Gilles Lemaire

N Uptake and Distribution in Plant Canopies

Nitrogen (N) is often considered to be the most important limiting factor, after water deficiency, for biomass production in natural ecosystems. In arable and forage cropping, N fertilization practices can provide a sufficient N supply for plants to achieve the potential growth allowed by the amount of energy intercepted by the crop. However, to ensure that this potential yield is reached, the N inputs are often higher than the minimum required for maximum crop growth: this is particularly true because N fertilizers are relatively cheap compared to the expected economic benefits from a maximized crop yield.

Relationships between dynamics of nitrogen uptake and dry matter accumulation in maize crops. Determination of critical N concentration

AbstractThe concept of critical nitrogen concentration(%Nc) has been proposed as the minimum%N in shoots required to produce the maximum aerial biomassat a given time. Several authors have shown that%Nc declines as a function of aerial biomassaccumulation (W) and the %Nc –W relationship has been proposed as a diagnostic tool of N statusin different crops, excluding maize. From data obtained in five nitrogenfertilisation experiments in irrigated maize crops, 26 critical data-pointswere selected with a precise statistical procedure. An allometric relationwas fitted and a critical %N−W relationshipmodel is proposed in maize as:If W < 1 t ha-1%Nc = 3.40If 1 t ha-1≤ W ≤ 22 t ha-1%Nc = 3.40(W)−0.37 The model is applicable to maize crop development between emergenceand silking + 25 days. The model was tested and validated with dataobtained in a network of 17 N fertilisation experiments conducted inFrance under contrasting pedoclimatic conditions. In only nineout of 280 data-points (3.2%), the plant N status was mispredictedwhen ±5% error around %Nc wasallowed. A critical N uptake model (Nuc, kg Nha-1) is proposed asNuc = 34 (W)0.63A comparison between Nuc and N uptake observedin N treatments giving the maximal grain yields has shown that maizecrops assimilate at least 30 kg N ha-1 in a storage N poolat the silking stage. The significance of the critical%N−W and Nu−W relationships is discussed in relation to theoretical models proposed inwhole plant ecophysiology. Different relationships calculated betweenleaf area index and aerial biomass accumulation, and between N uptakeand leaf area were consistent with previous results for other crops.This strengthens the interest of the critical%N−W relationship for use as diagnostictool of nitrogen status in maize crops.

Diagnosis of the nitrogen status in crops

I On the Critical N Concentration in Agricultural Crops.- 1 N Uptake and Distribution in Plant Canopies.- 2 Use of the Nitrogen Nutrition Index for the Analysis of Agronomical Data.- II The Nitrogen Requirement of Major Agricultural Crops.- 3 Grasslands.- 4 Wheat, Barley, and Durum Wheat.- 5 Maize and Sorghum.- 6 Grain Legumes.- 7 Potatoes.- 8 Mixed Crops.- III Management of N Nutrition.- 9 Nitrogen Diagnosis and Decision Support.- 10 Diagnosis Using Stem Base Extract: JUBIL Method.- 11 Leaf N Content as an Indicator of Crop N Nutrition Status.- 12 Radiometric Estimates of Nitrogen Status of Leaves and Canopies.- 13 Concluding Remarks: N Hazards to Crops and Environment.

Grassland ecophysiology and grazing ecology.

Environmental constraints and plant responses to defoliation morphogensis of pasture species and adaptation to defoliation animal interactions sustainable grazing management of natural pastures.

Chapter 8 – Quantifying Crop Responses to Nitrogen Deficiency and Avenues to Improve Nitrogen Use Efficiency

Publisher Summary This chapter develops a general theoretical framework of the regulation of N uptake and distribution at two levels of organization: the individual plant and the plant population (i.e., the crop). Its aim is to identify the buffering effect on some physiological traits when scaling up from organ to whole plant and to crop. From this theoretical framework, it derives a functional tool for the determination of N status of plant and crop, and then to quantify the degree of N deficiency. Using this diagnostic tool it analyzes quantitatively the response of important physiological processes to N deficiency. Further it examines the concept of nitrogen use efficiency (NUE) in light of the theory developed; avenues for improving NUE through plant breeding and agronomic strategies are identified. The nitrogen metabolism of plants is controlled by physiological process such as nitrate or ammonium transport through cell membranes in root, nitrate reduction in root and leaf, N2 fixation within nodules in legumes, and ammonium assimilation. Each of these metabolic processes is regulated at molecular levels with some degree of genetic variability. But when all these processes are integrated by scaling up to whole plant and plant population or community level the overall integrated regulation of N uptake and N use efficiency can be resumed by very general rules with very low interspecific variability.

Nitrogen, Plant Growth and Crop Yield

The use of fertilisers in agriculture and horticulture is the key to production of sufficient food (including the fodder for animals) to maintain the global human population (currently 6 billion; Evans 1998) and to permit its continued rapid growth to the expected 10 or even 12 billion (Bumb 1995). Nitrogen in a form which can be used by plants is essential to crop production, and application of N fertilisers, produced industrially by chemical reduction of atmospheric (gaseous) nitrogen, has enabled the enormous and unprecedented expansion of the world’s human population and the food supply (Bacon 1995; Evans 1998). The increased nitrogen supply is probably a consequence of population driven technological advances, in a complex interaction which is poorly understood (Evans 1998). Phosphate and potassium are also essential elements whose supply may not be sustainable in the longterm, as they are mined. However, the role of N in crop production is a critical aspect of crop production. Understanding the mechanisms by which crops respond to nitrogen is the key to maintaining and improving crop growth and yield, and the efficiency with which N is used and other resources also (Sinclair and Horie 1989; Bock and Hergert 1991; Lawlor et al. 1989; Grindlay 1997). This review analyses crop responses to N supply and integrates our knowledge of subcellular, cellular and organ processes to clarify the needs for N by plants and problems of quantification.

Campos in southern Brazil.

The Brazilian subtropical region is located between the extreme southern border ofthe country (approximately 33°S) and the Tropic of Capricorn. This chapter dis-cusses the main grazing ecosystems found in this region (states of Rio Grande doSul, Santa Catarina and Parana), based on a tradition of beef cattle livestock pro-duction, which started at the beginning of Brazilian colonization at Rio Grande doSul and, little by little, spread north to the grasslands of Santa Catarina and Parana.Few places in the world present such diversity in native forage species, withalmost 800 grasses and 200 legumes.

Effects of grazing on the roots and rhizosphere of grasses.

Fertilizer inputs are currently being reduced in many areas (Commission, European Communities, 1992) and the resultant drop in pasture fertility will reduce the stock carrying capacity. As a consequence, individual plants will be defoliated less frequently (Curll and Wilkins, 1982) and less nitrogen will be deposited as urine (Thomas et al., 1988), thus influencing plant competition and composition. Research to date has concentrated on the effects of grazing on above-ground aspects, and it has recently been stated that ‘the effects of herbivory on the timing, mass and quality of below-ground inputs remains one of the greatest unresolved issues of the dynamics of nutrient cycling’ (Ruess and Seagle, 1994). The soil microbiota in grasslands consists of populations of microorganisms, including bacteria, fungi, protozoa, nematodes, and microand macroarthropod groups (Ingham and Detling, 1986). These all rely for their growth, at least in part, on carbon or nitrogen substrates via litter, root production, sloughage and exudation. Figure 4.1 illustrates the main links in the detrital trophic food web and shows the primary role of plant roots, the connectivity and some of the many trophic interactions. Although nearly all soil organisms belong to the detrital food web, significant numbers of root herbivores exist in grassland soil (Curry, 1994), also relying on plant roots for their survival. Any alteration in plant-derived carbon, such as through defoliation, will have consequences at many levels in the food web (Fig. 4.1). Since microbial activity, supported in part by root-derived carbon, drives soil nutrient cycling, the production and use of carbon from root systems is also a key issue in the functioning of soil ecosystems (van Veen et al., 1989). 4

Quantifying crop responses to nitrogen and avenues to improve nitrogen-use efficiency

In the last 40 years, the global use of mineral N fertilizers has increased to support increasing food demand. Misuse of fertilizer has led to environmental problems in some regions and the rarefaction of energy leads to an increasing cost of mineral N fertilizers. For these reasons, improving N-use efficiency of crops and cropping systems is becoming a real challenge to growers, agronomists and breeders. Nitrogen-use efficiency depends on agronomic practices including mineral and organic nitrogen fertilization and the use of legumes in cropping systems, and genetic progress in nitrogen-use efficiency. The objective of this chapter is to develop a framework of the principles governing regulation of N uptake, N allocation and growth of plants and crops, and to apply these principles in tools for improving fertilization management and breeding. A theoretical analysis of the dynamics of plant and crop N demand in relation to growth potential during the crop cycle is developed. Agronomical tools to evaluate nitrogen status of plants and crops are presented and discussed. Physiological and morphological responses of plants and crops to N deficiency are then examined. Finally, the concept of nitrogen-use efficiency is considered in the light of the principles developed previously, and avenues for improving nitrogen-use efficiency through plant breeding and agronomy are discussed from a crop physiology point of view.

Effects of the previous shoot removal frequency on subsequent shoot regrowth in two Medicago sativa L. cultivars

The frequency of shoot removal in lucerne (Medicago sativa L.) has long been recognized as a key factor in its management and productivity. The present study was undertaken to determine the impact of cutting interval during spring (30 or 45 days) on the subsequent summer regrowth, in contrasting lucerne cultivars (cv. Europe and Lodi). In particular, the dynamics of shoot regrowth (leaf area index, radiation use efficiency, N accumulation in harvestable biomass) and its relationship with taproot organic reserves (starch and N contents) were studied. Results showed that increasing the duration of the spring regrowth had a positive effect on subsequent summer regrowth, but there were also effects of cultivars. During the first 14 days of summer regrowth, the “Lodi” cultivar showed higher leaf area index (LAI) and greater photosynthetic active radiation interception than the “Europe” cultivar. The organic reserve level was also affected by the length of the previous spring cutting treatment (45 days treatment > 30 days treatment) and cultivar (Lodi > Europe). Lodi accumulated a larger amount of starch and N reserves which were subsequently mobilized to a greater extent in the first three weeks of regrowth and this contributed to its faster initial shoot growth rate. Our results confirm the important role played by N and C taproot reserves in shoot growth rate in the first days following shoot removal. Results are discussed in relation to recent studies on the role of storage N compounds in regrowth, and the concepts of radiation use efficiency (RUE); nitrogen nutrition index (NNI), and the decline in N content seen during the accumulation of biomass in lucerne canopies.