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Background Low phytoavailability of phosphorus (P) limits crop production worldwide. Increasing seed P content can improve plant establishment and increase yields. This is thought to be a consequence of faster initial root growth, which gives seedlings earlier access to growth-limiting resources, such as water and mineral elements. It can be calculated that seed P reserves can sustain maximal growth of cereal seedlings for several weeks after germination, until the plant has three or more leaves and an extensive root system. Case study In this issue of Plant and Soil, Muhammad Nadeem and colleagues report (1) that measurable P uptake by roots of maize seedlings begins about 5 d after germination, (2) that the commencement of root P uptake is coincident with the transition from carbon heterotrophy to carbon autotrophy, and (3) that neither the timing nor the rate of uptake of exogenous P by the developing root system is influenced by initial seed P content. Hypothesis Here it is hypothesised that the delay in P acquisition by roots of maize seedlings might be explained if the expression of genes encoding phosphate transporters is not upregulated either (1) because the plant has sufficient P for growth or (2) because a systemic signal from the shoot, which relies on photosynthesis or phloem development, is not produced, translocated or perceived.

Selenium (Se) is an essential mineral element for animals and humans, which they acquire largely from plants. The Se concentration in edible plants is determined by the Se phytoavailability in soils. Selenium is not an essential element for plants, but excessive Se can be toxic. Thus, soil Se phytoavailability determines the ecology of plants. Most plants cannot grow on seleniferous soils. Most plants that grow on seleniferous soils accumulate <100 mg Se kg(-1) dry matter and cannot tolerate greater tissue Se concentrations. However, some plant species have evolved tolerance to Se, and commonly accumulate tissue Se concentrations >100 mg Se kg(-1) dry matter. These plants are considered to be Se accumulators. Some species can even accumulate Se concentrations of 1000-15 000 mg Se kg(-1 )dry matter and are called Se hyperaccumulators. This article provides an overview of Se uptake, translocation and metabolism in plants and highlights the possible genetic basis of differences in these between and within plant species. The review focuses initially on adaptations allowing plants to tolerate large Se concentrations in their tissues and the evolutionary origin of species that hyperaccumulate Se. It then describes the variation in tissue Se concentrations between and within angiosperm species and identifies genes encoding enzymes limiting the rates of incorporation of Se into organic compounds and chromosomal loci that might enable the development of crops with greater Se concentrations in their edible portions. Finally, it discusses transgenic approaches enabling plants to tolerate greater Se concentrations in the rhizosphere and in their tissues. The trait of Se hyperaccumulation has evolved several times in separate angiosperm clades. The ability to tolerate large tissue Se concentrations is primarily related to the ability to divert Se away from the accumulation of selenocysteine and selenomethionine, which might be incorporated into non-functional proteins, through the synthesis of less toxic Se metabilites. There is potential to breed or select crops with greater Se concentrations in their edible tissues, which might be used to increase dietary Se intakes of animals and humans.

Excessive N fertilization results in low N-use efficiency (NUE) without any yield benefits and can have profound, long-term environmental consequences including soil acidification, N leaching and increased production of greenhouse gases. Improving NUE in crop production has been a longstanding, worldwide challenge. A crucial strategy to improve NUE is to enhance N uptake by roots. Taking maize as a model crop, we have compared root dry weight (RDW), root/shoot biomass ratio (R/S), and NUE of maize grown in the field in China and in western countries using data from 106 studies published since 1959. Detailed analysis revealed that the differences in the RDW and R/S of maize at silking in China and the western countries were not derived from variations in climate, geography, and stress factors. Instead, NUE was positively correlated with R/S and RDW; R/S and NUE of maize varieties grown in western countries were significantly greater than those grown in China. We then testified this conclusion by conducting field trials with representative maize hybrids in China (ZD958 and XY335) and the US (P32D79). We found that US P32D79 had a better root architecture for increased N uptake and removed more mineral N than Chinese cultivars from the 0-60 cm soil profile. Reported data and our field results demonstrate that a large and deep root, with an appropriate architecture and higher stress tolerance (higher plant density, drought and N deficiency), underlies high NUE in maize production. We recommend breeding for these traits to reduce the N-fertilizer use and thus N-leaching in maize production and paying more attention to increase tolerance to stresses in China.

Limitation of grain crop productivity by phosphorus (P) is widespread and will probably increase in the future. Enhanced P efficiency can be achieved by improved uptake of phosphate from soil (P-acquisition efficiency) and by improved productivity per unit P taken up (P-use efficiency). This review focuses on improved (P-use efficiency, which can be achieved by plants that have overall lower P concentrations, and by optimal distribution and redistribution of P in the plant allowing maximum growth and biomass allocation to harvestable plant parts. Significant decreases in plant P pools may be possible, for example, through reductions of superfluous ribosomal RNA and replacement of phospholipids by sulfolipids and galactolipids. Improvements in P distribution within the plant may be possible by increased remobilization from tissues that no longer need it (e.g. senescing leaves) and reduced partitioning of P to developing grains. Such changes would prolong and enhance the productive use of P in photosynthesis and have nutritional and environmental benefits. Research considering physiological, metabolic, molecular biological, genetic and phylogenetic aspects of P-use efficiency is urgently needed to allow significant progress to be made in our understanding of this complex trait.

It has been estimated that one-third of the world's population lack sufficient Zn for adequate nutrition. This can be alleviated by increasing dietary Zn intakes through Zn biofortification of edible crops. Biofortification strategies include the application of Zn-fertilizers and the development of crop genotypes that acquire more Zn from the soil and accumulate it in edible portions. Zinc concentrations in roots, leaves, and stems can be increased through the application of Zn-fertilizers. Root Zn concentrations of up to 500-5000 mg kg(-1) dry matter (DM), and leaf Zn concentrations of up to 100-700 mg kg(-1) DM, can be achieved without loss of yield when Zn-fertilizers are applied to the soil. It is possible that greater Zn concentrations in non-woody shoot tissues can be achieved using foliar Zn-fertilizers. By contrast, Zn concentrations in fruits, seeds, and tubers are severely limited by low Zn mobility in the phloem and Zn concentrations higher than 30-100 mg kg(-1) DM are rarely observed. However, genetically modified plants with improved abilities to translocate Zn in the phloem might be used to biofortify these phloem-fed tissues. In addition, genetically modified plants with increased tolerance to high tissue Zn concentrations could be used to increase Zn concentrations in all edible produce and, thereby, increase dietary Zn intakes.

Contents Summary 677 I.  Physical and chemical properties of zinc 678 II.  Biochemical properties of zinc 678 III.  Proteins interacting with zinc 678 IV.  Zinc fluxes in the soil–root–shoot continuum 679 V.  Zinc in plants 684 VI.  Plant responses to elevated soil Zn 686 Acknowledgements 695 References 696 Summary Zinc (Zn) is an essential component of thousands of proteins in plants, although it is toxic in excess. In this review, the dominant fluxes of Zn in the soil–root–shoot continuum are described, including Zn inputs to soils, the plant availability of soluble Zn2+ at the root surface, and plant uptake and accumulation of Zn. Knowledge of these fluxes can inform agronomic and genetic strategies to address the widespread problem of Zn‐limited crop growth. Substantial within‐species genetic variation in Zn composition is being used to alleviate human dietary Zn deficiencies through biofortification. Intriguingly, a meta‐analysis of data from an extensive literature survey indicates that a small proportion of the genetic variation in shoot Zn concentration can be attributed to evolutionary processes whose effects manifest above the family level. Remarkable insights into the evolutionary potential of plants to respond to elevated soil Zn have recently been made through detailed anatomical, physiological, chemical, genetic and molecular characterizations of the brassicaceous Zn hyperaccumulators Thlaspi caerulescens and Arabidopsis halleri.

Phosphorus (P) and zinc (Zn) uptake and its physiological use in plants are interconnected and are tightly controlled. However, there is still conflicting information about the interactions of these two nutrients, thus a better understanding of nutritional homeostasis is needed. The objective of this work was to evaluate responses of photosynthesis parameters, P-Zn nutritional homeostasis and antioxidant metabolism to variation in the P × Zn supply of cotton ( Gossypium hirsutum L.). Plants were grown in pots and watered with nutrient solution containing combinations of P and Zn supply. An excess of either P or Zn limited plant growth, reduced photosynthesis-related parameters, and antioxidant scavenging enzymes. Phosphorus uptake favoured photochemical dissipation of energy decreasing oxidative stress, notably on Zn-well-nourished plants. On the other hand, excessive P uptake reduces Zn-shoot concentration and decreasing carbonic anhydrase activity. Adequate Zn supply facilitated adaptation responses to P deficiency, upregulating acid phosphatase activity, whereas Zn and P excess were alleviated by increasing P and Zn supply, respectively. Collectively, the results showed that inter ionic effects of P and Zn uptake affected light use and CO 2 assimilation rate on photosynthesis, activation of antioxidant metabolism, acid phosphatase and carbonic anhydrase activities, and plant growth-related responses to different extents.

Root hairs are a key trait for improving the acquisition of phosphorus (P) by plants. However, it is not known whether root hairs provide significant advantage for plant growth under combined soil stresses, particularly under conditions that are known to restrict root hair initiation or elongation (e.g. compacted or high-strength soils). To investigate this, the root growth and P uptake of root hair genotypes of barley, Hordeum vulgare L. (i.e. genotypes with and without root hairs), were assessed under combinations of P deficiency and high soil strength. Genotypes with root hairs were found to have an advantage for root penetration into high-strength layers relative to root hairless genotypes. In P-deficient soils, despite a 20% reduction in root hair length under high-strength conditions, genotypes with root hairs were also found to have an advantage for P uptake. However, in fertilized soils, root hairs conferred an advantage for P uptake in low-strength soil but not in high-strength soil. Improved root–soil contact, coupled with an increased supply of P to the root, may decrease the value of root hairs for P acquisition in high-strength, high-P soils. Nevertheless, this work demonstrates that root hairs are a valuable trait for plant growth and nutrient acquisition under combined soil stresses. Selecting plants with superior root hair traits is important for improving P uptake efficiency and hence the sustainability of agricultural systems.

Core Ideas Supplemental irrigation with micro‐sprinkling hose was suitable for wheat. The Fv/Fm, ΦPSII, and photosynthetic rate increased with increased hose sprinkling angle. Supplemental irrigation with 80° sprinkling angle hose increased dry matter, grain‐filling rate, and duration. All benefits were hypothesized to accrue through the increased soil water uniformity. Water‐saving irrigation techniques are critical for winter wheat (Triticum aestivum L.) development in the Huang‐Huai‐Hai Plain of China and assist in coping with increasingly serious water shortages. A micro‐sprinkling irrigation experiment was conducted from 2010 to 2012 to investigate how soil water distribution, photosynthesis, drymass accumulation, and grain yield responded to different sprinkling angles. Five treatments were designed: Rainfed, W35, W50, W65, and W80. The Rainfed treatment consisted of no irrigation throughout the growing season. The W35 to W80 treatments involved irrigation applied through micro‐sprinkling hoses with sprinkling angles of 35°, 50°, 65°, and 80°, respectively. Micro‐sprinkling irrigation could regulate soil water content to the target level of previous designs. After irrigation, distribution of soil water across the four inter‐rows was the most uniform in W80. Following the intermediate stage of grain filling, when increasing sprinkling angle from 35° to 80°, the mean actual photochemical efficiency, maximum quantum yield of the PSII, flag leaf photosynthetic rate, and canopy apparent photosynthetic rate (CAP) significantly increased. A high sprinkling angle is favorable in improving the grain‐filling rate as well as lengthening its duration. At maturity, the W80 treatment produced the highest dry matter (DM) in the stem plus sheath and the highest grain weight and total DM, leading to the highest 1000‐grain weight, grain yield, and water‐use efficiency (WUE). Overall, the optimum sprinkling angle of the micro‐sprinkling hose for supplemental irrigation (SI) of winter wheat after jointing is 80°, and the benefits are hypothesized to arise from an increased uniformity of soil water conditions.

Breeding for advantageous root traits will play a fundamental role in improving the efficiency of water and nutrient acquisition, closing yield gaps, and underpinning the \"Evergreen Revolution\" that must match crop production with human demand. This preface provides an overview of a Special Issue of Annals of Botany on \"Root traits benefitting crop production in environments with limited water and nutrient availability\". The first papers in the Special Issue examine how breeding for reduced shoot stature and greater harvest index during the Green Revolution affected root system architecture. It is observed that reduced plant height and root architecture are inherited independently and can be improved simultaneously to increase the acquisition and utilisation of carbon, water and mineral nutrients. These insights are followed by papers examining beneficial root traits for resource acquisition in environments with limited water or nutrient availability, such as deep rooting, control of hydraulic conductivity, formation of aerenchyma, proliferation of lateral roots and root hairs, foraging of nutrient-rich patches, manipulation of rhizosphere pH and the exudation of low molecular weight organic solutes. The Special Issue concludes with papers exploring the interactions of plant roots and microorganisms, highlighting the need for plants to control the symbiotic relationships between mycorrhizal fungi and rhizobia to achieve maximal growth, and the roles of plants and microbes in the modification and development of soils.