Research Article, Vegetos Vol: 31 Issue: 1

Taxonomical and Chemical Characterization of Panicum maximum L. from Different Agro- Ecological Zone of Sri Lanka

Dharanee Senanayake¹, Jayasekera LR^1* and Andreas Lucke²

¹Department of Genetics and Plant Breeding, BA College of Agriculture, Anand Agricultural University, Anand, Gujarat, India

²Main Vegetable Research Station, Anand Agricultural University, Anand, Gujarat, India

*Corresponding Author : Jayasekera LR
Department of Botany, University of Kelaniya, Sri Lanka
Tel: +94-11-2914480
Fax: +94-11-2914480
E-mail: ranjith@kln.ac.lk

Received: December 21, 2017 Accepted: February 23, 2018 Published: February 26, 2018

Citation: Senanayake D, Jayasekera LR, Lucke A (2018) Taxonomical and Chemical Characterization of Panicum maximum L. from Different Agro-Ecological Zone of Sri Lanka. Vegetos 31:1. doi: 10.4172/2229-4473.1000383

Abstract

Keywords: Guinea grass; Carbon isotope ratio; Leaf inorganic composition; Panicum maximum

Introduction

Grasses have been recognized as a unique lineage among the flowering plants for more than 200 years and they are the most diverse group of plants that can be found virtually in all habitats from freshwater to sand dunes. Approximately 11,313 plant species in the world have been recognized as grass species [1]. P. maximum L., is known as Guinea grass or the panic grass is frequently found in warmer climates and used as a fodder grass. Both annuals and perennials can be found in this species [2,3], and are widely distributed almost all over the tropical countries such as Africa, India, South America and Sri Lanka due to its capacity to grow even under harsh environmental conditions [3]. P. maximum is sometimes classified as an invasive species in the tropical countries [4].

Thermal responses of the Panicum grasses vary with their genotype [5]. Some are adaptable for the shade environment and some are highly adaptable for the warmer climates. Though the exposure to the 70% of full sunlight is the best condition for the growth, some records have revealed that certain varieties are capable of surviving even under highly shaded conditions [5]. Therefore, adaptive divergence in physiological traits such as the mode of photosynthesis between populations may be expected that may confer local adaptation. Although the greatest proportion of C4 plants occurs in the grass family, adaptation to more C3-C4 intermediate photosynthesis could be expected in the populations from the cooler climates. C3 and C4 plants are easily distinguishable on the basis of their carbon isotope ratios, expressed as δ¹³C values and quantified on a per mil (‰) basis [6,7]. The typical averages of δ¹³C for the C3 and C4 plants are -28‰ and -14‰ respectively. On a theoretical basis, the δ¹³C value for the C3-C4 intermediates could be expected between -14‰ and -28‰. In this study, we measured the δ¹³C of the Panicum maximum populations from cooler and warmer climates to see whether there are variations between populations that may confer local adaptation reflecting C3-C4 intermediates. It has also been recorded that C4 plants are characterized with lower leaf mesophyll to bundle sheath cell ratio compared to C3 plants [8,9]. In addition to leaf carbon isotope characteristics, a selected set of leaf nutrients were also measured to detect any location-dependent population variations in the leaf nutrient contents.

Materials and Methods

Leaf nutrient analysis

Four replicate leaf samples from each agro ecological zone were collected contamination-free in clean polythene bags from undisturbed locations, each with an approximate area of 500 m² representing the seven different agro ecological zones of Sri Lanka (Table 1). They were then brought to the laboratory and cleaned with distilled water followed by drying at 700 C for 72 h in an oven, and ground with a pre cleaned grinder contamination-free. The homogenized leaf samples were transferred to clean polythene bottles (50 ml) and stored in a refrigerator until analysis. Leaf nutrients, the total nitrogen (N) content was determined by the standard Kjeldahl digestion [10], and the phosphorus (P) by molybdo-vanadate method [11]. Contents of potassium (K), calcium (Ca), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn) were determined on a dry weight basis by atomic absorption spectrophotometry (Model: Spectraa 110) following a wet digestion with a tri acid mixture at 200°C [10]. Four replicates were analyzed from each agro ecological zone. To control and assess the quality of the analytical results, the rye grass certified reference material No. 281 from the Commission of the European Communities, Community Bureau of Reference (BCR) were analyzed simultaneously.

Table 1: Climatological data of the seven agro ecological zones of Sri Lanka*.

Carbon isotope analysis

A set of homogenized leaf material in sealed polythene bags were air mailed to the Institute of Bio-and Geosciences, Research Centre Juelich, Germany for the carbon isotope analysis. About 0.25 mg of a sample was weighed into tin capsules to yield approximately 100 μg of carbon, and combusted at 1080°C using an elemental analyzer (Model: Thermo Scientific Flash 2000) interfaced in-line with an isotope ratio mass spectrometer (Model: Thermo Scientific Delta V Advantage). The δ¹³C ratio was determined using the following equation and expressed in terms of δ values per mil (‰).

δ¹³C (‰) = [(Rsample - Rstandard)/Rstandard] × 10³

Rsample and Rstandard are the isotope ratios (¹³C/¹²C) of the samples and standards, respectively, reported on the VPDB scale. The standard represents the carbon isotope ratio in a fossil belemnite given by the PDB- formation. In addition to the calibration standards, additional laboratory standard materials were also inserted between samples to monitor the performance of the instrument. The overall precision of the replicate analyses was estimated as higher than ± 0.1‰ [12]. Four replicates per sample were analyzed from each agro ecological zone.

Leaf anatomy

Leaf tissues for all microscopic observations were obtained from the middle of the leaf lamina at a point equidistant between the midrib and the leaf margin. The leaf anatomy was observed using leaf transverse sections under the light microscope [8]. Light micrographs were used to determine the mesophyll to bundle sheath cell area ratio (M:BS) in a field of 0.25 cm² tissue area, and vein density measurement was done manually by counting the veins of each leaf under a microscope [13]. All the micrographs were processed by Image J software.

Statistical analysis

Statistical comparisons of the data obtained were conducted with MINITAB 14 version using one-way analysis of variance (ANOVA, p<0.05) followed by Tukey’s pair wise multiple comparison test. Pearson correlation test was employed to identify the correlations between the various nutrient elements.

Results and Discussion

As the element analysis in biological samples requires accurate determinations at their concentrations, the quality of the results (precision and accuracy) was confirmed by analyzing a certified reference material (BCR No. 281-rye grass) provided by simultaneously and the analytical results are compiled in Table 2. No certified values are available for N, P, K, Ca, Mg and Fe that are biologically significant. BCR No. 281 was selected in this study as it is floristically similar to Panicum maximum matrix to ensure the same sources of error caused by matrix effects. Copper, Mn and Zn of the BCR No. 281 are consistent with the measured values (Table 2).

Table 2: Analytical values of constituent elements for the rye grass certified reference material (BCR No. 281)*.

The highest leaf nitrogen (N) content was detected from the specimens of mid country-wet zone and the lowest from the low country-dry zone as shown in Table 3. Phosphorus (P) and potassium (K) contents were highest in the up country-wet zone samples, whereas the lowest in the mid country-wet zone and low countryintermediate zone respectively. The low country-wet zone samples were characterized with the highest calcium contents and the lowest content was found in the up country-wet zone. Magnesium (Mg) was highest in the specimens of mid country-intermediate zone and lowest in the specimens of up country-intermediate zone. Regarding micro elements, highest copper (Cu) content was detected from the specimens of mid country-intermediate zone and lowest was from the specimens of up country-wet zone. Highest iron (Fe), manganese (Mn) and zinc (Zn) contents were found in the specimens of low countrywet zone and the lowest contents were found in the specimens of low country-dry zone, up country-intermediate zone and mid countrywet zone respectively.

Table 3: Nutrient composition of P. maximum L. from different agro ecological zones in Sri Lanka*.

Except for few elements, a majority of the nutrient contents were higher in the samples from low country-wet zone than the other climatic zones, probably due to the higher annual rain fall in the wet zone resulting in a higher availability of most nutrients in the soil solution [14]. Even though the soil in dry zone is highly fertile, scarcity of water due to the low rain fall retard the nutrient absorption of plants [14]. Therefore the nutrient levels of the Panicum leaf samples from low country-dry zone were lower than those from the other climatic zones. It is known that the soils at higher elevations are poor in terms of fertility due to the landslides and soil erosion, fertility of the up country lands is much lower [14], resulting in less nutrient availability to plants [15].

Correlation analysis assuming linear relationships between elements was used to examine where there were similar patterns of elemental concentrations in the samples of P. maximum under different climatic conditions. The correlation matrix (Table 4) consists of element-by-element comparisons and shows a number of significant relationships. The synergistic accumulation effect of calcium-nitrogen and zinc-iron effect in the P. maximum leaves appear particularly remarkable. Apart from that, the competitive behavior in the absorption of phosphorus and nitrogen can be well recognized. The negative relationships between phosphorus, potassium and magnesium on the carbon, and calcium accumulation on phosphorus in the leaves are recognizable. Similar relationships can also be observed between potassium (K-Mn) and copper (Cu-Mn). Due to lack of sufficient information on the inte -element relationships of the essential elements in plant nutrients, a clear cut statement could not be made at this stage. Further detailed studies are necessary to verify the above significant inter-element relationships in P. maximum in terms of its chemical characterization. The results obtained for leaf δ¹³C values of P. maximum leaves originated from different agro ecological zones of Sri Lanka are summarized in Table 5. Irrespective of the site elevation, all samples have carbon isotope ratios (δ¹³C) typical of C4 mode of photosynthesis [7]. P. maximum samples collected from all seven agro ecological zones are functionally C4, as indicated by a C4 isotope ratio, the typical Kranz anatomy and the high C4-like vein density.

Table 4: Corelation matrix of elemental concentrations in leaves of P. maximum. L.

Table 5: Mean (±S.E) values of d¹³C values, mesophyll (M) to bundle sheath (BS) cell area ratios and the vein densities of P. maximum collected from different agro ecological zones of Sri Lanka.

Increased vein density was observed in the leaves of plants growing in arid environments. The typical C4 anatomy was observed in all Panicum leaf samples with high vein density associated with C4 photosynthesis. These results on vein density are consistent with the widely accepted hypothesis that the increased vein density is associated with C4 photosynthesis. Increased vein density enhances water use efficiency of plants growing in arid environments. It was interesting to note that with the increasing elevation, vein density was tended to increase. With respect to the M: BS ratios, no significant differences were observed between the different climatic zones. However, Panicum leaves from the up country-wet zone and low country-dry zone possessed relatively higher M: BS ratios. In leaf cross sections, BS cells occupied less than 10% of the leaf area, whereas the mesophyll cell area was about 70%. The M: BS ratios remained more or less similar in all P. maximum samples typical of a C4 species [6]. Increased BS size and high vein density have also been observed [6] in the C3 species of Panicum laxum and Panicum hylaeicum. They have reported a shift towards the C3-C4 intermediate condition in Panicum schenkii and Panicum milioides. According to a hypothesis put forward, high vein density may increase water use efficiency in arid environments.

In conclusion, all P. maximum samples collected from different climatic zones of Sri Lanka exhibited typical C4 Kranz anatomy where the vascular bundles are surrounded by chlorenchymatous bundle sheath cells. This is further supported by the anatomical C4 traits studied, i.e. high vein density and low M: BS area ratio of Panicum leaves. δ¹³C, a powerful diagnostic tool for C4 photosynthesis confirmed that P. maximum growing in Sri Lanka possesses the C4 mode of carbon assimilation. Within species variations of δ¹³C among the different climatic zones were minor of the order of 2.1‰ for all the Panicum samples, although carbon isotope discrimination is known to negatively correlate with the elevation [19]. Characterization of enzyme activities of the C4 cycle would provide more information as to whether P. maximum from Sri Lanka could be used as another model for C4 evolution.

Acknowledgement

Supports extended by the non-academic staff of the Department of Botany, University of Kelaniya are acknowledged.

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