Better understanding of root system structure and function is critical to crop improvement in water-limited environments. The aims of this study were to examine root system characteristics of two wheat genotypes contrasting in tolerance to water limitation and to assess the functional implications on adaptation to water-limited environments of any differences found. The drought tolerant barley variety, Mackay, was also included to allow inter-species comparison. Single plants were grown in large, soil-filled root-observation chambers. Root growth was monitored by digital imaging and water extraction was measured. Root architecture differed markedly among the genotypes. The drought-tolerant wheat (cv. SeriM82) had a compact root system, while roots of barley cv. Mackay occupied the largest soil volume. Relative to the standard wheat variety (Hartog), SeriM82 had a more uniform rooting pattern and greater root length at depth. Despite the more compact root architecture of SeriM82, total water extracted did not differ between wheat genotypes. To quantify the value of these adaptive traits, a simulation analysis was conducted with the cropping system model APSIM, for a wide range of environments in southern Queensland, Australia. The analysis indicated a mean relative yield benefit of 14.5% in water-deficit seasons. Each additional millimetre of water extracted during grain filling generated an extra 55 kg ha-1 of grain yield. The functional implications of root traits on temporal patterns and total amount of water capture, and their importance in crop adaptation to specific water-limited environments, are discussed.

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The role of root architectural traits in adaptation of wheat to water-limited environments

Publisher Summary This chapter reviews the wheat physiology/breeding work relative to the constraints of dryland cropping in Mediterranean environments and explores opportunities for additional yield improvement associated with morphological and physiological traits. The chapter discusses the environmental constraints to crop growth in the parts of the world that experience Mediterranean-type climates. Water stress is a major limitation to wheat growth and yield in Mediterranean and other environments. Grain yield (GY) is defined as the product of the biomass produced and the harvest index. Biomass can be increased by agronomic manipulation or by genetic means. Germination, ear initiation, terminal spikelet, and anthesis act as physiological switch for the allocation of assimilates to different organs of the plant, hence phenology and assimilate partitioning are closely related. Breeders have changed the structure of cereals considerably, both indirectly through changes in phenology and directly through the introduction of dwarfing genes. The chapter explains the effect of plant morphology and physiology on water usage. In water-limited environments, biomass production is a function of the water used by the crop (WU) and the efficiency with which it is converted into biomass (WUE). Biomass production can be defined by the amount of radiation intercepted (RI) and the radiation-use efficiency (RUE)—that is, the efficiency of the conversion of this radiation to dry matter.

Morphological and physiological traits associated with wheat yield increases in Mediterranean environments

A field study tested the hypothesis that modern wheat varieties invest a lesser proportion of the total dry matter (root plus shoot) in the root system compared to old varieties. The study was carried out on a duplex soil (sand over clay) at Merredin, Western Australia in a Mediterranean type environment. We also compared the root:shoot dry matter ratios of near-isogenic lines for Rht dwarfing genes.

Root:shoot ratios of old and modern, tall and semi-dwarf wheats in a mediterranean environment

At a whole plant level the effects of drought is usually perceived as a decrease in growth and photosyntheticcarbon assimilation. That is why this review is focused mainly on recent information about the effects ofdrought on plant growth, water relations and photosynthesis, as well as mechanisms of adaptation. It is shownthat plants have evolved a great number of adaptive mechanisms that allow the biochemical systems to copewith increased water deficit. The literature analyzed in this review shows the complexity of tolerance to waterdeficit and supports the statements of many authors that the flexibility of cell metabolism and its fastacclimation to changes in environmental conditions is a first essential step in stress avoidance.

An overview on drought induced changes in plant growth, water relationsand photosynthesis -

Water use and water use efficiency of old and modern wheat cultivars and one barley cultivar were measured in a Mediterranean environment at Merredin, W.A. Water use efficiency for grain increased substantially from old to modern cultivars, with little difference among modern cultivars. Water use efficiency for dry matter was similar between cultivars. Barley had the highest water use efficiency of both grain and dry matter. Improved water use efficiency for grain in modern cultivars was associated with faster development, earlier flowering, improved canopy structure and higher harvest index. Modern cultivars used slightly less water than old cultivars. The pattern of water use was also different, with late-maturing old cultivars using more water in the pre- than the post-anthesis period. The ratio of pre- to post-anthesis water use was highest with the late-maturing, old cultivar Purple Straw (5.2:1) and lowest with early-maturing, modern cultivar Gutha (3.0:1). Soil evaporation estimates showed that modern cultivars had lower rates of soil evaporation in the early part of the growing season. This was associated with their faster leaf area development and improved light interception. About 40% of the total water use was lost by soil evaporation with very little difference between wheat cultivars. Barley had 15% less soil evaporation than wheat. Water use efficiency for grain based on transpiration (transpiration efficiency) for the four modern cultivars was 15.8 kg ha-1 mm-1, similar to other studies in comparable environments. Some further improvement in water use efficiency appears possible through improvement in crop biomass and harvest index. However, given the frequent and severe limitations of total water supply at low rainfall sites such as Merredin, there appears to be more scope for improvement in yield and water use efficiency in the medium and high rainfall areas of the wheatbelt.

Water Use and Water Use Efficiency of Old and Modern Wheat Cultivars in a Mediterranean-type Environment

Spring wheat cv. ‘Gutha’ was grown in continuous wheat (W/W) and narrow-leafed lupin (L. angustifolius L. cv. Yandee)-wheat (L/W) rotation on a yellow earth over mottled clay (Arenic Fragiudult) in a mediterranean climate for two years. The first year had a higher than average rainfall with adequate soil water until anthesis. The second year was very dry (only 232 mm total rainfall) and soil water contents were low throughout the growing season. Nitrogen fertilizer (+N) treatments were included in both years. In the first year an adjacent experiment compared the effects of loosening a pronounced traffic pan which existed on the site (LS)versus unloosened (US).

The cost of stress: Dry matter partitioning changes with seasonal supply of water and nitrogen to dryland wheat

. Spring wheats are sown in autumn and mature under conditions of spring water deficits in Western Australia. While grain yields are very small (averaging 1.05t ha-1), water-use efficiencies (WUE) of crops grown with good management compare favourably with the rest of Australia at 10 kg grain ha-1 mm-1. Major water loss, calculated at an average 40% of growing-season rainfall, occurs through evaporation from the soil surface, and in some years significant losses also occur through deep drainage on sandy soils. Evaporation losses can be reduced by stubble retention and adding gypsum to improve filtration on structurally unstable, fine-textured soils. More general increases in crop water use occur through all agronomic factors which result in rapid and uniform plant cover as early as possible after break of season. This includes adequate fertilization, weed control and narrow row-width. Wheat crops sown after pasture and grain legumes consistently outyield all-cereal rotations.

Ann Hamblin

Papers

The cost of stress: Dry matter partitioning changes with seasonal supply of water and nitrogen to dryland wheat

Management of soil water for wheat production in Western Australia