Chunjian Li

Potassium nutrition of crops under varied regimes of nitrogen supply

Nitrogen (N) over-application is a serious problem in intensive agricultural production areas with consequent large N losses and environmental pollution. In contrast to N, potassium (K) application has been neglected in many developing countries and this has resulted in soil K depletion in agricultural ecosystems and prevented increases in crop yields. Nitrogen-potassium interaction is currently a topic of interest in many studies and the focus of this review is K nutrition under varied N regimes. Nitrogen form and application rate and time influence soil K fixation and release, as well as K uptake, transport, cycling and reutilization within crops. High yielding quality crops can be obtained by optimal N: K nutritional ratios. High rates of applications of N and K do not necessarily lead to increased yield increments and may even reduce yield. Yield response to K uptake depends on N nutritional status and the interaction is usually positive when NO3−-N is supplied. Antagonism between NH4+ and K+ in uptake was mostly attributed to simple competitive effects in the past while evidence showing mixed-noncompetitive interactions existed. Two components of membrane transport systems for K uptake by plants are a high-affinity K+ transport system which is inhibited by NH4+ and a low-affinity K+ transport system which is relatively NH4+ insensitive. Potassium is highly mobile within plants but its flow and partitioning can change depending on the forms of N supply. NH4+ nutrition in comparison to NO3−-supply results in more K translocation to leaves. A better understanding of the mechanism of N-K interaction can be a useful guide to best nutrient management in agricultural practice in order to achieve high yields with high nutrient use efficiency.

Temporal and spatial profiling of root growth revealed novel response of maize roots under various nitrogen supplies in the field.

A challenge for Chinese agriculture is to limit the overapplication of nitrogen (N) without reducing grain yield. Roots take up N and participate in N assimilation, facilitating dry matter accumulation in grains. However, little is known about how the root system in soil profile responds to various N supplies. In the present study, N uptake, temporal and spatial distributions of maize roots, and soil mineral N (Nmin) were thoroughly studied under field conditions in three consecutive years. The results showed that in spite of transient stimulation of growth of early initiated nodal roots, N deficiency completely suppressed growth of the later-initiated nodal roots and accelerated root death, causing an early decrease in the total root length at the rapid vegetative growth stage of maize plants. Early N excess, deficiency, or delayed N topdressing reduced plant N content, resulting in a significant decrease in dry matter accumulation and grain yield. Notably, N overapplication led to N leaching that stimulated root growth in the 40–50 cm soil layer. It was concluded that the temporal and spatial growth patterns of maize roots were controlled by shoot growth and local soil Nmin, respectively. Improving N management involves not only controlling the total amount of chemical N fertilizer applied, but also synchronizing crop N demand and soil N supply by split N applications.

Phenotypic plasticity of the maize root system in response to heterogeneous nitrogen availability

Mineral nutrients are distributed in a non-uniform manner in the soil. Plasticity in root responses to the availability of mineral nutrients is believed to be important for optimizing nutrient acquisition. The response of root architecture to heterogeneous nutrient availability has been documented in various plant species, and the molecular mechanisms coordinating these responses have been investigated particularly in Arabidopsis, a model dicotyledonous plant. Recently, progress has been made in describing the phenotypic plasticity of root architecture in maize, a monocotyledonous crop. This article reviews aspects of phenotypic plasticity of maize root system architecture, with special emphasis on describing (1) the development of its complex root system; (2) phenotypic responses in root system architecture to heterogeneous N availability; (3) the importance of phenotypic plasticity for N acquisition; (4) different regulation of root growth and nutrients uptake by shoot; and (5) root traits in maize breeding. This knowledge will inform breeding strategies for root traits enabling more efficient acquisition of soil resources and synchronizing crop growth demand, root resource acquisition and fertilizer application during crop growing season, thereby maximizing crop yields and nutrient-use efficiency and minimizing environmental pollution.

The Mucilage Proteome of Maize (Zea mays L.) Primary Roots

Maize (Zea mays L.) root cap cells secrete a large variety of compounds including proteins via an amorphous gel structure called mucilage into the rhizosphere. In the present study, mucilage secreted by primary roots of 3-4 day old maize seedlings was collected under axenic conditions, and the constitutively secreted proteome was analyzed. A total of 2848 distinct extracellular proteins were identified by nanoLC-MS/MS. Among those, metabolic proteins (approximately 25%) represented the largest class of annotated proteins. Comprehensive sets of proteins involved in cell wall metabolism, scavenging of reactive oxygen species, stress response, or nutrient acquisition provided detailed insights in functions required at the root-soil interface. For 85-94% of the mucilage proteins previously identified in the relatively small data sets of the dicot species pea, Arabidopsis, and rapeseed, a close homologue was identified in the mucilage proteome of the monocot model plant maize, suggesting a considerable degree of conservation between mono and dicot mucilage proteomes. Homologues of a core set of 12 maize proteins including three superoxide dismutases and four chitinases, which provide protection from fungal infections, were present in all three mucilage proteomes investigated thus far in the dicot species Arabidopsis, rapeseed, and pea and might therefore be of particular importance.

Proteomic Analysis Revealed Nitrogen-mediated Metabolic, Developmental, and Hormonal Regulation of Maize (Zea mays L.) Ear Growth

Optimal nitrogen (N) supply is critical for achieving high grain yield of maize. It is well established that N deficiency significantly reduces grain yield and N oversupply reduces N use efficiency without significant yield increase. However, the underlying proteomic mechanism remains poorly understood. The present field study showed that N deficiency significantly reduced ear size and dry matter accumulation in the cob and grain, directly resulting in a significant decrease in grain yield. The N content, biomass accumulation, and proteomic variations were further analysed in young ears at the silking stage under different N regimes. N deficiency significantly reduced N content and biomass accumulation in young ears of maize plants. Proteomic analysis identified 47 proteins with significant differential accumulation in young ears under different N treatments. Eighteen proteins also responded to other abiotic and biotic stresses, suggesting that N nutritional imbalance triggered a general stress response. Importantly, 24 proteins are involved in regulation of hormonal metabolism and functions, ear development, and C/N metabolism in young ears, indicating profound impacts of N nutrition on ear growth and grain yield at the proteomic level.

A novel morphological response of maize (Zea mays) adult roots to heterogeneous nitrate supply revealed by a split-root experiment

Approximately 35-55% of total nitrogen (N) in maize plants is taken up by the root at the reproductive stage. Little is known about how the root of an adult plant responds to heterogeneous nutrient supply. In this study, root morphological and physiological adaptations to nitrate-rich and nitrate-poor patches and corresponding gene expression of ZmNrt2.1 and ZmNrt2.2 of maize seedlings and adult plants were characterized. Local high nitrate (LoHN) supply increased both lateral root length (LRL) and density of the treated nodal roots of adult maize plants, but only increased LRL of the treated primary roots of seedlings. LoHN also increased plant total N acquisition but not N influx rate of the treated roots, when expressed as per unit of root length. Furthermore, LoHN markedly increased specific root length (m g(-1)) of the treated roots but significantly inhibited the growth of the lateral roots outside of the nitrate-rich patches, suggesting a systemic carbon saving strategy within a whole root system. Surprisingly, local low nitrate (LoLN) supply stimulated nodal root growth of adult plants although LoLN inhibited growth of primary roots of seedlings. LoLN inhibited the N influx rate of the treated roots and did not change plant total N content. The gene expression of ZmNrt2.1 and ZmNrt2.2 of the treated roots of seedlings and adult plants was inhibited by LoHN but enhanced by LoLN. In conclusion, maize adult roots responded to nitrate-rich and nitrate-poor patches by adaptive morphological alterations and displayed carbon saving strategies in response to heterogeneous nitrate supply.

Transport and partitioning of phosphorus in wheat as affected by P withdrawal during flag-leaf expansion

Increasing P-use efficiency within the plant is one of the acclimations to P-limiting conditions. In this work, we studied the effects of P withdrawal during flag-leaf expansion on sink-source relationships and P-use efficiency in two detillered wheat cultivars (Triticum aestivum L., CA9325 and JM2) under controlled conditions. The study period was divided into two phases of one month each. In the first period after withdrawing P from the medium, the rates of dry weight gain were unaffected compared with the control plant. However, the net dry matter deposition in the ear, and P remobilization within the plant were accelerated in both cultivars. In control plants and in the first period, P transported in the xylem came mainly from the roots’ current uptake in both cultivars; in the second period, however, phloem retranslocation of P from the shoot and cycling through the root contributed 86% in CA9325 and 95% in JM2 to the xylem-transported P. In the P-deficient plants of both cultivars, almost all of the P transported in the xylem was remobilized, exported from vegetative organs and recycled through the phloem. Over the entire duration of the experiment, the net dry matter deposition and P allocation to grains were not synchronous, indicating independent regulatory processes. Although withdrawing P from the medium markedly reduced the net dry weight gain of whole plants in both cultivars, the final dry weight of the grains was hardly influenced. The percentage of grain dry weight to whole plant dry weight increased from 42.5% in control plants to 44.7% in P-deficient plants in CA9325, and from 41.0% to 45.0% in JM2, and that of P increased from 24.8% to 87.7% and from 25.5% to 84.3%, respectively. The results showed that withdrawing P from the medium during flag-leaf expansion did not influence grain growth and its final P content. The possible mechanisms to regulate P redistribution and reutilization in plants are discussed.

Genetic Control of Lateral Root Formation in Cereals

Cereals form complex root systems composed of different root types. Lateral root formation is a major determinant of root architecture and is instrumental for the efficient uptake of water and nutrients. Positioning and patterning of lateral roots and cell types involved in their formation are unique in monocot cereals. Recent discoveries advanced the molecular understanding of the intrinsic genetic control of initiation and elongation of lateral roots in cereals by distinct, in part root-type-specific genetic programs. Moreover, molecular networks modulating the plasticity of lateral root formation in response to water and nutrient availability and arbuscular mycorrhizal fungal colonization have been identified. These novel discoveries provide a better mechanistic understanding of postembryonic lateral root development in cereals.

Nicotine Concentration in Leaves of Flue‐cured Tobacco Plants as Affected by Removal of the Shoot Apex and Lateral Buds

It is believed that the nicotine concentration in tobacco is closely correlated with the amount of nitrogen (N) supplied. On the other hand, N uptake mainly occurs at the early growth stage, whereas nicotine concentration increases at the late growth stage, especially after removing the shoot apex. To identify the causes of the increased nicotine concentration in tobacco plants, and to compare the effects of different ways of mechanical wounding on nicotine concentration, field experiments were carried out in Fuzhou, Fujian Province in 2003 and 2004. Excision of the shoot apex had almost no influence on N content in the plant; however, it caused dramatic increases in nicotine concentration in leaves, especially in the middle and upper leaves. An additional increase of the nicotine concentration was obtained by removal of axillary buds. The wounding caused by routine leaf harvests, however, did not change the leaf nicotine concentration, and neither did reducing leaf harvest times. The present results revealed no direct relationship between N supply and nicotine concentration in tobacco leaves, and indicate that not all kinds of mechanical wounding were capable of stimulating nicotine synthesis in tobacco plants. Since nicotine production is highly dependent on the removal of apical meristems and hence on the major sources of auxin in the plant, and application of 1-naphthylacetic acid onto the cut surface of the stem after removing the shoot apex markedly decreased the nicotine concentration in different leaves and the total nicotine content in the plant, the results suggest that decreased auxin supply caused by removal of the shoot apex as a kind of mechanical wounding might regulate nicotine synthesis in the roots of tobacco plants.

Effects of different nitrogen forms and combination with foliar spraying with 6-benzylaminopurine on growth, transpiration, and water and potassium uptake and flow in tobacco

NH4+-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO3−, NH4NO3, NH4+ or NH4++6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO3−-N by NH4+-N (NH4NO3) did not change growth, transpiration, uptake and flow of water and K+ compared with the NO3−-N treatment. However, NH4+-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO3−-N and NH4NO3 treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (δ13C) compared with the NO3− treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K+ uptake to 10%. Under NH4+ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K+ currently taken up, indicating massive mobilisation of K+ from lower leaves and even the roots. Phloem retranslocation of K+ from the shoot and cycling through the root contributed 67% to the xylem transport of K+, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K+) caused by NH4+-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10−8M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH4+-fed plants are discussed.