Abstract
Calcium-rich vegetables in diet could minimize calcium deficiency and maximize good health and well-being. The aim of the present study was to determine the effect of different levels of foliar application of CaCl2 on lettuce (Lactuca sativa L.) growth and calcium concentrations with the application of organic and conventional fertilizers. Pot experiments were conducted with three calcium levels (60, 120, and 180 mg·L−1 of CaCl2) of an organic fertilizer (3N–0.8P–3.5K) and commercial conventional fertilizer (15N–15P–15K). Calcium in whole oven-ashed samples of shoots was analyzed by atomic absorption spectrometry. Nine lettuce cultivars including butterhead, romaine, and loose-leaf phenotype were tested. These results revealed that the calcium concentration in lettuce significantly increased as calcium levels increased from 60 to 180 mg·L−1. Elevated calcium concentrations in organic and conventional fertilizers increased the concentration of calcium in lettuce from 1.82% at 60 mg·L−1 to a mean of 2.15% at 120 and 180 mg·L−1. The concentration of calcium in the loose-leaf phenotype was 2.17%, 2.47%, and 3.80% higher than that in the butterhead and romaine phenotypes at 60, 120, and 180 mg Ca/L, respectively. Furthermore, the significant difference in calcium concentration among cultivars ranged from 1.27% to 3.05%. ‘Perilla Green’, ‘Breen lettuce’, and ‘Salinas’ had the highest calcium concentrations followed by ‘Jericho lettuce’, ‘Salad Bowl’ and ‘Crisp’, and ‘Kaiser’, whereas ‘Valmaine’ and ‘Rosa Green’ had the lowest calcium concentrations. The present study revealed that selecting fertilizers and cultivars with high calcium concentration can increase the total calcium content of lettuce.
Mineral nutrients required in the human body are mainly obtained through the consumption of vegetables. Calcium is an essential and major nutrient in vegetable. Adequate calcium intake is important for constructing and maintaining bones, the clotting of blood, and the function of hormones and enzymes (Department of Health & Human Services, 2000; Ervin et al., 2004). For most men and women, the recommended daily intake of calcium is 1000 mg·d−1 (Meacham et al., 2008). However, the required calcium intake should total 1300–1700 mg·d−1 for the elderly (Heaney, 2001). A study suggested that it is possible to improve dietary calcium intake by increasing the consumption of calcium-rich vegetables (Bernstein et al., 2002). Calcium-rich vegetables in diet could ameliorate the potential for calcium deficiency in human nutrition. Fresh leafy vegetables such as celery, collard, chinese cabbage, and soybean sprouts contain high levels of calcium, which are major sources of minerals (Kamchan et al., 2004). Increasing the calcium content in leafy vegetables could further improve its nutritional benefits to consumers, considering that calcium is the mineral nutrient most commonly deficient in modern diets (Grusak, 2002).
Lettuce (L. sativa L.) is an annual plant of the family Asteraceae. It is an important leafy vegetable that has high market value, it is grown around the world and is used particularly as the base for salads. It can be easily cultivated because it has a short vegetation period. Lettuce is a good source of vitamin A and potassium, as well as a minor source of several other vitamins and nutrients (Niari et al., 2012), and its nutritional characteristics have been studied throughout the world (Ashkar and Ries, 1970; Keat et al., 1999). Calcium has been considered an important nutrient in lettuce. The calcium content of lettuce is affected by calcium uptake. This is associated with the water uptake of its roots and in turn affects the calcium distribution in the shoots (Collier and Tibbitts, 1984; Schlagnhaufer et al., 1987).
The aim of the present study was to determine the effect of the foliar application of CaCl2 on lettuce growth and calcium concentrations with organic and conventional fertilizer applications and determine whether the calcium content could be increased through nutritional regimes.
Materials and Methods
Experimental site and plant materials.
Nine lettuce cultivars, including butterhead (‘Salinas’, ‘Crsip’, and ‘Kasier’), romaine (‘Breen lettuce’, ‘Jericho lettuce’, and‘Valmaine’) and loose-leaf (‘Perilla Green’, ‘Salad Bowl’, and ‘Rosa Green’) phenotypes, were tested. All seeds were obtained from Hubei Eshu Agricultural Technology Co. Ltd. (Wuhan, China). Peregrinated seeds were sown in seedling trays to produce uniform seedlings. Seedlings at the three leaf stage were manually transplanted on 4 Feb. 2016. These seedlings were transplanted on a hill spaced at 20 × 20 cm.
Experimental design and management.
The experiments were conducted using three calcium levels (60, 120, and 180 mg·L−1 of CaCl2) and two fertilizer forms, viz. organic fertilizer (3N–0.8P–3.5K, with micronutrients reported; Wuhan Green Chemical Co., Ltd., Wuhan, China) and commercial conventional fertilizer (15N–15P–15K, with micronutrients reported; Wuhan Green Chemical Co., Ltd.). These fertilizers were manually broadcast during basal application. These experiments were laid out in a randomized complete block design with four replications for each treatment. The foliar application of CaCl2 fertilizers was performed in three concentrations, and plants were sprayed three times every 20 d, and watered once every 3 d at 5:00 pm. Standard cultural management practices were followed to achieve maximum yield.
Samplings and measurements.
After harvesting the plants, the fresh weights were recorded. Then, the sampled plants were washed with tap water, oven-dried to a constant weight at 70 °C, and the oven dry weight was recorded. Afterward, calcium concentration and total calcium content were analyzed by atomic absorption spectrometry (Meagy et al., 2013).
Statistical analysis.
The statistical analysis included analysis of variance, and Statgraphics version 3.1 for Windows was used. The means were separated by Duncan’s new multiple range test for the main effects of treatments and by least significant difference for the interactions of treatments (Steel and Torrie, 1980).
Results
Head weights.
The fresh and dry weights of heads varied with cultivars, as shown in Table 1. The mean fresh weight and mean dry weight of the three largest headed cultivars was 22.80% and 33.20% larger than the three smallest headed cultivars, respectively. Furthermore, among cultivars, ‘Perilla Green’, ‘Breen lettuce’, and ‘Salinas’ were classified with the highest dry weights, whereas ‘Kasier’, ‘Valmaine’, and ‘Rosa Green’ were classified with the lowest dry weights.
The fresh and dry weights of heads of the cultivars and phenotypes of lettuce.
There was a significant difference in fresh weight and dry weight between the romaine and loose-leaf phenotypes (Table 2). The romaine phenotype had the highest fresh weight (158 g/head), followed by butterhead (156 g) and loose leaf (154 g). The dry weights in these phenotypes significantly differed in the same order as the fresh weights. The organic and conventional regimes of fertilization had no significant effect on head weight and calcium concentration (Table 2). The fresh and dry weights were slightly higher at 120 mg Ca/L than at 60 or 180 mg Ca/L (Table 2). Calcium concentration in the loose-leaf phenotype was 2.17%, 2.470%, and 3.80% higher than that in butterhead and romaine phenotypes at 60, 120, and 180 mg Ca/L, respectively. Calcium concentration and total calcium content was the highest at 120 mg Ca/L followed by 180 mg Ca/L and 60 mg Ca/L. The elevated calcium levels in organic and commercial conventional fertilizers increased the calcium concentration in lettuce from 1.82% at 60 mg·L−1 to a mean of 2.15% at 120 and 180 mg·L−1.
The fresh weight, dry weight, calcium concentration, and total calcium content of the three phenotypes with organic and conventional fertilizers and calcium levels.
The type of fertilizer and the interaction between fertilizer and phenotype had no effect on the head weights (Table 3). Furthermore, there was no significant difference in the interaction of phenotypes and fertilizers in terms of the head fresh weights of lettuce. However, the 60 or 120 mg Ca/L concentration in conventional fertilizer resulted in high fresh or dry weights compared with 180 mg Ca/L (Tables 4 and 5). With the organic regime, the 180 mg-Ca/L concentration suppressed the growth relative to the low supply of calcium.
The interaction of phenotypes and fertilizers in terms of the head fresh weights of lettuce.
The interaction between fertilizers and calcium concentration in terms of fresh weight of lettuce are listed in descending order according to the mean weights of cultivars.
Interactions between calcium concentrations and fertilizers in terms of the mean dry weights of cultivars.
Calcium concentration.
As presented in Table 6, significant differences in calcium concentration occurred among individual cultivars, which ranged from 1.22% to 2.52% with organic fertilizers and from 1.47% to 3.53% with conventional fertilizer. A significant difference was found in the interaction of fertilizers by variety, but the ranking of cultivars did not differ with regard to these fertilizers. Total calcium content varied widely among cultivars, ranging from 0.17 to 0.13 g/head with organic fertilizers and from 0.16 to 0.19 g/head with conventional fertilizers (Table 7). Furthermore, there was no significant difference in the interaction of fertilizers by variety. However, there was a significant difference in the interaction of cultivars with calcium concentration, in which cultivars exhibited different trends in total calcium content when calcium concentration was increased (Table 8).
The interaction between cultivars and fertilizers in terms of the calcium concentration of lettuce.
The interaction between cultivars and fertilizers in terms of total calcium content in lettuce cultivars.
The interaction between cultivars and calcium concentrations in terms of calcium concentrations in lettuce.
Total calcium content.
The interaction of cultivars and calcium concentration with calcium accumulation in lettuce is illustrated in Table 9. Among these cultivars, total calcium content varied from 0.13 to 0.28 g/head. Cultivars with notably high calcium content were ‘Perilla Green’, ‘Breen lettuce’, and ‘Salinas’, whereas cultivars with notably low calcium content were ‘Kasier’, ‘Valmaine’, and ‘Rosa Green’. Hence, there is a significant interaction between cultivars and calcium concentration. This shows that the response of cultivars increase according to the increase in calcium concentration. That is, total calcium content reached a peak at 120 mg Ca/L when the calcium concentration increased from 60 to 180 mg Ca/L (Table 9).
The interaction between cultivars and calcium concentrations in terms of total calcium content in lettuce listed in descending order.
Discussion
Because of genetic improvements in plant head size, the romaine phenotype had larger fresh and dry weights than the butterhead and loose-leaf phenotypes. In the present study, results revealed that the calcium concentration did not differ in overall plant head size, although the total calcium content (g/head) varied. That is, total calcium content increased as plant dry weight increased. This conclusion is consistent with the study conducted by Meagy et al. (2013). Furthermore, there was no significant difference in plant head weight between the organic and chemical nutritional regimes. Moreover, plant head size significantly increased as calcium concentration increased from 60 to 180 mg Ca/L in the chemical regime. However, little effect was observed in the organic regime. ‘Perilla Green’ and ‘Breen’ lettuce had larger fresh weights compared with other cultivars, and may be attributed to the difference in transpiration and the resulting nutrient content (Mou, 2009). Loose-leaf lettuce cultivars contained higher calcium concentration and total calcium content compared with the butterhead or romaine lettuce cultivars. It is possible that loose-leaf phenotypes transpired more water than the romaine and butterhead phenotypes because of morphology, thereby increasing nutrient uptake transport and exhibiting higher growth.
There were significant differences in nutritional value among cultivar phenotypes. Compared with butterhead and romaine cultivars, loose-leaf cultivars had higher calcium concentration and total calcium content. According to Collier and Tibbitts (1982, 1984), this transpiration affects the delivery and distribution of calcium to lettuce leaves because young leaves with developing heads can develop tipburn. There reason for this is that older leaves transpire more than young leaves, thereby depriving young leaves of calcium. Loose-leaf cultivars are less sensitive to tipburn than romaine or butterhead cultivars, showing the different magnitudes of calcium delivery to young leaves by transpiration (Hylmö, 1953).
Significant variations in calcium content were observed among cultivars. ‘Perilla Green’, ‘Breen lettuce’, and ‘Salinas’ had the highest calcium concentration. The possible reason is that cultivars allow the transpiration of water into the plant head, contributing to the increase in calcium concentration. By contrast, among romaine cultivars, Jericho lettuce, Salad bowl lettuce, and Crisp lettuce accumulate moderate levels of calcium concentration. This may be because of the thick and semi-open leaves of romaine plant heads, which obstruct the transpiration of water, thereby leading to lower calcium concentration, when compared with loose-leaf and butterhead cultivars (Collier and Tibbitts, 1984). Furthermore, ‘Kasier’, ‘Valmaine’, and ‘Rosa Green’ had lower calcium concentrations compared with other cultivars. Moreover, ‘Kasier’, ‘Valmaine’, and ‘Rosa Green’ have partially closed heads, whereas others have open heads. It is possible that these closed heads obstruct the transpiration of water, resulting in lower calcium concentration in the leaves (Barta and Tibbitts, 1991).
Conclusions
Fresh and dry weight was higher in the romaine phenotype than in the butterhead and loose-leaf phenotypes. Furthermore, loose-leaf cultivars had higher calcium concentrations and total calcium content when compared with butterhead or romaine cultivars. Furthermore, total calcium content was higher when commercial conventional fertilizers than when organic fertilizers. These cultivars differed widely in total calcium content. ‘Perilla Green’, ‘Breen lettuce’, ‘Salinas’, and ‘Perilla Green’ had the highest calcium concentrations among all cultivars. A wide range of variability in calcium concentration occurred among the different cultivars of lettuce, including the different phenotypes and introductions. Therefore, breeding and selection can potentially improve nutrient density in lettuce. Enhancing the mineral nutrition levels of lettuce can improve nutrient uptake without the need to increase produce consumption.
Literature Cited
Ashkar, S.A. & Ries, S.K. 1970 Lettuce tipburnas related to nutrient imbalance and nitrogen composition. Michigan State University, East Lansing, MI
Barta, D.J. & Tibbitts, T.W. 1991 Calcium localization in lettuce leaves with and without tipburn: Comparison of controlled-environment and field-grown plants J. Amer. Soc. Hort. Sci. 116 870 875
Bernstein, M.A., Nelson, M.E., Tucker, K.L., Layne, J., Johnson, E., Nuernberger, A., Castaneda, C., Judge, J.O., Buchner, D. & Singh, M.F. 2002 A home-based nutrition intervention to increase consumption of fruits, vegetables, and calcium rich foods in community dwelling elders J. Amer. Diet. Assn. 102 1421 1427
Collier, G.F. & Tibbitts, T.W. 1982 Tipburnoflettuce Hort. Rev. 4 49 65
Collier, G.F. & Tibbitts, T.W. 1984 Effects of relative humidity and root temperature on calcium concentration and tipburn development in lettuce J. Amer. Soc. Hort. Sci. 109 128
Department of Health & Human Services 2000 Healthy People 2010: Objectives for improving health (Part B: Focus areas 15–28). Vol. II. U.S. Government Printing Office, Washington, DC
Ervin, R.B., Wang, C.Y., Wright, J.D. & Kennedy Stephenson, J. 2004 Dietary intake of selected minerals for the United States population: 1999–2000. Advance data from vital and health statistics. Vol. 341. National Center for Health Statistics, Hyattsville, MD
Grusak, M.A. 2002 Enhancing mineral content in plant food products J. Amer. College Nutr. 21 178 183
Heaney, R.P. 2001 Calcium needs of the elderly toreduce fracture risk J. Amer. College Nutr. 20 192 197
Hylmö, B. 1953 Transpiration and ion absorption Physiol. Plant. 6 333 405
Kamchan, A., Puwastien, P., Sirichakwal, P.P. & Kongkachuichai, R. 2004 In vitro calcium bioavailability of vegetables, legumes and seeds J. Food Compos. Anal. 17 311 320
Keat, C., Meng-Wei, L. & Ling, C. 1999 Effects of nutrient composition on butterhead lettuce (Lactuca sativa L. cv. Panama) grown in deep flow technique in the tropics. ISHS Seed Symposium 504: VI Symposium on Stand Establishment. p. 135–146
Meacham, S., Grayscott, D., Chen, J.J. & Bergman, C. 2008 Review of the dietary reference intake for calcium: Where do we go from here? Crit. Rev. Food Sci. Nutr. 48 378 384
Meagy, M.J., Eaton, T.E. & Barker, A.V. 2013 Nutrient density in lettuce cultivars grown with organic or conventional fertilization with elevated calcium concentrations HortScience 48 1502 1507
Mou, B. 2009 Nutrient content of lettuce and its improvement Curr. Nutr. Food Sci. 5 242 248
Niari, S.M., Bahri, M.H. & Rashidi, M. 2012 Chemical materials application and storage periods effects on vitamin C of ambient stored lettuce Amer.-Eurasian J. Agr. Environ. Sci. 12 8 1081 1084
Schlagnhaufer, B.E., Holcomb, E.J. & Orzolek, M.D. 1987 Effects of supplementary light, solution heating, and increased solution calcium levels on lettuce production in the nutrient film technique Appl. Agr. Res. 2 124 129
Steel, R.G.D. & Torrie, J.H. 1980 Principle sand procedures of statistics: A biometrical approach. 2nd ed. McGraw-Hill, New York, NY