Evaluation of drought tolerance in bread wheat using water relations

Introduction

Climate change, climate variability and environmental stress play an important role in determining physiological performance, vulnerability and productivity of crop plants in their environment (Marian Brestic and Marek Zivcak, 2013).

Drought is considered as the main environmental factor limiting plant growth and yield worldwide. Drought stress results in stomatal closure and reduced transpiration rates; a decrease in the water potential of plant tissues; growth inhibition and decrease in photosynthesis; accumulation of abscisic acid (ABA), sorbitol, mannitol and proline; and formation of radical scavenging compounds, e.g. ascorbate, glutathione and a-tocopherol (Yordanov et al., 2003). Besides these physiological responses, plants also undergo morphological changes (Vassileva et al., 2012).

The impact of water shortage and lower rainfall during the sowing period seems to be the main reason for lesser acreage under wheat crop and reduction in wheat production. Therefore, breeding for drought tolerant wheat is an important task and objective in the present scenario (Anwar et al., 2011).

Since the yield is a trait with low heritability, therefore selection based on that alone can not be reliable (Blum, 1992). Many morphological, biochemical and molecular responses on crop plants, such as stomatal closure and reduced transpiration rates contribute towards adaptation to such unavoidable environmental constraints (Sairam, 1994; Mohamed and Ismail, 2009).

Breeding for drought tolerance by selecting solely for yield may not be successful, because the heritability of yield under drought conditions is controlled at independent genetic loci. Therefore, plant breeders have always looking appropriate and repeatable indicators to screening for drought tolerance (Zobel et al., 1988; Hasheminasab et al., 2014). Physiologists have often suggested that the detection and selection of physiological traits related to plant water status are reliable methods to breeding for higher yield, and could be a valuable strategy for use in conjunction with normal methods of plant breeding (El Jaafari et al., 1993; Blum, 2005). RWP is an important physiological indicator in assessing the degree of water stress. RWP is also among the main physiological criteria that influence plant water relations and have been using for screening drought tolerant genotypes (Hasheminasab et al., 2012).

Relative water content (RWC), Relative Water Loss (RWL), Excised leaf water retention (ELWR), Excised Leaf Water Loss (ELWL), Initial Water Content (IWC), Leaf Water Content (LWC), Water Saturation Deficit (WSD), Leaf Water Loss (LWL) are among the main physiological criteria that influence plant water relations and have been used for assessing drought tolerance (Barrs, 1968; Clark and Maccaig, 1982; Manette et al., 1988; Xing et al., 2004). The objective of the present investigation were to:

i) better understand the effect of drought stress on some physiological traits associated with leaves water status (ii) identify the efficent physiological traits for screening drought tolerant genotypes and (iii) introduction of a new integrated selection index for screening drought tolerant genotypes.