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Non-destructive Proximal Sensing for Early Detection of Citrus Nutrient and Water Stress Paolo Menesatti, Federico Pallottino, Francesca Antonucci, Giancarlo Roccuzzo, Francesco Intrigliolo, and Corrado Costa
Abstract
This chapter reports the application of non-destructive optical-based technologies for the rapid and efficient assessment of the nutritional status and water stress detection improving their use efficiency. In the proximal sensing section, it was presented the use of spectral and hyperspectral imaging to evaluate the plant nutritional status. Proximal sensing offers the opportunity to rapidly collect a huge amount of crop canopy information. In the infrared thermography and thermometry section, results about their use to assess plant water stress analysing canopy and soil temperature variation were reported. Finally, the use of spectrophotometry and of the chlorophyll meter for the citrus nutrient detection is presented. The analyses of data were carried out by linear regressions and by multivariate statistics. Keywords
Spectroscopy • Infrared thermography and thermometry • Nutritional plant • Water soil detection • Multivariate analysis
9.1
Introduction
Water and nutrient uptake are closely related and they are both essential for plant growth and productivity. The advent of precision production methods and the increase of both environmental and quality standard issues imply the necessity to improve nutrient and water use efficiency. These objectives and the related need for automation have led to the development of rapid methods for plant monitoring (Jones 2004). The current worldwide guidelines for citrus fertilisation for young and bearing trees are based on soil and leaf analyses
P. Menesatti (*) • F. Pallottino • F. Antonucci • C. Costa CRA-ING (Agricultural Engineering Research Unit of the Agricultural Research Council), Via della Pascolare 16, 00015 Monterotondo Scalo (RM), Italy e-mail:
[email protected];
[email protected] G. Roccuzzo • F. Intrigliolo CRA-ACM (Citrus and Mediterranean Crops Research Centre of the Agricultural Research Council), Corso Savoia 190, 95024 Acireale (CT), Italy e-mail:
[email protected];
[email protected]
and yield expectancy. In fact, soil testing and leaf analysis are the tools normally used to monitor the nutrient status of citrus orchards and modify the fertiliser management. Leaf analysis is the most widespread procedure, and it is performed since a long time by comparing results to well-established reference values of standard age spring-cycle leaves (Embleton et al. 1973a). Destructive sampling is necessary to examine dry mass and nutrient content of leaves, but this procedure is quite labour intensive and time consuming. Hydric status in the soil-plant system can be monitored by means of soil water measurements (water content or potential), soil water balance calculations or plant stress sensing (tissue water status or physiological responses). Water potential, Y, is defined as the potential energy per unit mass of water with reference to pure water at zero potential, and it is expressed in units of pressure (MPa). In most biological systems, water has less potential energy than in the pure state, thus resulting in negative values for water potential. The pressure chamber or Scholander bomb is the reference technique for Y determination. By means of this instrument, it is possible to measure the approximate water potential in plant tissues. A leaf attached to a stem
A.K. Srivastava (ed.), Advances in Citrus Nutrition, DOI 10.1007/978-94-007-4171-3_9, © Springer Science+Business Media B.V. 2012
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(or the stem itself) is placed inside a sealed chamber, and pressurised gas is added to the chamber slowly. As the pressure increases, at some point, sap will be forced out of the xylem and will be visible at the cut end of the stem. The pressure that is required to do so is equal and opposite to the water potential of the leaf (Scholander et al. 1965). Stem water potential is highly correlated with stomatal conductance and consequently with transpiration rate and with assimilation rate that is known to limit yield. Leaf water potential and predawn leaf water potential are also highly correlated with stomatal conductance, but the correlation of stomatal conductance with stem water potential has been found to be higher than that with leaf water potential (Naor 2000). Surface temperature measured with infrared thermometers is an important tool to diagnose plant water stress and thus for irrigation scheduling which has been in practice for some decades. The introduction of thermal cameras made the wider use of such approaches feasible, especially when combined with automated image analysis (Jones 2004). This chapter reports the application of non-destructive opticalbased (spectral and thermal) technologies, for ground-based proximal sensing that could be used in farm, for rapid, efficient and low-cost nutritional status and for the water stress detection in order to optimise orchards management.
9.2
Proximal Sensing
9.2.1
Ground-Based Hyperspectral Imaging as Proximal Sensing Technology
Spatial heterogeneity of physical and chemical properties represents a crucial factor with an impact on crop response and final production in terms of quantity and quality. Precision agriculture aims to a focused use of any input, in the right place, time and amount, in order to improve farmer’s income and reduce the adverse environmental impact on crop production. There is a growing demand for rapid and non-invasive acquisition of fine-scale information on soil properties and plant physical-chemical status for site-specific management. Proximal sensing through different technologies can help offer the opportunity to rapidly collect a huge amount of information regarding the crop canopy and therefore to identify the plants needs. Many different types of proximal sensors can be used to measure plant, among which, multi- or hyperspectral imaging and thermographic imaging. For nearly 20 years, the environmental monitoring techniques have made many important strides, thanks to innovations in electronics and information technology. In agriculture, e.g. collecting short optical parameters, i.e. spectral reflectance data, related to vegetation cover, allowed the determination of plant distress signals before
their onset was visible to the naked eye (Massantini et al. 1992; Enterline 2001). The visible near-infrared (VIS-NIR) spectral analysis appears to be one of the most innovative and interesting being non-destructive. The information content of such a technique is very high being based on the principle that every molecule absorbs or reflects only specific wavelengths, hence it is possible to obtain correlations between the amount of a particular compound and the standard quantity of reflected light (reflectance), absorbed (absorbance) or released (transmittance) (Urbani et al. 2002). These techniques, coupled with precise optical systems calibrated to estimate the chlorophyll content of plants (SPAD), can provide important support to agriculture from both a technical and agronomic point of view and from an economic one; indeed, the real-time diagnosis of the nutritional status of plants leads to targeted interventions for the prevention and resolution of problems as well as to rationalised ones. An example of how suitable these technologies can be is represented by the cases of study which follow.
9.2.2
Plant Nutritional Status Evaluation Through Canopy Spectral Image Analysis
Two distinct sets of tests are presented in the following case study, the first carried out in November 2001, the second in July 2002, respectively. The tests were conducted by the CRA-ING at the farm ‘Palazzelli Experimental Institute’ for citrus (Lentini, eastern Sicily) on nine adult plants of orange [Citrus sinensis (L.) Osbeck]. The system used in the research consists of an optical spectrometer that captures the picture cleanly and disperses, pixel by pixel, the spectrum on a camera matrix (V1-IMSPECTOR SPECIM Finland). For the acquisition of horizontal two-dimensional spectral images, the spectrometer is mounted on a horizontal controlled handling system (DV-Spectral Scanner Padova); a tripod (with electronic control) enables movement on a vertical axis of the spectrometer, to take images of very large objects varying the resolution and scan speed, and to synchronise the acquisition with the corresponding angle (Pan & Tilt-DV-Padova). Images have an optical resolution of 570 dots per 300 vertical lines. The reflectance values were measured only in regions of interest (ROI) corresponding to the densest part of the crown. The calibration of the instrument in this regard required special attention because it is a delicate operation which depends on the success and reliability of the measurements. In fact, the optical characteristics of the plants found in reflection strongly depend on the intensity and angle of incident sunlight. Since field surveys have no artificial lighting, it can happen to perform measurements with different
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Non-destructive Proximal Sensing for Early Detection of Citrus Nutrient and Water Stress
lighting conditions due to the sun light variability and weather conditions. In order to reduce the variability of measurement due to different lighting conditions, the spectral optical system is calibrated to a black and white reference, which occupies the lower and upper limits of the dynamic response of the instrument. The blank sample is in turn standardised by accurate survey performed in the laboratory. This procedure led to good repeatability of measurement in the first test of an experimental survey. Moreover, to further evasluate the light variability, during the second test a white rectangular sheet was used as reference nearby each plant sample. Thus, each spectral image includes a constant white standard reference at about the same distance (and optical plane) of the object to be measured. The set of spectral information involved 115 different values of wavelengths from 400 to 970 nm with step 5 nm. The most interesting reports (minimum or maximum) between individual wavelengths were placed in conjunction with analytical measurements on leaves, plants and soil. Leaves were sampled from the nine plants, taken from non-interesting bearing terminal branches, and on these were carried out the analysis for the determination of N, P, K, Ca, Mg, Fe, Mn, Zn and intensity of staining Green (SPAD-502 Chlorophyll Meter, Minolta). Moreover, the plant hydric status (xylem index) and soil chemical-physical characteristics were determined. Afterwards, spectral images of the 20 leaves sampled from each of the plants subjected to in-field hyperspectral measurements were acquired. From the findings on the foliage of citrus for the first test, there were significant values of simple linear correlations between reflectance at individual wavelengths and N content of leaves (all lengths in the range 750–795 nm), in Mn (range 930–950 nm), and more importantly, P (range 400–700 nm, 880–970 nm). The in-field spectral detection system showed promising performances, although the first test was not an easy to operate and calibrate. This also required a significant amount of light to work with a good signal-to-noise ratio. These difficulties have greatly limited the number of photographs. With the spectral range used, however, images of high spectral resolution were obtained. The spatial resolution was low, but enough to make the hyperspectral analysis afterwards. The data acquired during the second experimental trial are still being processed.
9.3
Infrared Thermography to Assess Plant Water Stress
Infrared thermography (IT) is a non-invasive and nondestructive method based on measuring specific electromagnetic radiation emitted by any object, according to the Stefan-Boltzmann’s and Planck’s laws (see Maldague 1994; Rahkonen and Jokela 2003 for basic theory). All objects emit
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heat (energy) waves. If an object is cold, its molecules vibrate slower and energy of longer wavelengths is emitted. When the temperature of the object rises, its molecules vibrate faster and the wavelength becomes shorter. Every particular energy wavelength has a temperature associated with it. Among the different image analysis techniques and technologies, thermography has the capability to associate to the image information, the thermal punctual information, which is the temperature of each single pixel, in order to operate comparisons between objects inside the same image. Moreover, a good thermographic system could record until 50 frames per second; in this way, it is possible to operate dynamic measurements observing temporal modification of the object. This information could also be used to study biological phenomena. Those systems could calculate the surface temperature of the object, operating corrections due to many factors: physical, environmental and instrumental. However, one of the most important factors for a correct infrared thermal measurement is the emissivity that measures the capability of an object to adsorb or emit the thermal radiation. This capability is equal to 1 only for ideal objects (black objects) but is, in general, very low (
