Chemical composition of two semi-aquatic plants for food use

Transcription

1 Plant Pbods for Human Nutrition 44:11-16, t993 Kluwer Academic Publishers. Printed in the Netherlands. Chemical composition of two semi-aquatic plants for food use A. DEWANJI, S. MATAI, L. SI, S. BARIK & A. NAG Biological Sciences Division, Indian Statistical Institute, 203 Barrackpore Trunk Road, Calcutta , India Received 10 October 1991; accepted 2 March 1992 Key words: Chemical composition, Enhydrafluctuans, Marsilea quadrifolia, Seasonal variation Abstract, The seasonal variation in the nutrient composition of Enhydrafluctuans and Marsilea quadrifolia, two edible semi-aquatic plants, was studied in order to promote their consumption as green leafy vegetables. Both plants had a high crude protein content throughout all harvesting seasons. Enhydrafluetuans had a low ash content and was a good source of/?-carotene (3.7 to 4.2mg/100g on a fresh weight basis). Marsilea quadr~lia exhibited wide fluctuations between seasons and was not very promising in nutrient composition when compared to other commonly used green leafy vegetables. Introduction Large growths of aquatic plants in lakes and waterways of tropical countries are a menace but they also represent a natural resource of green leaves [1]. In view of the world wide need for additional sources of food, the exploitation of these plants which are otherwise of no economic importance would not only be a step towards better resource utilization but would also help to solve the weed eradication problem. Not all water plants are fit for human consumption but many have reported food uses. Amongst the semi-aquatic plants, Enhydra fluctuans is known to be consumed as a vegetable or salad in many parts of India [2] and has reported medicinal values [3]. The stalk and leaves of another emergent weed Marsilea quadrifolia is also reported to be eaten as a pot herb [4]. No detailed information is available on the nutritional composition of these two plants. This study was undertaken to explore the nutritive potential of the two semi-aquatic plants. Since aquatic weeds are known to differ widely in their chemical composition depending upon species, season and location [5], the monthly variation in nutrient composition of plants growing at the same site was also studied.

2 12 Materials and methods Description of plants. Enhydrafluctuans Lour of the family Compositae is the only species of this genus recorded in India. It is a prostrate herb with opposite, sessile, linear-oblong leaves and is commonly known as the 'water cress' or 'marsh herb' [3]. Marsilea quadrifolia is a genus of aquatic or subaquatic ferns from the family Marsileaceae and is widely distributed in tropical and temperate regions. This herb has a creeping rhizome, quadrifoliate leaves and bean shaped sporocarps. It is found throughout India usually at the edges of ponds, irrigation canals and as a weed in wet fields [4]. Method of collection. For each species, sites were marked where the plants were found to grow luxuriently. Harvests were conducted at monthly intervals from the same site. As per availability, samples of Enhydra fluctuans were harvested from February to July 1989 while Marsilea quadrifolia samples were available from March to September During each harvest, approximately one kg of plant species was collected from four accessible parts of the waterbody to form representative samples. Chemical analysis. The plants were thoroughly washed and dried at 80 C for 48 hours. The dried samples were ground to a fine powder and stored for analysis. At each harvest, a part of the fresh sample was used directly for flcarotene estimations. Nitrogen (N), ash and fl-carotene were determined by standard methods [6]. Crude protein was calculated as N x Analytical determination of sodium (Na), potassium (K), and calcium (Ca) was made through flame emission using a Systronics 121 flame photometer. Inorganic phosphorus was determined following the method of Fiske & Subbarow [7]. Stat&tical analys&. To test seasonal differences in nutrient composition, an analysis of variance (ANOVA) was applied to the data set [8]. When significance was observed at the 5% level, the least significant difference (LSD) for the same significance level was determined. Results and discussion Seasonal changes in the dry matter, crude protein, ash and /~-carotene content of Enhydra fluctuans are given in Table 1 while those of Marsilea quadrifolia are given in Table 2. Beginning from the first season, an increase in dry matter content was observed for both plants which reached its peak

3 13 Table 1. Seasonal variation in the chemical composition of Enhydrafluctuans a Month of Dry matter Crude protein* Ash* fl-carotene harvest % % % mg/100g February March April May June July LSD a The results are mean values of four replicates. * dry weight basis. Table 2. Seasonal variation in the chemical composition of Marsilea quadrifolia a Month of Dry matter Crude protein* Ash* fl-carotene harvest % % % rag/100g March April ,6 May , ,7 June ,1 July ,4 August ,3 September LSD The results are mean values of four replicates. * dry weight basis. in the month of May. Both plants seemed to have attained maximum maturity during the summer season. Significant differences between seasons was found in the dry matter content of Marsilea quadrifolia for all harvests excluding the month of September. Wide variation between seasons was observed in the crude protein content of both plants which could be related to seasonal differences in their growth rates. Enhydrafluctuans exhibited values ranging from 25.3% in February to 32.6% in April while Marsilea quadrifolia had a minimum protein content of 24.5% in June and a maximum of 36.2% in September. In a study on green leafy vegetables, Gupta & Wagle [9] reported crude protein values ranging from 21.6% in spinach to 29.8% in mustard leaves. Crude protein is the component that is reported to exhibit most drastic variation both between sites and with the stage of maturity [10].

4 14 Enhydrafluctuans had higher ash values (13.7% to 18.3 %) than Marsilea quadrifolia (7.5% to 15.8%). Except for the month of February in Enhydra fluetuans, both plants had values below 16%. Over the seven-month harvesting period, Marsilea quadrifolia showed an alternate increasing and decreasing trend. In a study on green leafy vegetables, ash values ranged from 12.5% in cauliflower greens to 26.6% in spinach [9]. fl-carotene, serving as precursors of Vitamin A, ranged from 3.1 mg/100 g to 4.2rag/100 g in Enhydra fluctuans while Marsilea quadrifolia had lower values ranging from 2.1 rag/100 g to 2.8 rag/100 g in March. Vitamin A is of special importance as more than five million children in Asia are affected by Vitamin A deficiency every year and many of them become blind [11]. In a study by Lala & Reddy [12], a daily intake of 40 g of green leaves even for a short period was found to improve the serum Vitamin A levels of preschool children. The increased consumption of green leaves should, therefore, be encouraged as a potential source of fl-carotene. An increased intake of green and yellow vegetables was also found to lower cancer deaths in an elderly population [13]. The seasonal changes in the mineral composition of Enhydrafluetuans and Marsilea quadrifolia are reported in Tables 3 and 4 respectively. The sodium content of Enhydrafluctuans ranged from a minimum of 1.6% in June to a maximum of 2.6% in February. Marsilea quadr~folia exhibited lower values for all months. The sodium contents of both plants were found to be relatively high when compared to reported values for terrestrial plants [14]. Enhydra fluetuans showed an increasing trend in potassium content from March to July. Maximum variation among seasons was observed in Marsilea quadrifolia which had potassium contents ranging from 1.1% to 2.8%. In both plants, calcium levels for all harvests were below 1% which is low when compared to that of other green leafy vegetables [9]. These differences could be attributed to analytical procedures used, the age and kind of plant part sampled as well as the mineral availability in the environment [15]. Seasonal variations in the mineral content of other emergent aquatic plants have been reported [10]. The phosphorus content of Enhydra fluctuans (ranging from 0.5% to 0.7%) and that of Marsilea quadrifoh:a (ranging from 0.1% to 0.8%) were comparable to reported values of other commonly used green leafy vegetables [16]. Results of chemical analysis suggest no particular trend to variations common to the chemical composition of both plants with progress in time of harvesting. This was probably because the date of harvesting did not necessarily represent increasing age of the plant. Moreover, all plants studied came from a natural stand and were undoubtedly subject to variations in nutrient availability.

5 1"able 3. Seasonal variation in the mineral composition of Enhydra fluctuans (dry weight basis) a Month of Sodium Potassium Calcium Phosphorus harvest % % % % February March April May June July LSD a The results are mean values of four replicates. 15 Table 4. Seasonal variation in the mineral composition of Marsilea quadrifolia (dry weight basis) a Month of Sodium Potassium Calcium Phosphorus harvest % % % % March April May June July August September LSD a The results are mean values of four replicates. Conclusions Marsilea quadrifolia had a high protein content but low mineral and //-carotene values. This plant also exhibited wide fluctuations between harvest. Enhydrafluctuans, on the other hand, had a high protein content, low ash values and was a good source of//-carotene which makes it promising for use as a leafy vegetable. The widespread consumption of this plant in places where it is found to grow should be encouraged. As burgeoning populations force utilization of marginal agricultural areas, edible aquatic plants should be used more intensively especially as a cheap source of nutritious greens which would enrich the diets of the local populace.

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