annual precipitation is about 800 mm. Soils are predominantly deep, dark-gray, lime clay montmorillonite vertisols (Gonzalez et al., 2004).

7.2.2 PLANT MATERIAL AND XYLEM WATER POTENTIAL MEASUREMENTS

Four co-existing shrub species, representative of the native plant community and of importance for browsing animals (Dominguez-Gomez et al., 2014) and other multipurpose uses such as wood, charcoal, and timber, have been chosen: Condalia hookeri M.C. Johnst. (Rhamnaceae), Cordia boissieri A.DC. (Boraginaceae), Prosopis laevigata (Humb. & Bonpl. ex Willd) M.C. Johnst. (Fabaceae), and Celtis pallida Torr. (Ulmaceae). Five plants of each species were randomly selected within a 20 m><20 m undisturbed thomscrub plot (Gonzalez et al., 2004) for xylem water potential (MPa) measurements. Xylem water potential measurements were performed on terminal twigs immediately after cutting the sample, at 15-days intervals (between February 21 and June 30, 2017), at 06:00 h (predawn) and 14:00 h (midday), using a Scholander pressure bomb (Model 3005, Soil Moisture Equipment Corp., Santa Barbara, CA) (Ritchie and Hinckley, 1975).

7.2.3 ENVIRONMENTAL DATA

Air temperature (°C) and relative humidity (%) were registered daily using a HOBO Pro Data Logger (HOBO Pro Temp/RH Series, Forestry Suppliers, Inc., Jackson, MS). Daily rainfall (mm) was obtained from a Tipping Bucket Rain Gauge (Forestry Suppliers, Inc.). Gravimetric soil water content (kg kg'1 dry soil) on each sampling date was determined in soil cores at layers (four replications) of 0-10, 10-20, 20-30, 30-40, and 40-50 cm.

7.2.4 STATISTICAL ANALYSES

Since xylem water potential and soil water content data were not normally distributed (Kolmogorov-Smimov test) and variances were not homogeneous for most sampling dates (Levene test), predawn, midday, and soil water content experimental data were subjected to the Kruskal-Wallis nonpara- metric test (Ott, 1993). The relationships between predawn and midday water potentials and prevailing environmental variables were determined by the Spearman’s rank-order correlation analysis since the null hypothesis of normality was rejected at p = .05. All applied statistical methods were according to the SPSS® software package (standard released version 13.0 for Windows, SPSS Inc., Chicago, IL).

 
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