Short communication
Predicting nitrogen, magnesium and iron nutritional
status in some perennial crops using a portable
chlorophyll meter
Mahmoud M. Shaahan
*, A.A. El-Sayed, E.A.A. Abou El-Nour
Botany Department, National Research Centre, El-Tahrir St., Dokki, Cairo, Egypt
Accepted 25 March 1999
Abstract
A field study was conducted through the years 1996 and 1997 in Ismailia and Sharkia governorates, Egypt, with the horticultural crops: mango, mandarin, guava and grapevine. The study aimed at predicting nitrogen, magnesium and iron-status under field conditions using the pocket apparatus Hydro N-Tester (Minolta, Japan). The data showed that the apparatus can be used as an easy and fast tool to predict both chlorophyll and N-status in the leaves of fruit trees. The N-Tester readings in the range of 110±610 for mango, 735±820 for mandarin, 249±332 for guava and 285±345 for grapevine were considered to be in the range for nitrogen sufficiency at the recommended growth stages for each crop. Lower readings were considered to be in the deficiency range and higher ones in the excess range. The N-Tester can also be used to assess Mg-status for some fruit trees. Measurement of Fe-status using the N-Tester chlorophyll meter under field conditions was not possible.#1999 Elsevier Science B.V. All rights reserved.
Keywords: Horticultural crops; N-Tester; Chlorophyll; Nitrogen; Magnesium; Iron
1. Introduction
It is well know that nutrient deficiency in most cultivated crops during the growth season causes imbalances, leading to reduced yield. Among the essential macronutrients, nitrogen is described as the most important element for vegetative
* Corresponding author. Tel.: +20-2-336-5223; fax: +20-2-361-0850
E-mail address:sh0207@soficom.com.eg (M.M. Shaahan)
growth, flowering and fruit bearing of fruit trees (Mengel and Kirkby, 1987). One of the results of nitrogen deficiency is lack of chlorophyll formation and a low chlorophyll density in plant leaves (Thomson and Weier, 1962). Low levels of magnesium (Neals, 1956), iron (Barton, 1970), and other nutrients such as sulphur, calcium, manganese and zinc can also affect chlorophyll formation leading to different degrees of chlorosis (Mengel and Kirkby, 1987). Consequently, chlorophyll density in plant leaves can theoretically be used as a tool to determine the nutritional status for some nutrients. Chlorophyll determination in the laboratory is a destructive measurement and requires effort, time and chemicals. The pocket apparatus Hydro N-Tester has been designed to measure the density of the green color in the leaves which is considered to be proportional to the chlorophyll content. SPAD chlorophyll meters have been used to determine N-status in several crops (Takebe and Motomatsu, 1987 with corn, Takebe and Yoneyama, 1989; Takebe et al., 1990 with rice; Wood et al., 1992, 1994 with corn, Kantety et al., 1996 with tall fescue).
The present work aimed at investigating the possible correlations between the N-Tester readings and the total chlorophyll, total nitrogen, total magnesium and total iron contents in the leaves of mango, mandarin, guava and grapes in an attempt to find out an easy and fast tool to determine the nutritional requirements of these perennial crops under field conditions.
2. Materials and methods
The study was conducted at different locations in Ismailia and Sharkia governorates, Egypt, during 1996 and 1997.
2.1. Crops
The crops under investigation were chosen to represent different fruit groups, evergreen and deciduous fruits and different types, i.e. Stone fruits, Citrus trees and Pome fruits, and they were: mango (Mangifera indicacv. Zypdya), mandarin (Citrus reticulata cv. Clamentinier mandarina), guava (Psidium guajava) and grapevine (Vitis viniferacv. Thomson seedless).
2.2. The chlorophyll meter used
The apparatus is the portable Hydro N-Tester (Minolta, Japan) with a reading checker. It is battery energized and weighed ca. 225 g. Each reading is an average of 30 measurements.
2.3. Readings and sampling
Mandarin: Readings were taken on 5±7-month-old healthy mature leaves from the middle of non-fruiting spring extension growth.
Guava: The third fully developed pair of leaves from the tip of the fruiting terminal shoot were measured.
Grapevine: Readings were carried out on the leaf stalk of the basal leaf opposite to a bunch cluster.
A total of 20 readings, equivalent to 600 measurements, was carried out for each crop. All the measured leaves were freshly picked, kept in an ice box and transported to the laboratory for analysis of total chlorophyll, total nitrogen (N), total magnesium (Mg) and total iron (Fe).
2.4. Determinations and measurements
2.4.1. Chlorophyll determination
Part of the sample was rinsed with distilled water and dried with clean tissue paper. The chlorophyll content was determined according to the method of Maclachlan and Zalik (1963). The rest of the sample was washed with tap water, 0.01 N HCI, distilled water and then oven dried at 708C and ground.
2.4.2. Nutrient determination
Nitrogen was determined in the dry plant sample using a Bauschi digestion and N2-distillation unit. A part of the dried sample was dry digested using a Muffle
furnace at 5508C (Chapman and Pratt, 1978). Mg and Fe were determined by Atomic absorption spectrophotometry.
2.5. Data analysis
Data were subjected to the NCSS-computer program (Hintze, 1990) to calculate the possible correlations, r2 and regression formula. The significance levels of Snedecor and Cochran (1967) were used as references.
3. Results
3.1. Chlorophyll content
3.2. Nitrogen
Results presented in Fig. 2 illustrate nitrogen concentrations in the leaves of the trees as correlated with chlorophyll meter readings. Regression analysis of the
data showed that nitrogen concentrations in the leaves of mango, mandarin, guava and grapevine are linearly correlated with the chlorophyll meter readings (r0.95 andr2 <0.92 for all measurements,p<1). Thus, the meter readings can fairly describe the N-status of these crops in the field and practically determine whether N-fertilizer application is needed.
3.2.1. Mango
According to Robinson (1986), a concentration of 1.0±1.5% N (on dry weight basis) in mango leaves after flowering is considered as the `Sufficient' range. A range of 110±610 chlorophyll meter readings was found to correspond to that range (Fig. 2). We consider that a reading of 110 is proportional to a critical N-concentration (1.0%) in the leaves below which, N-fertilization is needed. A reading of >610 corresponds to an excessive N-level where further nitrogen fertilization is wasteful and can lead to groundwater pollution.
3.2.2. Mandarin
Nitrogen values in 5±7-month-old mandarin leaves (Robinson, 1986) are considered deficient below 2.2% (Fig. 2). The range 2.2±2.4% (meter readings 650±735) is considered marginal, while the range 2.4±2.6% (meter readings 735± 820) is sufficient. Meter readings of >821 correspond to undesirably high N-levels.
3.2.3. Guava
Here, 1.3±1.6% N (249±332 meter reading) in the third fully developed pair of leaves from the tip of a fruiting terminal shoot of guava was reported to be sufficient (Robinson, 1986) (Fig. 2). Thus, chlorophyll meter readings below 249 can be used as an indicator for deficiency, whilst those >332 indicate high N-levels in the leaves.
3.2.4. Grapevine
Sufficient nitrogen values in the grapevine leaves were reported in the range 2.6±3.0% (Robinson, 1986) corresponding to chlorophyll meter readings of ca. 285±345.
3.3. Magnesium
As is shown in Fig. 3, significant positive correlations were found between the chlorophyll meter readings and magnesium concentrations in the leaves of all the trees except mandarin, where they were significantly negative. So, it seems that the meter can be also used to predict Mg status in some, but not all fruit trees.
3.4. Iron
4. Discussion
The linear correlations found between the N-Tester readings and leaf chlorophyll content showed that the apparatus can easily be used as a chlorophyll meter for the
studied crops under field conditions. Differences in the regression formula from crop to crop can probably be attributed to differences in leaf anatomy (i.e. leaf thickness, cuticle thickness, presence of waxy layer, etc.). Variation in chlorophyll concentration according to plant genotype is also a reason for these differences.
Leaf chlorophyll concentration has been found to be directly related to N-concentration by many investigators (Takebe and Yoneyama, 1989; Takebe
et al., 1990 with rice; Wood et al., 1992, 1994 with corn; Kantety et al., 1996 with tall fescue) and so, the chlorophyll meter can be used to assess the N-status of the plant. Great differences in the chlorophyll meter readings associated with deficient, sufficient and excess N-level ranges in the leaves of the studied trees were observed (Fig. 2). Narrow-deficient, and wide-sufficient ranges were observed with mango, while the opposite was observed with mandarin. Intermediate trends were recorded for guava and grapevine. This can be explained by the genotype differences concerning nitrogen requirement in relation to chlorophyll density in the leaves and by the extent to which the crop is sensitive to N-shortage. Mandarin can be considered as the most sensitive in this group followed by guava and grapes, while mango is the least sensitive.
The N-tester, therefore, can serve as an easy and fast tool to diagnose N-status to remedy N-deficiency and to avoid excessive N-fertilization and pollution of the environment.
Significant positive correlations between Mg-concentrations in the leaves of mango, guava and grapevine and the chlorophyll meter readings suggested that Mg concentration also affected leaf chlorosis. Mg deficiency can be distinguished by the interveinal chlorosis of the old leaves down the trees. However, in the case of N-deficiency, the whole leaves turned pale yellow and may cover the chlorosis due to deficiency of other nutrients. Mg concentration in the leaves of these trees was found to correlate with N-concentrations. In contrast, Mg appears to accumulate in the leaves of mandarin with low N-levels and in this case the N-tester cannot be used to determine its real need. However, addition of N-fertilizers may automatically solve the problem as a result of the better growth due to N-sufficiency.
Iron deficiency can also cause significant interference in the measurements by the chlorophyll meter. In contrast to N and Mg, the interveinal chlorosis of Fe-deficiency can be seen on new leaves. Fe chlorosis can also be overcome by N-fertilization. The weak response of the chlorophyll meter to the total iron concentrations might be explained by the findings of Dekock et al. (1960), namely that the haem pigments contribution is only ca. 0.1% of the total iron in the plant leaves.
5. Conclusions
chlorophyll meter is possible for some fruit trees. Using, the N-Tester to determine the Fe-status of fruit trees is not possible.
Acknowledgements
The authors thank Hydro-Agri Company, Germany, for its cooperation supplying the N-Tester. They wish also to thank the staff of the Program `Micronutrients and other Plant Nutrition Problems in Egypt' and its Principal investigator Prof. Dr. Mohamed M. El-Fouly for their help in sample analysis.
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