Does Morphological and Anatomical Plasticity during the Vegetative Stage Make Wheat More Tolerant of Water Deficit Stress Than Rice?  

Abstract

District of Columbia 20005 (P.S.B.)Water scarcity and the increasing severity of water deficit stress are major challenges to sustaining irrigated rice (Oryza sativa) production. Despite the technologies developed to reduce the water requirement, rice growth is seriously constrained under water deficit stress compared with other dryland cereals such as wheat (Triticum aestivum). We exposed rice cultivars with contrasting responses to water deficit stress and wheat cultivars well adapted to water-limited conditions to the same moisture stress during vegetative growth to unravel the whole-plant (shoot and root morphology) and organ/tissue (root anatomy) responses. Wheat cultivars followed a water-conserving strategy by reducing specific leaf area and developing thicker roots and moderate tillering. In contrast, rice 'IR64' and 'Apo' adopted a rapid water acquisition strategy through thinner roots under water deficit stress. Root diameter, stele and xylem diameter, and xylem number were more responsive and varied with different positions along the nodal root under water deficit stress in wheat, whereas they were relatively conserved in rice cultivars. Increased metaxylem diameter and lower metaxylem number near the root tips and exactly the opposite phenomena at the rootshoot junction facilitated the efficient use of available soil moisture in wheat. Tolerant rice 'Nagina 22' had an advantage in root morphological and anatomical attributes over cultivars IR64 and Apo but lacked plasticity, unlike wheat cultivars exposed to water deficit stress. The key traits determining the adaptation of wheat to dryland conditions have been summarized and discussed.Among cereals, rice (Oryza sativa) and wheat (Triticum aestivum) are the most important staple food crops, and they belong to the family Poaceae. These two cereals share a common ancestor and diverged about 65 million years ago (Sorrells et al., 2003). Rice eventually developed strong adaptation potential for fully flooded conditions across tropical to temperate environments, while wheat became well adapted to aerobic conditions mostly restricted to temperate environments. Rice, with a semiaquatic behavior, consumes about 30% of the total fresh water available for agricultural crops worldwide, which equates to a 2-to 3-fold higher consumption than other cereals such as wheat and maize (Zea mays; Peng et al., 2006). Despite a significantly lower water requirement, the potential yield of wheat in a favorable environment (9 tons ha 21 ) is comparable with the yield of fully flooded rice (9 tons ha 21 ) in the dry season at the International Rice Research Institute (IRRI; Fischer and Edmeades, 2010). Hence, rice records very low water productivity compared with wheat and other dryland cereals. Because of growing concerns about water scarcity and the increased frequency and magnitude of water deficit stress events under current and future climates, increasing or even sustaining rice yield under fully flooded conditions is highly challenging. To minimize the total water requ...

Genetic Control of Plasticity in Root Morphology and Anatomy of Rice in Response to Water Deficit

“…The rice root anatomy is adapted to semiaquatic conditions with characteristic outer sclerenchymatous layer, large cortex diameter, small stele, and xylem (Coudert et al, 2010;Kadam et al, 2015). However, to what extent natural and human selection has shaped root anatomical plasticity in response to water-deficit stress remains to be elucidated.…”

“…For instance, the plasticity of root length density in water-deficit stress contributes to rice grain yield stability (Sandhu et al, 2016). Similarly, the comparative analysis between water-deficit tolerant rice and wheat (Triticum aestivum) has demonstrated the functional relevance of plasticity in shoot and root traits to better adapt to water-deficit stress (Kadam et al, 2015). However, phenotypic traits that express constitutively with no plasticity also could provide stress adaptation.…”

“…Therefore, current rice breeding programs are striving to develop cultivars that are productive under water-deficit conditions (Bernier et al, 2009;Kumar et al, 2014;Sandhu et al, 2014). This will require a suite of morphological, anatomical, and physiological adjustments of shoot and root traits (Kadam et al, 2015;Sandhu et al, 2016). Interactions among these traits in response to water deficit are complex, rendering effective knowledge-intensive breeding strategies.…”

Rice Root Architectural Plasticity Traits and Genetic Regions for Adaptability to Variable Cultivation and Stress Conditions

“…Under different types of drought stress, plasticity in root length density or total root length (Kano-Nakata et al, 2011;Tran et al, 2015) and lateral root length and/or branching (Suralta et al, 2010;Kano-Nakata et al, 2013) has been observed to improve shoot biomass, water uptake, and photosynthesis under drought in rice. Plasticity in the level of root aerenchyma development (measured as root porosity) was reported to result in higher shoot dry matter (Niones et al, 2013) and grain yield (Niones et al, 2012) under transient drought stress in rice, and plasticity in other anatomical traits has been hypothesized as a major reason for wheat (Triticum aestivum) being more drought tolerant than rice (Kadam et al, 2015). In a set of 42 native and crop species, plasticity in root depth was a better predictor of shoot response to drought than absolute root depth (Reader et al, 1993).…”

Emerging tools, concepts and ideas to track the modulator genes underlying plant drought adaptive traits: An overview

“…16 A considerable number of such component traits are responsible for the quantitative regulation of utilization of water, the most critical resource under moisture deficit stress conditions, which could vary across diverse plant species and types. [17][18] Tissue water status determines the overall metabolic activity of the cells constituting it, which ultimately underlies the efficacy of various CT mechanisms. Cellular tolerance mechanisms equip each cell to build up a force against drought.…”