Estimation of light interception and biomass of the potato (Solanum tuberosum L.) from reflection in the red and near-infrared spectral bands

Agricultural and Forest Meteorology, 53 ( 1990 ) 19-31

19

Elsevier Science Publishers B.V., Amsterdam

Estimation of light interception and biomass of the potato (Solanum tuberosum L. ) from reflection in the red and near-infrared spectral bands P. Millard l, G.G. Wright 1, M.J. Adams 2, R.V. B i r n i e 1 a n d P. W h i t w o r t h ~ 1Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, AB9 2QJ (U.K.) eSchool of Applied Sciences, The Polytechnic, Wulfuna Street, Wolverhampton, WEV1 2SB (U.K.) (Received and accepted June 10, 1990)

ABSTRACT Millard, P., Wright, G.G., Adams, M.J., Birnie, R.V. and Whitworth, P., 1990. Estimation of light interception and biomass of the potato (Solanum tuberosum L.) from reflection in the red and near-infrared spectral bands. Agric. For. Meteorol., 53:19-31. Radiance measurements of the potato crop at near-infrared (NIR) and red (R) wavelengths were related to ground cover, light interception and destructive growth measurements in 1986. The relationship between the ratio NIR/R and crop ground cover was dependent upon crop nitrogen (N) status, as a result of fertilizer applications increasing the chlorophyll concentration in leaves. Calculation of cumulative light interception, assuming a 1:1 relationship between light interception and ground cover, showed a curvilinear relationship with cumulative NIR/R (YNIR/R), with N application increasing the slope of the line. Values of ~NIR/R were related to total crop dry weight, giving a separate quadratic equation for crops grown with or without N fertilizers. These relationships were tested by predicting, retrospectively, the biomass of crops grown in 1985 with several different levels of fertilizer nitrogen. During the period of canopy expansion, radiometric data overestimated crop growth. After canopy closure, the mean differences between measured and estimated values of crop dry weight were not significantly (P> 0. l ) different from zero, as long as the quadratic equation derived from the N-fertilized crops was used. The implications of using radiometric data for modelling crop growth are discussed.

INTRODUCTION

Total dry matter production by the potato crop has been shown to be linearly related to intercepted solar radiation (Allen and Scott, 1980; Khurana and McLaren, 1982 ). Crop growth can, therefore, be modelled and yield potentials estimated from light interception data (Mackerron and Waister, 1985 ). Direct methods of crop light interception can be expensive in terms of equipment in an experiment with many plots. A single solarimeter samples 0168-1923/90/$03.50

© 1 9 9 0 - - Elsevier Science Publishers B.V.

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P. M I L L A R D ET AL.

only a small area of canopy, so may be unrepresentative during senescence or if the crop lodges. In addition, the use of many solarimeters in an experiment precludes access of vehicles for crop spraying. Indirect methods based upon ground cover, a more easily measured crop variable, are often used and assume that there is a 1:1 relationship between percentage ground cover and radiation interception (Burstall and Harris, 1983; Millard and Marshall, 1986). Conventional methods for measuring percentage ground cover of a potato canopy are time consuming. Such methods include visual scoring (Burstall and Harris, 1983 ), vertical photography followed by random point sampling (Steven, et al., 1986 ) or area digitising (Birnie et al., 1987 ). What is required is a field technique that provides a more direct and rapid measurement of light interception, to provide an input to models of crop growth. This paper examines the value of potato canopy reflectance measurements, using field radiometers, as a rapid means of obtaining estimates of light interception. Of particular interest is the use of reflectance measurements made at near-infrared (NIR) and red (R) wavelengths. Such measurements exploit two well known spectral properties of green leaves: that NIR reflectance.is positively related to leaf biomass and R reflectance is negatively related to chlorophyll concentration (Gausman, 1974; Whittingham, 1974; Curran, 1983). There is an extensive literature on the use of field radiometry for monitoring crop growth. Tucker et al. (1979), Holben et al. (1980), Kimes et al. ( 1981 ), Markham et al. ( 1981 ) and Aase et al. ( 1984 ) have, amongst others, correlated R and NIR spectral data with crop variables like green leaf area index and biomass. Aase and Siddoway ( 1980, 1981 a, b) showed that it was possible to estimate plant population density, biomass and grain yield of wheat crops using the N I R / R ratio or the normalised difference ( N D ) vegetation index of Rowse et al. (1973), where ND is expressed as ( N I R - R ) / (NIR + R). The relationship between crop biomass and the vegetation index is, however, growth stage dependent, at least for cereals (Tucker et al., 1980; Aase and Siddoway, 1981a). Steven and Demetriades-Shah (1987) also showed that most of the relationships are crop or experiment specific and argued that a more fundamental, physiologically based approach to remote sensing of crops be adopted. Previous work using a two-band radiometer showed that the N I R / R ratio of the potato crop is highly correlated with ground cover up to the time of canopy closure (Birnie et al., 1987). The present investigation extends this work to examine the relationship between NIR and R canopy radiance measurements and light interception in a second growing season. The aim of the experiment was to establish a relationship between NIR and R canopy radiance and destructive crop growth measurements, which could be tested by predicting, retrospectively, the crop biomass measured in the previous grow-

LIGHT INTERCEPTION ESTIMATED BY RADIOMETRY

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ing season. A second aim was to investigate the effect of nitrogen (N) rate on leaf chlorophyll concentration and N I R / R ratio. MATERIALS AND METHODS

Crops Solanum tuberosum L. cv. Maris Piper was grown in 1985 in a freely drained till derived from basic igneous material (Insch series; Glentworth and Muir, 1963 ). Crops were fertilized with either 80 or 120 kg N h a - 1 with applications of fertilizer either at planting or at tuber initiation. In 1986, plants were grown in a freely drained soil derived from granite and granitic gneiss (Countesswells series; Glentworth and Muir, 1963 ). Two treatments of either no N or 200 kg N h a - ~applied at planting were given. The nitrogen supply to the fertilized crops ensured that the crops were nitrogen replete since the recommendation for N application to a main crop grown on a soil of moderate N status is 150 kg h a - 1 (Scottish Agricultural Colleges, 1985 ). In both years, the fertilizer treatments were randomised in blocks with four replicates. Other details of the sites and crop husbandry are given in Table 1. Measurements of plant dry weight were made at eight harvests over the growing season, in both experiments, by sampling 10 plants from each plot as described by Millard and Marshall ( 1986 ). During 1986, ground cover measurements were taken every 3 days until canopy closure, and thereafter once a week, by vertical photography using a 35-mm diapositive film (Kodachrome 64). Ground cover values were computed from the photographs by tracing the complex outline of the canopy and digitising the area, as described previously (Birnie et al., 1987). The chlorophyll concentration in leaflets TABLE 1 Description of experimental sites and crop husbandry 1985

1986

Grid reference Soil series Previous crop Planting density (plants m - z ) P fertilizer at planting (kg h a - ~) K fertilizer at planting (kg ha -~ )

NJ 717 308 Insch Spring barley 4.5 87 167

NJ 905 045 Countesswells Spring barley 4.5 87 167

Date of Planting Emergence N applied at tuber initiation Final harvest

12 April 17 June 18 July 17 September

16 April 12 June 30 September

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P. MILLARDETAL.

sampled from the top of the canopy was measured weekly, using dimethyl sulphoxide to extract the chlorophyll and attenuation of photon detection from a sealed miniature ~4C standard for quantification (Millard and Robinson, 1987 ). The rapidity of this technique allowed 10 leaflets to be analysed from each plot every time chlorophyll measurements were made. The mean of these 10 values was used as the 'plot value' for subsequent calculations.

Radiometry A two-band radiometer was used to take measurements of crop radiance. The radiometer had a red channel extending from 595 to 635 n m and a nearinfrared channel between 800 and 930 n m (Adams et al., 1985 ). Both detectors shared the same field of view (1.33 m diameter at 3.0 m elevation) through a single collecting lens using a neutral density beam splitter m o u n t e d at 45 ° to the optical axis. In 1986, the radiometer was m o u n t e d beside the camera used for ground cover determinations so that photographs and radiance values could be recorded simultaneously for the same ground area. Crop radiance was measured from a fixed peg located at either side of the plot by operators wearing dark clothing. The quadrats selected for radiometry were those selected randomly within the plots for the final destructive harvest in each experiment. The radiometer was m o u n t e d at the end of a 1-m side arm 3 m above the upper surface of the crop, using a surveying staff with a spirit level to ensure that the radiometer was presented to the crop canopy at a constant angle. To minimise the effect of changing spectral characteristics of the crops at different times of the day ( K u m a r and Monteith, 1981 ), standard precautions, as described by Milton (1987 ), were taken. All measurements on each sample day were taken with the sensor head and all other equipment in the same position relative to the sun. Rapidly changing cloud conditions were avoided by making measurements with a clear sky, or on days with little wind or overcast conditions. All measurements were made within 2 h of solar noon. The instantaneous red and near-infrared crop radiance readings were corrected for instrumental dark-current values and the signal voltages expressed as a ratio to produce a dimensionless function, the near-infrared/red ratio ( N I R / R ) . This ratio is highly sensitive to crop development, but has a reduced sensitivity to the magnitude of solar radiation compared with the individual radiance values (Lord et al., 1985 ). RESULTS

Crop growth and radiance During 1986, daily measurements of crop radiance were taken in order to determine the relationship between crop radiance and growth. Details of crop

23

LIGHT INTERCEPTION ESTIMATED BY RADIOMETRY

/ iI

/ i 11 t t

J

; 20

I 40

I 60

I 80

I 100

Days after emergence

Fig. 1. Crop percentage ground cover measurements for the fertilized ( • ) and unfertilized ( 0 ) crops in 1986. Each line is the mean of four replicates. Vertical bars represent the standard errors of the treatment means (3 d.f.). 8.0-~

7.0-

6.0-

5,0-

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....

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Days after emergence

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Fig. 2. Crop near-infrared/red reflectance ratio for the fertilized ( - ) and unfertilized ( .... ) crops in 1986. Each line is the mean of four replicates. Vertical bars represent the standard errors of the treatment means ( 3 d.f.).

24

P. MILLARD ET AL.

8.0-

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0

0

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Fig. 3. The relationship between crop ground cover and near-infrared/red reflectance ratio for the fertilized ( • ) and unfertilized ( O ) crops in 1986. Values are the means of four replicates. Arrows indicate 40 DAE for both treatments.

dry matter production, N uptake and partitioning have been given elsewhere (Millard et al., 1989). Briefly, the crops emerged on 12 June and the final harvests were made on 30 September, 110 days after 50% emergence (DAE). At the final harvest, the tuber dry matter yields for the unfertilized and fertilized treatments were 9.6 and 11.9 ( _ 0.22, standard error with 3 d.f.), respectively. During canopy expansion, plants in the fertilized treatment were burned by the large amounts of calcium nitrate and suffered an initial check in their growth. However, by 40 DAE, the fertilized crops had recovered sufficiently to produce a greater ground cover than the unfertilized plants. (Fig. 1 ) and thereafter grew to produce full ground cover. In contrast, the unfertilized crops achieved a maximum ground cover of only 85% (Fig. 1 ). Values of N I R / R increased rapidly up to 40 DAE for both treatments (Fig. 2 ). Thereafter, there was a small upward trend in the N I R / R values for the high-N crop until late on in the season when, at ~ 90 DAE, leaves started to senesce and the N I R / R values fell. In contrast, the unfertilized crops showed a gradual decline in N I R / R from 40 DAE to final harvest. Because of these differences in N I R / R between the two treatments, the relationship between N I R / R and ground cover was dependent upon the N status of the crop after

LIGHT INTERCEPTION

E S T I M A T E D BY R A D I O M E T R Y

25

2.5-

~ 2.0s

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:r

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after

emergence

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Fig. 4. Leaf chlorophyll concentrations in the fertilized ( • ) and unfertilized ( O ) crops in 1986. Values are the means of four replicates and vertical bars represent the standard errors of the treatment means (3 d.f.).

40 DAE (Fig. 3 ). Nitrogen was found not to have any obvious influence upon this relationship by Birnie et al. ( 1987 ). However, in the present investigation the range of N application was much greater ( 0 - 2 0 0 kg N h a - 1 compared with 0 - 1 2 0 kg N h a - 1) and the corresponding growth response by the crop was larger than those found by Birnie et al. ( 1987 ). It is well established that N supply can affect the chlorophyll concentration in the leaves of plants (Moorby and Besford, 1983). Nitrogen application significantly ( P < 0 . 0 5 ) increased the concentration of total chlorophyll recovered from the leaves throughout the experiment (Fig. 4 ). Both treatments showed a peak in chlorophyll concentration around 40 DAE, while the canopy was still expanding. Thereafter, the chlorophyll concentrations remained fairly stable, until late on in the season when there was a rapid decline due to leaf senescence. The differences in leaf chlorophyll concentration between the two treatments were reflected in their differing relationships between N I R / R and ground cover (Fig. 3 ).

26

P. MILLARD ET AL 400-

300-

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~e'''

200-

.O'" ,O'''"

100-

,Z~"

I

I

180

I

360

540

Accumulated

intercepted

rodiation

9~

I

720 (MJ m -2)

Fig. 5. The relationship between cumulative light interception and cumulative near-infrared/ red reflectance ratio in 1986. Values are for the fertilized ( • ) and unfertilized ( © ) treatments, and are the mean of four replicates.

1600-

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Fig. 6. The relationship between total crop dry matter and the cumulative near-infrared reflectance ratio. Values are for the fertilized ( • ) and unfertilized ( O ) treatments, and are the means of four replicates.

27

LIGHT INTERCEPTION ESTIMATED BY RADIOMETRY

Radiometric measurement oflight interception and crop growth Daily measurements of N I R / R in 1986 were used to calculate cumulative values Z N I R / R . Ground cover measurements and daily solar radiation data were used to calculate cumulative light interception values, assuming a 1:1 relationship between ground cover and light interception. Plotting Z N I R / R against cumulative light interception for the two treatments showed that the relationship was not linear and was affected by the N fertilizer (Fig. 5 ). The differences in the relationship due to the N status of the crop implied that radiance ratio measurements were not dependent solely on the leaf area of the crop and the incident radiation, but also the chlorophyll status of the leaves. Since light interception data can be used to predict crop growth, values of ~ N I R / R were plotted against total crop dry weight for the two N treatments A

B

1200

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800

800

400-

400,

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800

12100

400

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400

I

1

i

i

i

400

800

1200

400

800

Measured

crop

dry

weight

12 IO0

(g/m 2)

Fig. 7. Comparison of crop weight (g m - 2 ) measured throughout 1985 with values estimated from ZNIR/R, using the relationship between YNIR/R and crop weight determined for the fertilized crop in 1986. Values are the mean of four replicates for crops grown with: ( A ) 80 kg N h a - l applied at planting; ( B ) 120 kg N h a - l applied at planting; ( C ) 120 kg N h a - l applied at tuber initiation. Lines represent the 1 : 1 relationship.

28

P. MILLARD ET AL.

(Fig. 6). In both cases, the relationships gave a quadratic line, with the following equations D--6.86+4.47 ( ~ N I R / R ) - 0 . 0 0 3 4 ( E N I R / R ) 2

(1)

D=-52.90+6.14 (ENIR/R)-0.0083 (~NIR/R) 2

(2)

for the fertilized and unfertilized crops, respectively, where D = total dry weight of the crop (kg m - 2 ) . To determine if Z N I R / R could be used as a substitute for light interception in the modelling of crop growth, weekly measurements of N I R / R from the crops grown in 1985 were used to calculate values of ZNIR/R. Values of N I R / R were assumed to represent those of the 3 days either side of the observation and used to calculate daily values of N I R / R . These daily values of N I R / R were then summed to calculate E N I R / R . Using the relationship determined between E N I R / R and total crop weight for the fertilized plants in 1986 (eq 1 ), the growth of the 1985 crops was estimated, retrospectively, from their E N I R / R values (Fig. 7). Comparison of the estimated values with those measured by growth analyses showed that during the period of canopy expansion (0-37 DAE), the radiometric data overestimated crop growth. However, after the attainment of complete ground cover (42 DAE in each treatment), a paired Student's t-test showed that the mean differences between the measured and estimated values of crop dry weight were not significantly ( P > 0.1 ) different from zero. Therefore, crop radiance data successfully estimated crop growth after canopy closure, irrespective of the timing or rate of N fertilizer application (Fig. 7 ). DISCUSSION

For crops that were not N stressed, the relationship between crop weight and canopy radiance (eqn. 1 ) allowed the prediction of crop weight to be made from canopy reflectance measurements. However, prior to canopy closure, values of E N I R / R significantly overestimated crop weight (Fig. 7). In addition, the weight of a N-stressed crop is likely to be overestimated where the equation derived from a stress-free crop is used. The results indicate that the value of canopy radiance values in predicting crop weight is limited in two ways. Firstly, the relationship between N I R / R and crop weight is growth stage dependent since estimates of crop weight prior to canopy closure were inaccurate; secondly, the relationship can be altered by crop husbandry. These conclusions are similar to those made by Aase and Siddoway (1981a) and Tucker et al. (1980). Where a canopy does not completely cover the soil, the measured radiance values derive from both soil and plant. Radiometric measurements were made over bare soil controls throughout both the 1985 and 1986 growing seasons. Using the percentage plant cover values and these soil background readings,

LIGHT INTERCEPTION ESTIMATED BY RADIOMETRY

29

the component of Z N I R / R derived from soil was removed by subtracting the product of the bare soil measurement and the proportion of bare soil found in the plot from the N I R / R value. This procedure had little effect upon the N I R / R ratio (maximum of 0.9 at emergence, see Fig. 2) and no effect upon the pattern of N I R / R over time. Therefore, soil background effects are assumed to be of little importance in explaining the overestimation of light interception during canopy expansion. This assumption is possible because the crops in both years were grown on soils with similar spectral properties; the assumption would not be valid if the soils were markedly different (Curran, 1983). The initial expansion of the potato canopy is closely related to air temperature. Waister and Ross ( 1981 ) found the relative growth rate of leaves to be linearly dependent upon mean air temperature, above a base temperature of 2.4 ° C. The mean June air temperature at the experimental sites, during canopy expansion, was 10.5°C in 1985 and 13.2°C in 1986 (Meteorological Ofrice, 1985-1986). The lower mean air temperature in 1985 than 1986 would have increased the slope of the relationship between Z N I R / R and accumulated intercepted radiation. This in turn would have led to an overestimation of light interception at the start of the season, and hence growth in 1985, when the equations from 1986 were used. Nitrogen status altered the relationship between Z N I R / R and crop weight. In 1986, the unfertilized crops were N deficient (Millard et al., 1989), while none of the crops in 1985 were N stressed (Millard and Robinson, 1990). Therefore, the use of eq. 2 to estimate crop dry weight from 1985 ~ N I R / R data resulted in an overestimation of crop growth. At the final harvest in 1985, values of crop weight predicted using eqn. 2 had a mean value higher than those estimated using eqn. 1 (data not shown). The Monteith ( 1977 ) model of crop growth determines the growth of stress-free crops as the product of the fraction of solar radiation being intercepted (f) and the efficiency of radiation use for dry matter production (the dry matter:radiation quotient, e ). Steven and Demetriades-Shah (1987) have shown that e can be affected by stress, with a yellow virus infection of sugar beet reducing e from 1.35 to 1.09 g MJ -1. However, these workers found that N fertilization of barley had no significant effect on e (Steven and Demetriades-Shah, 1987 ). Similarly, e was little affected by a wide range of N applications to the potato crop (Millard and Marshall, 1986). The main effect of N deficiency on the relationship between Z N I R / R and crop weight is likely to be due to the N-deficient plants having lower leaf chlorophyll concentrations, thereby increasing the red reflectance from the canopy (Curran, 1983). Hay and Galashan (1989) suggested that the most economic method of monitoring farm yields of the potato crop may be remote sensing of radiation interception. Knowledge of e has allowed radiometric measurements o f f to be used in modelling the growth of sugar beet (Kumar and Monteith, 1981;

30

P. MILLARD ET AL.

Steven et al., 1983 ). The present investigation has used the relationship between Z N I R / R and crop growth to estimate growth in other crops without prior knowledge of e. Similarly, Markham et al. ( 1981 ) and Baret et al. ( 1989 ) showed that for wheat crops, accumulated radiometric data were more closely related to dry matter production than instantaneous measurements. Provided that crops are not N deficient, it should be possible to use weekly measurements of crop radiance after canopy closure as a direct input to growth models such as that of MacKerron and Waister ( 1985 ) and MacKerron ( 1987 ). Further work is required to use radiometry to model crop growth prior to canopy closure, or in N-deficient crops.

REFERENCES Aase, J.K. and Siddoway, F.H., 1980. Determining winter wheat stand densities using spectral reflectance measurements. Agron. J., 72:149-152. Aase, J.K. and Siddoway, F.H., 198 la. Spring wheat yield estimates from spectral reflectance measurements. In: Inst. Electronic Electric. Eng. Trans. Geosci. Remote Sensing, 19: 78-84. Aase, J.K. and Siddoway, F.H., 1981 b. Assessing winter wheat dry matter production via spectral reflectance measurements. Remote Sensing Environ., 11: 267-277. Aase, J.K. Siddoway, F.H. and Millard, J.P., 1984. Spring wheat leaf phytomass and yield estimates from airborne scanner and hand-held radiometer measurements. Int. J. Remote Sensing, 5: 771-781. Adams, M.J. Ewen, G.J. and Birnie, R.V., 1985. A portable two-band radiometer. Int. J. Remote Sensing, 6: 963-966. Allen, E.J. and Scott, R.K., 1980. An analysis of growth of the potato crop. J. Agric. Sci., 94: 583-606. Baret, F., Guyot, G. and Major, D.J., 1989. Crop biomass evaluation using radiometric measurements. Phytogrammetria, 43:241-256. Birnie, R.V., Millard., P., Adams, M.J. and Wright, G.G., 1987. Estimation of percentage ground cover in potatoes by optical radiance measurements. Res. Dev. Agric., 4: 33-35. Burstall, L. and Harris, P.M., 1983. The estimation of percentage light interception from leaf area index and percentage ground cover in potatoes. J. Agric. Sci., 100: 241-244. Curran, P.J., 1983. Problems in the remote sensing of vegetation canopies for biomass estimation. In: R.M. Fuller (Editor), Ecological Mapping from Ground, Air and Space. Institute of Terrestrial Ecology Symposium No. 19, National Environmental Research Council, pp. 84-100. Gausman, H.W., 1974. Leaf reflectance of near infrared Photogramm. Eng. Remote Sensing, 40: 183-191. Glentworth, R. and Muir, J.W., 1963. The Soils of the country round Aberdeen, Inverurie and Fraserburgh (Sheets 77, 76 and 87/977). HMSO, Edinburgh. Hay, R.K.M. and Galashan, S., 1989. The yields of arable crops in Scotland 1978-82: actual and potential yields of potatoes. Res. Dev. Agric., 6: 1-5. Holben, B.N., Tucker, C.J. and Fan, C., 1980. Spectral assessment of soybean leaf area and leaf biomass. Photogramm. Eng. Remote Sensing, 46:651-656. Khurana, S.C. and McLaren, J.G., 1982. The influence of leaf area, light interception and season on potato growth and yield. Potato Res., 25: 329-342. Kimes, D.S,, Markham, B.L., Tucker, C.J. and McMurtey, J.G., III, 1981. Temporal relation-

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ships between spectral response and agronomic variables of a corn canopy. Remote Sensing Environ., 11 : 401-411. Kumar, M. and Monteith, J.L., 1981. Remote sensing of crop growth. In: H. Smith (Editor), Plants and the Daylight Spectrum. Academic Press, London, pp. 133-144. Lord, D., Desjardino, R.L., Dube, P.A. and Brach, E.J., 1985. Variations of crop canopy spectral reflectance measurements under changing sky conditions. Photogramm Eng. Remote Sensing, 51: 689-695. MacKerron, D.K.L., 1987. A weather-driven model of potential yield in potato and its comparison with achieved yields. Acta Hortic., 314: 85-94. MacKerron, D.K.L. and Waister, P.D., 1985. A simple model of potato growth and yield. I. Model development and sensitivity analysis. Agric. For., Meteorol., 34:241-252. Markham, B.L., KJmes, D.S., Tucker, C.J. and McMurtey, J.E., III, 1981. Temporal spectral response of a corn canopy. Photogramm. Eng. Remote Sensing, 47:1599-1602. Meterological Office, 1985-86. Monthly weather reports for 1985-1986. HMSO, London. Millard, P. and Marshall, B., 1986. Growth, nitrogen uptake and partitioning within the potato (Solanum tuberosum L.) crop, in relation to nitrogen application. J. Agric. Sci., 107:421429. Millard, P. and Robinson, D., 1987. Colorimetric determination of the total chlorophyll concentrations in potato leaves by liquid scintillation counting. Potato Res., 30:491-494. Millard, P. and Robinson, D., 1990. Effect of the timing and rate of nitrogen fertilization on the growth and recovery of fertilizer nitrogen with the potato (Solanum tuberosum L.) crop. Fert. Res., 21: 133-140. Millard, P., Robinson, D. and Mackie-Dawson, L.A., 1989. The regulation of nitrogen partitioning within the potato (Solanum tuberosum L.) plant by nitrogen supply. Ann. Bot., 63: 289-296. Milton, E.J., 1987. Principles of field spectroscopy. Int. J. Remote Sensing, 8:1807-1827. Monteith, J.L., 1977. Climate and the efficiency of crop production in Britain. Phil. Trans. R. Soc. London Ser. B, 281: 277-294. Moorby, J. and Besford, R.T., 1983. Mineral nutrition and growth. In: A. Lauchli and R.L. Bieleski (Editors), Inorganic Plant Nutrition. Encyclopedia of Plant Physiology New Series Vol. 15B. Springer-Verlag, Berlin pp. 481-529. Rowse, Jr., J.W., Haase, R.H., Schell, J.A. and Deering, D.W., 1973. Monitoring vegetation systems in the Great Plains with ERTS. In: Third Earth Resources Technology Satellite-1 Symposium, NASA, pp. 309-317. Scottish Agricultural Colleges, 1985. Fertilizer recommendations. Publ. No. 160, Scottish Agricultural Colleges and The Macaulay Institute for Soil Research. Steven M.D. and Demetriades-Shah, T.H., 1987. Spectral indices of crop productivity under conditions of stress. In: Advances in Digital Image Processing. Proc. 13th Annual Conf. Remote Sensing Soc., Nottingham, pp. 593-601. Steven, M.D., Biscoe, P.V. and Jaggard, K.W., 1983. Estimation of sugar beet productivity from reflection in the red and infrared spectral bands. Int. J. Remote Sensing, 4: 325-334. Steven, M.D., Biscoe, P.V., Jaggard, K.W. and Paruntu, J., 1986. Foliage cover and radiation interception. Field Crops Res., 13: 75-78. Tucker, C.J., Elgin, J.H. and McMurtey, J.E., III, 1979. Temporal spectral measurements of corn and soybean crops. Photogramm. Eng. Remote Sensing, 45: 643-648. Tucker, C.J. Holben, B.N., Elgin, Jr., J.H. and McMurtey, J.E., III, 1980. Relationship of spectral data to grain yield variation. Photogramm. Eng. Remote Sensing, 46: 657-666. Waister, P.D. and Ross, H.A., 1981. Seasonal changes in net assimilation rate and relative growth rate of stem cuttings of potato. Potato Res., 24:221. Whittingham, C.P., 1974. The Mechanisms of Photosynthesis. Edward Arnold, London.