Victoria González-Dugo

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.

Reflections on food security under water scarcity

Forecasts on population growth and economic development indicate that there will be substantial increases in food demand for the forthcoming decades. We focus here on the water requirements of food production, on the issue of whether there would be enough water to produce sufficient food in the future, and we offer options to face this challenge based on recent trends observed in some agricultural systems. Given the competition for water faced by the agricultural sector, and the uncertainties associated with climate change, improving the efficiency of water use in both rain-fed and irrigated systems is the main avenue to face the challenge. In rain-fed agriculture, managing the risk associated with rainfall variability is a promising option to increase productivity. In irrigated systems, a case study on the improvements in water productivity in Andalusia, Spain, is used to illustrate some of the opportunities to make progress. Progress in reducing irrigation water use in recent decades has been substantial, but decreasing the consumptive use of crops is a much more difficult challenge. The need for more research and technology transfer on improving water-limited crop production is highlighted, and emphasis is placed on interdisciplinary approaches to gain the insight needed to achieve new breakthroughs that would help in tackling this complex problem.

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.

Spatial Resolution Effects on Chlorophyll Fluorescence Retrieval in a Heterogeneous Canopy Using Hyperspectral Imagery and Radiative Transfer Simulation

Increasing attention is being given to chlorophyll fluorescence (F) for global monitoring of vegetation due to its relationship with physiology. New progress has been made in the methodological and technical aspects of signal retrieval with the recently published low-resolution global maps of fluorescence. Nevertheless, little progress has been made in the interpretation of the F signal when quantified in large pixels, an important issue due to the effects of structure, percentage cover, shadows, and background. High-resolution (40 cm) airborne hyperspectral imagery is used in this letter to assess the retrieval of fluorescence by the Fraunhofer line depth method from pure tree crowns and aggregated pixels. Due to canopy heterogeneity, the F signal extracted from aggregated pixels is highly degraded. A poor relationship is obtained between fluorescence extracted from pure tree crowns (Fcrown) and that quantified from pixels aggregating pure tree crowns, shadows, and background (Faggregated) (R2 = 0.25; p <; 0.01). The relationship between F and stomatal conductance (used as a physiological indicator) decreases as a function of aggregation, yielding R2 = 0.69 (p <; 0.01) when calculated from pure tree crowns and R2 = 0.38 (p <; 0.05) from pixels containing crown, shadows, and soil. This letter demonstrates the need for methods to accurately retrieve a pure-vegetation fluorescence signal from aggregated pixels. The FluorMODleaf and FluorSAIL models were combined with the geometric forest light interaction model (FLIM) model and led to the “FluorFLIM” model developed for this letter. Simulations conducted with FluorFLIM obtain predictive relationships between Fcrown and Faggregated pixels as a function of percentage cover, enabling the estimation of pure-crown F from aggregated pixels (R2 = 0.72, p <; 0.01).

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.

Improving Productivity to Face Water Scarcity in Irrigated Agriculture

This chapter provides an overview of how irrigated agriculture is facing increased competition for water, focusing on the role of crop physiology and its interactions with farm irrigation management in coping with water scarcity. Principles of dry-land agronomy and crop physiology inform and reciprocally illustrate how understanding of irrigated systems may shed light on water management in dry-land cropping. Farmers in irrigated agriculture tend to apply excess water to avoid the yield loss associated with the risks of water deficits of an unknown magnitude. Recent and future advances in crop water status monitoring should permit future improvements in precision irrigation, by combining very uniform water application with the accurate determination of water needs. However, in situations of water scarcity, improving irrigation efficiency (IE) by combining advances in engineering and management would not be sufficient, simply because water supply would not be enough. In such cases, the use of deficit irrigation strategies will be the only viable alternative to sustain irrigated production, particularly in drought years.

Using High-Resolution Hyperspectral and Thermal Airborne Imagery to Assess Physiological Condition in the Context of Wheat Phenotyping

Abstract: There is a growing need for developing high-throughput tools for crop phenotyping that would increase the rate of genetic improvement. In most cases, the indicators used for this purpose are related with canopy structure (often acquired with RGB cameras and multispectral sensors allowing the calculation of NDVI), but using approaches related with the crop physiology are rare. High-resolution hyperspectral remote sensing imagery provides optical indices related to physiological condition through the quantification of photosynthetic pigment and chlorophyll fluorescence emission. This study demonstrates the use of narrow-band indicators of stress as a potential tool for phenotyping under rainfed conditions using two airborne datasets acquired over a wheat experiment with 150 plots comprising two species and 50 varieties (bread and durum wheat). The flights were performed at the early stem elongation stage and during the milking stage. Physiological measurements made at the time of flights demonstrated that the second flight was made during the terminal stress, known to largely determine final yield under rainfed conditions. The hyperspectral imagery enabled the extraction of thermal, radiance, and reflectance spectra from 260 spectral bands from each plot for the calculation of indices related to photosynthetic pigment absorption in the visible and red-edge regions, the quantification of chlorophyll fluorescence emission, as well as structural indices related to canopy structure. Under the conditions of this study, the structural indices

Airborne Thermal Imagery to Detect the Seasonal Evolution of Crop Water Status in Peach, Nectarine and Saturn Peach Orchards

In the current scenario of worldwide limited water supplies, conserving water is a major concern in agricultural areas. Characterizing within-orchard spatial heterogeneity in water requirements would assist in improving irrigation water use efficiency and conserve water. The crop water stress index (CWSI) has been successfully used as a crop water status indicator in several fruit tree species. In this study, the CWSI was developed in three Prunus persica L. cultivars at different phenological stages of the 2012 to 2014 growing seasons, using canopy temperature measurements of well-watered trees. The CWSI was then remotely estimated using high-resolution thermal imagery acquired from an airborne platform and related to leaf water potential (ѰL) throughout the season. The feasibility of mapping within-orchard spatial variability of ѰL from thermal imagery was also explored. Results indicated that CWSI can be calculated using a common non-water-stressed baseline (NWSB), upper and lower limits for the entire growing season and for the three studied cultivars. Nevertheless, a phenological effect was detected in the CWSI vs. ѰL relationships. For a specific given CWSI value, ѰL was more negative as the crop developed. This different seasonal response followed the same trend for the three studied cultivars. The approach presented in this study demonstrated that CWSI is a feasible method to assess the spatial variability of tree water status in heterogeneous orchards, and to derive ѰL maps throughout a complete growing season. A sensitivity analysis of varying pixel size showed that a pixel size of 0.8 m or less was needed for precise ѰL mapping of peach and nectarine orchards with a tree crown area between 3.0 to 5.0 m2.

Balancing crop yield and water productivity tradeoffs in herbaceous and woody crops

The links between water and crop yield are well known. In agricultural systems, maximum yield and maximum water productivity (WP; yield divided by water use) are not always compatible goals. In water-limited situations, optimal solutions must be reached by finding a compromise between the levels of crop production and WP. The tradeoffs between production and WP are reviewed here and the dominant effects of the environment on WP are examined. Genetic improvement for WP generally has yield tradeoffs, whereas management measures devised to improve WP also enhance yield. It is shown that partial closure of the stomata in response to environmental stimuli has a variable impact on canopy transpiration, depending on the degree of coupling between the canopy and the atmosphere. In contrast to the behaviour of the major herbaceous crops, WP increases in some woody crops in response to water stress, suggesting that biomass and transpiration are not linearly related, and that deficit irrigation should be successful in these species. Avoiding high evaporative demand periods (e.g. through tolerance to low temperatures) is an important option that aims to increase production and WP. A case study is presented for improving sunflower (Helianthus annuus L.) yield and WP in temperate environments.

Early Detection and Quantification of Almond Red Leaf Blotch Using High-Resolution Hyperspectral and Thermal Imagery

Red leaf blotch is one of the major fungal foliar diseases affecting almond orchards. High-resolution thermal and hyperspectral airborne imagery was acquired from two flights and compared with concurrent field visual evaluations for disease incidence and severity. Canopy temperature and vegetation indices were calculated from thermal and hyperspectral imagery and analyzed for their ability to detect the disease at early stages. The classification methods linear discriminant analysis and support vector machine, using linear and radial basis kernels, were applied to a combination of these vegetation indices in order to quantify and discriminate between red leaf blotch severity levels. Chlorophyll and carotenoid indices and chlorophyll fluorescence were effective in detecting red leaf blotch at the early stages of disease development. Linear models showed higher power to separate between asymptomatic trees and those affected by advanced stages of disease development while the non-linear model was better in discriminating asymptomatic plants from those at early stages of red leaf blotch development. Leaf-level measurements of stomatal conductance, chlorophyll content, chlorophyll fluorescence, photochemical reflectance index, and spectral reflectance showed no significant differences between healthy leaves and the green areas of symptomatic leaves. This study demonstrated the feasibility of early detecting and quantifying red leaf blotch using high-resolution hyperspectral imagery.