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5Journal of Siberian Federal University. Engineering & Technologies 2015 8. Issue 8. 1096-1102
УДК 528. 855
Relationships Between Chlorophyll Potential of Wheat Crops and Leaf Area Index of the Plants Based on Reflectance Spectra Obtained by Ground-Based Remote Measurements
Alexander F. Sid'ko*, Tamara I. Pisman, Irina Yu. Botvich and Anatoly P. Shevyrnogov
Institute of Biophysics of the Siberian Branch of RAS 50 Akademgorodok, Krasnoyarsk, 660036, Russia
Received 17.06.2015, received in revised form 10.08.2015, accepted 11.11.2015
The purpose of this study was a comparative study of seasonal dynamics of chlorophyll potential, leaf area index and fresh biomass of different varieties of wheat based on reflectance spectra obtained by ground-based remote measurements. The study reports data on seasonal dynamics of reflectance of wheat canopies composed of various wheat cultivars in the Krasnoyarskii Krai. The study presents an optical remote method for determining chlorophyll potential, S(t), of wheat plants, which is calculated based on reflectance spectra of the crops during the growing season. Chlorophyll potential of wheat plants, S(t), may serve as an indicator of their physiological state and potential biological productivity; also, it can be used to predict grain yield of wheat fields. Different wheat cultivars have different chlorophyll potentials, S(t), during the growing season. There is high positive correlation between dry and fresh wheat biomass, and chlorophyll potential, S(t), of wheat plants. A digital database has been compiled, which contains spectral data of the wheat canopies studied.
Keywords: chlorophyll potential, leaf area index, wheat.
DOI: 10.17516/1999-494X-2015-8-8-1096-1102.
© Siberian Federal University. All rights reserved Corresponding author E-mail address: sidko@ibp.krasn.ru
*
Связь хлорофилльного потенциала посева пшеницы и листового индекса
при наземном дистанционном зондировании
А.Ф. Сидько, Т.И. Письман, И.Ю. Ботвич, А.П. Шевырногов
Институт биофизики СО РАН 50 Aкадемгородок, Kрасноярск, 660036, Rоссия
Целью данного исследования является сравнительное изучение сезонной динамики хлорофилльного потенциала, индекса листовой поверхности и свежей биомассы различных сортов пшеницы на основе спектров отражения, полученных с помощью наземных дистанционных измерений на территории Красноярского края. Исследование представляет собой оптический дистанционный способ определения хлорофилльного потенциала, S (t), из растений пшеницы, который рассчитывается на основе спектров отражения культур в период вегетации. Хлорофилльный потенциал растений пшеницы, S (t), может служить индикатором их физиологического состояния и биологической продуктивности. Различные сорта пшеницы имеют различные хлорофилльные потенциалы в течение вегетационного сезона. Существует положительная корреляция между сухой и свежей биомассой пшеницы и хлорофилльным потенциалом. Была составлена цифровая база данных, которая содержит спектральные показатели изученных посевов пшеницы.
Ключевые слова: хлорофилльный потенциал, листовой индекс, пшеница.
Introduction
The ratio of leaf surface area to unit ground surface area, called leaf area index (LAI) [1]. Monitoring of LAI is mandatory for spatially distributed modeling of vegetation productivity. In addition, LAI is an important variable in climatic, ecological and agronomical research studies [2, 3]. Measuring LAI on fields is time consuming and requires a great amount of labor and hence cost. For these reasons, many studies have sought to discover relationships between LAI and remote sensing data for its rapid and objective estimation [4].
Remote sensing data and ground-based remote measurements can be used to determine the relationship between the reflectance of the crops and parameters characterizing their physiological state. By now, these methods have been developed to such an extent that the plant spectral data can be used to evaluate physiological and biological parameters of the plants both at a single point in time and in different stages of their growth [5, 6]. The most convenient optical characteristic for identification of optical properties of plant canopies is their spectral reflectance. Spectral reflectance of the crops contains a considerable amount of information on the physiological state of the plants. For instance, the intensity and the spectral composition of the light reflected by the plant canopy contain information about the species composition of the plants, their moisture content (fresh biomass), and the state of the surface (LAI) [7, 5]. Considerable changes in reflectance spectra of the crops during their growing season are associated with accumulation and destruction of plant pigments, structural changes, and species composition of the plants [8-10].
The use of the reflectance factor, px of leaves at different wavelengths may be insufficient for accurate evaluation of certain physiological parameters of plants [11]. Thus, new remote sensing techniques need to be developed to obtain more data on the physiological state of the; plants. Their development is, however, hindered by the lack erf experimental data collected in the field throughout the growing season.
The purpose of this study was a comparative study of seasonal dynamics of chlorophyll potential, leaf area index and fresh biomass of different varieties of wheat based on reflectance spectra obtained by ground-based remote me asurements .
Material and methods
The method of determining chlorophyll potential is based on the analysis of field studies of wheat crop reflectance that have been regularly conducted for many years, by using remote sensing techniques. Field studies were performed on test plots situated in the KrasnoyarskiiKrai, during the growing season. Wheat (Triticum acstivum L.) cultivars Skala, Ravnina, Saratovskaya, etc. were used as model crops. The areas of the test plots were 200 m2 (4x50) and 600 m2 (10x60). The photometered area varied between 0.5 and 2 m2; generally, nadir measurements were taken. The plots were located rather clo se to each otht r, i.e. inthe tame climafic conditions [12].
Reflectance spectra of wheat canopies were renorded from the elevated work platform at heights of 2 to 18 m under sunny conditions, using a PDSP doublerbcam spectrophoeomefer [13] (Sid'ko et al., 2000). It recondt d the differentin[ reflectance factor of the object studie d and the refere nte surface. The scanning rate in the spectrd range ftom 400 to 850 nm was 60 s [14].
The reflectance factor of tine study object (turface) p , (J,cp) is defined as 1he ratio of the brightness of the study surface, BJp) to the brightness of the etalon, B0(J,p).
pJpp) — Bf(ircp)/B0(ir,cp), (1)
where (J,p) are the polar and azimuth angles, tespectively [15] (Sid'ko et oo, 2013).
From the values oo ohe reflectance factorf p, calculated chlorophyll potential - parameter S. Parameter S is chlorophyll potential, determtned by the amount ot light in the 550-730 nm range absorbedby planfr in certain periods of rhe growing season. Parameter S is the best indtcator of the interrelationship between reflectance factor and physiological and biometric parameters of the plants. Parameter S is determined from the following equation:
730
5 = 90 ((i) + P55o(t))- ¡P(¿,t)dX (2)
550
where 90 is the multiplier equa1 to the half-width o. the chlorophyll absorption band between 550 and 730 nm, p 550 and p 730 are average values of the reflectance fector of the wheat canopy at l — 550 nm and l— 7r;3 0 nm, ond t and t0 are boundarier of the time interval of summing Si (t) for a certain period of time [10]). The method of recording was described in detail elsewhere [15].
In addition to recording of the reflectance factors (p,) of the wheat fields, we determined dry and fresh biomass of the plants, leaf area index (LAI), and projective cover (B). The dry and fresh biomass of wheat plants was determined gravimetrically: we cut off the aboveground parts of the plants growing on the plot of an area between 0.25 and 2 m2, weighed them and dried in a drying
cabinet at a temperature of 100-102 °C. Wheat productivity on the reference and experimental plots was evaluated by batch measutement, by weighing tine grain collected by the Sampo - 1200 selection combine with grasp width of 1 m, and by weighing grainsmanually picked from 2 m2 plots. The relative error of determination of the production was 5 %.
Leaf area index -LAI (m2/ m2), or relative surface area of a plant canopy, is defined as the total one-sided leaf area per unit ground surfac e are a :
LAI = kldN,
(3)
Where l is leaf length, d n width of the broadest part of the leaf, k is coefficient of proportionality determined experimentally for leaves tf a given species, N is the number of leaves per unit area of ground.
Projective cover, B, or leaf avea density, LAD is define d as the ground aoea covered by plants when viewed vertically downward (nadir-viewed), which is ttsentially determined by the leal. aiea index of plants and inclination angCes of plant parts. In the general case, B is determined as spaces between plants per unit area of ground:
B = 1 - e
1-(1/D)-(G / cts( j))
(4)
where L=(LAI), D is relative variance—the parameter characterizing the type of spatial arrangement of plant parts; j is viewmg angle; G is distributio n function of inclination angles of plant parts. The projective cover was determined experimentally and varied from 0.3 to 1 during the plant glowing season.
3. Results and discussion
Calculation of chlorophyll potential, S(t) based on the curves of reflectance spectra for the wheat cultivars studied during the growing season showed clear cultivar-related differences (Fig. 1). The highest S(t) was obtained foc wheat cv. "hasatovskayn" - a dark-geeen cultivfe, and the lowest - for "Skala" - a light-green eultivar. Analysis of cttlorophyll potentiat, S(t) ot wheat canopies showed that
Fig. 1. Chlorophyll potential, S(t) for different wheat cultivars. The starting point is 1 June
reflectance factors in the wavelength range between 550 and 730 nm demonstrate the most considerable variations throughout the growing season.
A study of the relationship between wheat biomass and chlorophyll potential showed that larger biomass corresponded to higher S(t) (Fig. 1). Results of field measurements of the biomass correlated with the evaluation of plant productivity based on S(t) [12]. The following productivities were obtained for different wheat cultivars: for Saratovskaya - 0.285 kg^m-2, for Ravnina - 0.249 kg^m-2, and for Skala - 0.229 kg^m-2.
At the same time, we determined leaf area index, LAI, for the wheat fields (Fig. 2). Dark-green broad-leaved cultivars (Ravnina, Saratovskaya) had the highest LAI - 5.0 - 6.0 m2/m2. For other wheat cultivar (Skala), LAI varied between 3 and 4.5 m2/m2. There were 370 - 460 shoots per m2. The number of leaf layers was no more than 5 or 6. The projective cover, B varied from 0.3 (shoot emergence) to 1 (tillering and leaf-tube formation), and in the latter stage, the magnitude of B no longer influenced the reflectance factor, px.
The table gives results of the analysis of the relationship between chlorophyll potential, S(t), dry and fresh biomass of wheat plants. Chlorophyll potential, S(t), had the correlation with dry and wheat biomass.
Dynamics of fresh biomass of different varieties of wheat during the growing season is shown in Fig. 3. There are clear differences in the accumulation of fresh biomass for different wheat cultivars.
Fig. 2. Leaf area index, LAI for different wheat cultivars
Table. Correlation coefficient between chlorophyll potential, S(t) and dry biomass, Mdry, and fresh biomass, Mfresh, of wheat plants
"Wheat cultivar Mfresh Mdry
Skala 0.861 0.81
Ravnina 0.85 0.82
Saratovskaya 0.815 0.795
10 ZJÖ 30 10 20 50 10 Z0 30
lung Kuly augusl
Fig. 3. Fresh biomass for different wheat cultivars
Dark-green broad-leaved cultivars (Ravnina, Saratovskaya) liad the higliest fresh biomass, about 7.0 -8.0 g/m2, light-green cultivar (Skala) - about 66.0 g/m2.
Having iummarized thr wheat crop data accumulaOed over many years, such as the effects of soil treatment technologiev employed before and during planting, amounOs of aertilizera (N, K, P) added to the soil per m2 during the growing feason, and thr use of wheat cultivars with diiferenr maturation times and productivities, wo correlated chlorophyll potential, ES(t), with the grain yield. Our results showed a high correlation between ES(t)and grain yield for different wheat cultivars and cultivation conditions.
4. Conclusion
The study of reflectance factors, px of different wheat canopies and simultaneous determination of their physiological parameters suggested a definite conclusion about the usefulness of remote sensing techniques to solve many practical issues. Among them are evaluation of the physiological state of farm crops and prediction of tliati biological pvoductiviry [16, 17]. Monitoring of changes in the; reflectance factors of whrat canopies during; the growing season showed thcir high information valua in the red side of chloaophyll absorption band, which may be used as a clue to decode aenrtpaae multispectral images and evaluate grain yield of wheat fields [18].
1. Chlorophyll potential of wheat plants, S (t), may be used as an indicator of their physiological state and polential biological productivity.
2. Chlonophyll potential, S(t), in different lor different wheat cultivars during the growing season.
3. There is positive correlation between dry and fresh wheat biomass and chlorophyll potential, S(t), for different wheat cultivars.
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