François Gastal

Water deficit and nitrogen nutrition of crops. A review.

Among the environmental factors that can be modified by farmers, water and nitrogen are the main ones controlling plant growth. Irrigation and fertilizer application overcome this effect, if adequately used. Agriculture thus consumes about 85% of the total fresh water used worldwide. While only 18% of the world’s cultivated areas are devoted to irrigated agriculture, this total surface represents more than 45% of total agricultural production. These data highlight the importance of irrigated agriculture in a framework where the growing population demands greater food production. In addition, tighter water restrictions and competition with other sectors of society is increasing pressure to diminish the share of fresh water for irrigation, thus resulting in the decrease in water diverted for agriculture.The effect of water and nutrient application on yield has led to the overuse of these practices in the last decades. This misuse of irrigation and fertilizers is no longer sustainable, given the economic and environmental costs. Sustainable agriculture requires a correct balance between the agronomic, economic and environmental aspects of nutrient management. The major advances shown in this review are the following: (1) the measurement of the intensity of drought and N deficiency is a prerequisite for quantitative assessment of crop needs and management of both irrigation and fertilizer application. The N concentration of leaves exposed to direct irradiance allows both a reliable and high-resolution measurement of the status and the assessment of N nutrition at the plant level. (2) Two experiments on sunflower and on tall fescue are used to relate the changes in time and irrigation intensity to the crop N status, and to introduce the complex relationships between N demand and supply in crops. (3) Effects of water deficits on N demand are reviewed, pointing out the high sensitivity of N-rich organs versus the relative lesser sensitivity of organs that are poorer in N compounds. (4) The generally equal sensitivities of nitrifying and denitrifying microbes are likely to explain many conflicting results on the impact of water deficits on soil mineral N availability for crops. (5) The transpiration stream largely determines the availability of mineral N in the rhizosphere. This makes our poor estimate of root densities a major obstacle to any precise assessment of N availability in fertilized crops. (6) The mineral N fluxes in the xylem are generally reduced under water deficit and assimilation is generally known to be more sensitive to water scarcity. (7) High osmotic pressures are maintained during grain filling, which enables the plant to recycle large amounts of previously assimilated N. Its part in the total grain N yield is therefore generally higher under water deficits. (8) Most crop models currently used in agronomy use N and water efficiently but exhibit different views on their interaction.

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.

The coordination of leaf photosynthesis links C and N fluxes in C3 plant species

Photosynthetic capacity is one of the most sensitive parameters in vegetation models and its relationship to leaf nitrogen content links the carbon and nitrogen cycles. Process understanding for reliably predicting photosynthetic capacity is still missing. To advance this understanding we have tested across C3 plant species the coordination hypothesis, which assumes nitrogen allocation to photosynthetic processes such that photosynthesis tends to be co-limited by ribulose-1,5-bisphosphate (RuBP) carboxylation and regeneration. The coordination hypothesis yields an analytical solution to predict photosynthetic capacity and calculate area-based leaf nitrogen content (N a). The resulting model linking leaf photosynthesis, stomata conductance and nitrogen investment provides testable hypotheses about the physiological regulation of these processes. Based on a dataset of 293 observations for 31 species grown under a range of environmental conditions, we confirm the coordination hypothesis: under mean environmental conditions experienced by leaves during the preceding month, RuBP carboxylation equals RuBP regeneration. We identify three key parameters for photosynthetic coordination: specific leaf area and two photosynthetic traits (k3, which modulates N investment and is the ratio of RuBP carboxylation/oxygenation capacity () to leaf photosynthetic N content (N pa); and J fac, which modulates photosynthesis for a given k 3 and is the ratio of RuBP regeneration capacity (J max) to). With species-specific parameter values of SLA, k 3 and J fac, our leaf photosynthesis coordination model accounts for 93% of the total variance in Na across species and environmental conditions. A calibration by plant functional type of k 3 and J fac still leads to accurate model prediction of N a, while SLA calibration is essentially required at species level. Observed variations in k3 and Jfac are partly explained by environmental and phylogenetic constraints, while SLA variation is partly explained by phylogeny. These results open a new avenue for predicting photosynthetic capacity and leaf nitrogen content in vegetation models.

Short-term response of the nitrogen nutrition status of tall fescue and Italian ryegrass swards under water deficit

Grasslands are rarely irrigated, thus water deficits often induce a reduction of the nitrogen nutrition index (NNI) during summer. This is measured using the ratio between the actual N concentration and the minimum N concentration required to achieve the maximum growth rate. NNI is derived from the standing biomass by a simple relationship. This paper details the results of a field experiment, combining 2 levels of irrigation with 2 levels of nitrogen fertilisation during the summer, on 2 commonly cultivated grass species in pure swards (tall fescue, Festuca arundinacea L., and Italian ryegrass, Lolium multiflorum). Plant water status, NNI, root length density (RLD), soil volumetric water content (θv), and mineral nitrogen concentration (N) were followed under water deficit and recovery. In both species, RLD was high (>6 cm/cm 3 ) in the 0-0.25 m soil layer. Whereas the NNI of tall fescue responded strongly to its water status, Italian ryegrass was most often above optimal nitrogen nutrition because of its slow growth in that particular season and its higher superficial RLD. However, its NNI generally followed the θv closely, whereas tall fescue exhibited a delay in response of NNI upon rewatering, suggesting lasting effects of water deficits on the absorption capacity of roots in that species.

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.

Drought effects on growth and carbon partitioning in a tall fescue sward grown at different rates of nitrogen fertilization

Abstract The effects of drought on plant water and nitrogen status, shoot growth and carbon partitioning between root and shoot, were studied during two summer regrowth cycles of a tall fescue sward grown at different rates of application of nitrogen fertilizer. Nitrogen fertilizer was the main factor influencing the relative partitioning of the newly assimilated 14C. Drought did not prevent the expression of the effect of nitrogen fertilization on carbon partitioning. The highest carbon partitioning coefficients to roots (15–30 per cent) were measured in swards having the lowest nitrogen status. With the high nitrogen fertilization rate, water shortage increased the carbon partitioning to roots from 6 to 13 per cent (means for both regrowths, before rewatering). This increase mainly occurred in the surface horizon of soil (0–20 cm). However, an estimate in absolute terms showed that root growth was not increased by drought. Water shortage impaired the sward nitrogen status. It is therefore proposed that at least part of the change in the carbon allocation pattern induced by drought was mediated via an effect on nitrogen nutrition. The importance of the drought-induced modification of carbon partitioning is discussed.

How much do water deficits alter the nitrogen nutrition status of forage crops

Water deficits alter the nitrogen nutrition of crops. In grasslands, this has a major impact on both forage yield and nitrogen fluxes in the soil. It is important to assess the N balance in order to adjust fertilization to the expected needs of the crop and thus minimize any environmentally negative impacts of crops. Grassland species, including grasses, display a diverse ability to utilise soil resources. Nitrogen fluxes and the nitrogen absorption by grass swards of two species with contrasting rooting depths were computed using the appropriate module from the STICS simulation platform. In the case of the deep-rooted species, tall fescue, soil mineral N fluxes to the roots were very close to N uptake values, consistent with its nitrogen nutrition index being lower than one. In the case of the shallow-rooted species Italian ryegrass, there was a large excess in terms of N supply, which was also consistent with its non-limiting nitrogen nutrition index. In both species, and even when nitrogen demands for growth were fully satisfied, the nitrogen nutrition index was closely and linearly related to the soil mineral N flux to roots.

Responses of plant traits of four grasses from contrasting habitats to defoliation and N supply

The objective of the study was to identify specific plant traits determining adaptation of grass species to defoliation and N availability, and thus having a major impact on species dynamics, primary productivity, and on nutrient cycling in grassland ecosystems. It was specifically examined whether the response of species to defoliation is related to their plasticity in leaf growth and in leaf growth zone components, and whether the response of species to nitrogen is related to their plasticity in root morphology and subsequent N acquisition, and to N losses through leaf senescence. The study was conducted on L. perenne and D. glomerata, two grazing tolerant species from fertile habitats, and on F. arundinacea and F. rubra, two less grazing tolerant species from less fertile habitats. Plants were subjected to repeated defoliation at three cutting heights under both high N and low N supply. Biomass allocation, leaf elongation, characteristics of the leaf growth zone (height and relative growth rate), and root morphology, N uptake and N losses through leaf senescence were evaluated. Under high N supply, L. perenne and D. glomerata showed the greatest tolerance to defoliation, due to a large plasticity in the height of the leaf growth zone and due to compensatory growth, either within the leaf growth zone or between growing leaves. Under low N supply, F. rubra was the only species with the ability to develop a more branched root system and a greater length of tertiary roots than under high N. As a consequence, under low N supply F. rubra had a higher specific N uptake and a higher growth rate than the other species. This slow growing species also showed a higher nitrogen allocation to dead leaves and subsequently a higher potential N loss to leaf litter.

Water Deficit and Nitrogen Nutrition of Crops

Among the environmental factors that can be modified by farmers, water and nitrogen are the main ones controlling plant growth. Irrigation and fertilizer application overcome this effect, if adequately used. Agriculture thus consumes about 85% of the total fresh water used worldwide. While only 18% of the world’s cultivated areas are devoted to irrigated agriculture, this total surface represents more than 45% of total agricultural production. These data highlight the importance of irrigated agriculture in a framework where the growing population demands greater food production. In addition, tighter water restrictions and competition with other sectors of society is increasing pressure to diminish the share of fresh water for irrigation, thus resulting in the decrease in water diverted for agriculture.The effect of water and nutrient application on yield has led to the overuse of these practices in the last decades. This misuse of irrigation and fertilizers is no longer sustainable, given the economic and environmental costs. Sustainable agriculture requires a correct balance between the agronomic, economic and environmental aspects of nutrient management. The major advances shown in this review are the following: (1) the measurement of the intensity of drought and N deficiency is a prerequisite for quantitative assessment of crop needs and management of both irrigation and fertilizer application. The N concentration of leaves exposed to direct irradiance allows both a reliable and high-resolution measurement of the status and the assessment of N nutrition at the plant level. (2) Two experiments on sunflower and on tall fescue are used to relate the changes in time and irrigation intensity to the crop N status, and to introduce the complex relationships between N demand and supply in crops. (3) Effects of water deficits on N demand are reviewed, pointing out the high sensitivity of N-rich organs versus the relative lesser sensitivity of organs that are poorer in N compounds. (4) The generally equal sensitivities of nitrifying and denitrifying microbes are likely to explain many conflicting results on the impact of water deficits on soil mineral N availability for crops. (5) The transpiration stream largely determines the availability of mineral N in the rhizosphere. This makes our poor estimate of root densities a major obstacle to any precise assessment of N availability in fertilized crops. (6) The mineral N fluxes in the xylem are generally reduced under water deficit and assimilation is generally known to be more sensitive to water scarcity. (7) High osmotic pressures are maintained during grain filling, which enables the plant to recycle large amounts of previously assimilated N. Its part in the total grain N yield is therefore generally higher under water deficits. (8) Most crop models currently used in agronomy use N and water efficiently but exhibit different views on their interaction.