Foliar Fertilization of Muskmelon: Effects of Potassium Source on Market Quality and Phytochemical Content of Field-Grown Fruit 1 TX 78596

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1 Foliar Fertilization of Muskmelon: Effects of Potassium Source on Market Quality and Phytochemical Content of Field-Grown Fruit 1 John L. Jifon 2 and Gene E. Lester 3 2 Texas A&M University, Texas Agricultural Expt Station, 2415 E. Business Hwy 83 Weslaco, TX USDA-ARS, Kika de la Garza Subtropical Agricultural Research Center, 2413 East Highway 83, Building 200, Weslaco, Texas jljifon@agprg.tamu.edu ABSTRACT Muskmelon [Cucumis melo L. (Reticulatus Group)] fruit quality attributes such as sweetness, aroma and texture are directly related to potassium (K)-mediated processes. However, during fruit growth and maturation, soil K supply alone is often inadequate to satisfy K requirements. Previous studies have demonstrated that supplemental foliar K applications can overcome this apparent deficiency however, the suitability of potential K salts as foliar sources is still uncertain. In this study, we evaluated the effects of six foliar K sources (potassium chloride - KCl, potassium nitrate - KNO 3, monopotassium phosphate MKP, potassium sulfate - K 2 SO 4, potassium thiosulfate - KTS, and a glycine amino acid-complexed K- Potassium Metalosate, KM) on fruit quality parameters of field-grown muskmelon Cruiser. Experiments were conducted during the spring of 2005 and 2006 at 2 locations (Weslaco and Santa Ana) in south Texas. Weekly foliar K applications were established starting at fruit set and continuing to fruit maturity. Leaf, stem, petiole and fruit tissues from plots receiving supplemental foliar K treatments generally had higher K concentrations than those from control plots but this effect was not consistent among the K sources and between locations. Even though pre-plant soil K concentrations were very high, supplemental foliar K treatments resulted in generally higher K concentrations in plant tissues, suggesting that plant K uptake from the soil solution was not sufficient to saturate tissue K accumulation. Fruit from plots receiving supplemental foliar K generally had higher soluble solids concentrations (SSC), sugars, and the human wellness compounds - ascorbic acid and β-carotene than control fruit. However, this trend was more evident only at Weslaco and there were generally no consistent trends among K sources except for KNO 3 which tended to result in less firm fruit. Fruit yields were not affected by supplemental foliar K applications. These results generally support our previous controlled environment findings that supplementing soil K supply with foliar K applications during fruit development and maturation can improve muskmelon fruit quality by increasing firmness, sugars, ascorbic acid and β-carotene contents. The current data also provide additional evidence Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the authors and/or their affiliations. 1 This material is based upon work supported in part by the Cooperative State Research, Education, and Extension Service, U.S.D.A. under Agreement No , Designing Foods for Health through the Vegetable & Fruit Improvement Center and by the Potash & Phosphate Institute- Foundation for Agronomic Research, Fluid Fertilizer Foundation, Tessenderlo Kerley, Inc, and Rotem BKG LLC. 1

2 of differences among potential foliar K sources, with KNO 3 consistently emerging as a less desirable source of K during fruit development and maturation. INTRODUCTION Potassium (K) is an essential plant mineral nutrient involved in numerous physiological processes vital to plant growth and productivity (Marschner, 1995; Wyn Jones et al., 1979). Fruiting crops such as tomatoes, melons and watermelon have high potassium requirements to maintain optimal leaf function and also to facilitate sugar transport and storage. Hence, adequate plant K content is correlated with sweetness, an important trait for consumer preference of such fruit (Lester, 2005). Almost all K found in plant tissues is taken up by roots from the soil solution in its ionic form (K + ). Even though, K is abundant in most soils in Texas (and western USA), the bulk of soil K is unavailable to plants. Soil K supply and plant uptake are regulated by plant and environmental factors (Mengel and Kirkby, 1980). For instance, in many species, K uptake occurs mainly during the vegetative stages of plant growth when root growth is not inhibited by carbohydrate supply (Fisher et al., 1970). During reproductive development, competition for photoassimilates between developing fruits and vegetative organs may limit root growth and nutrient uptake, including K (Ho, 1988). Therefore, soil derived K, which is essential for sugar transport and unloading into fruit during fruit growth and development is not always optimal during the critical fruit development period, and this is partly responsible for poor fruit quality, and yield. Our previous controlled environment studies (Lester et al., 2005, 2006) have demonstrated that supplementing soil K supply with foliar K applications during fruit development and maturation can improve muskmelon fruit quality parameters such as fruit firmness, sugar content, ascorbic acid and beta-carotene levels. Lin et al., (2004) found that increasing the K supply from 120 to 240 mg L -1 in a hydroponics system resulted in higher muskmelon fruit sugar concentrations. For field-grown plants, increasing soil fertilizer inputs may not be enough to alleviate the developmentally-induced K deficiency partly because of reduced root growth during reproductive development and also because of competition for binding sites on roots from cations such as NH 4 + (Clarkson, 1989; Engels and Marschner, 1993). These K-induced benefits can also be affected by several factors including source of K, dosage, plant development stage, weather conditions and soil physical and chemical characteristics. Data of Lester et al. (2005) suggested that the relative fruit quality enhancement of supplemental foliar K application was greater when an organic form of K (Metalosate-K) was used compared to an inorganic (potassium chloride, KCl) source. Potassium chloride is the most widely used source of fertilizer K, however, its relatively high salt index (~120, Mortvedt, 2001) and its high point of deliquescence (POD, 86%, Schönherr and Luber, 2001) can limit its use for foliar nutrition. A low POD reduces the risk of crystallization and residue formation following foliar sprays. The objective of this field study was to evaluate the effects of mid-to-late season supplemental foliar K fertilization on muskmelon fruit quality parameters - fruit size, weight, firmness, sugars, and concentration of phytochemicals ascorbic acid, beta-carotene. Six K sources and a control were evaluated in field studies conducted over a two year period at two locations. MATERIALS AND METHODS This research was conducted during the spring growing seasons of 2005 and 2006 in commercial fields in the Lower Rio Grande Valley region of Texas, an area with a semiarid climate 2

3 averaging approximately 558 mm of rainfall. Netted, orange-flesh muskmelon ( Cruiser ) was planted in early February of each study year in two locations: Santa Ana (longitude 26 " 5' N, latitude 98 " 8' W; soil type: Harlingen clay), and Weslaco (longitude 26 " 9' N, latitude 97 " 57' W; soil type: Hidalgo sandy clay loam soil). The Weslaco site was on the Texas A&M University Research & Extension Center, and the Santa Ana site was in a commercial grower s field. Pre-plant soil tests indicated adequate soil K levels ( ppm); consequently, no soil K was added. Standard commercial practices for spring muskmelon production including irrigation, nutrient management, and pest control were followed at each site. In each experiment, 20 ft-long plots were established in a completely randomized design and replicated four times. Starting at fruit set, and continuing till fruit maturation, the following weekly foliar K treatments were established: (1) control (no K, de-ionized water), (2) potassium chloride (KCl) (3) potassium nitrate (KNO 3 ), (4) Monopotassium phosphate (MKP Mora-Leaf P&K 30% K 2 O, Rotem BKG LLC, Ft Lee, NJ) (5) potassium sulfate (K 2 SO 4 ), (6) potassium thiosulfate (KTS, 25% K 2 O, Tessenderlo Kerley Inc., Phoenix, AZ) and (7) a Glycine amino acid-complexed K (Potassium Metalosate, KM, 24% K 2 O; Albion Laboratories, Inc, Clearfield, Utah). A non-ionic surfactant (Silwet L-77; Helena Chem. Co., Collierville, TN) was added to all spray treatments at 0.3% (v/v). Proprietary fertilizer K sources were formulated according to manufacturer recommendations and calibrated to supply ~4 lbs K 2 O per acre during each foliar application. All treatments were applied between 0500 and 0800 HR on each spray event. A total of 5 foliar K applications (~20 lbs K 2 O/ac) per treatment were made at each location. At maturity, ten uniform fruit were harvested from each plot and analyzed for fruit sugars, fruit firmness, K concentration, soluble solids concentration, ascorbic acid, and β-carotene following the procedures previously described by Lester et al. (2005). Fruit yield was measured in 2006 at the Weslaco site. RESULTS AND DISCUSSION Leaf, stem, petiole and fruit tissues from plots receiving supplemental foliar K treatments generally had higher K concentrations than those from control plots but this effect was not consistent among the K sources and between locations (Table 1). However, KNO 3 generally had the least beneficial effect on tissue K concentrations, perhaps a dilution effect resulting from NO 3 -enhanced vegetative growth. Even though pre-plant soil K concentrations were very high, supplemental foliar K treatments resulted in generally higher K concentrations in plant tissues, suggesting that plant K uptake from the soil solution was not sufficient to saturate K accumulation. It is also possible that restricted root growth and activity during the fruit development period and the high demand for nutrients by the developing fruit contributed, at least in part, to this apparent limitation on root K uptake from the soil solution (Oosterhuis, 1993). Significant treatment effects on fruit SSC, sugars, and human wellness compounds were more prevalent at the Weslaco location than at Santa Ana (Table 2). Fruit from plots receiving supplemental foliar K generally had higher SSC than those from the control plots. Similar trends were observed for total and component sugars (glucose, sucrose and fructose) but variability in individual observations resulted in inconsistent trends among the K sources. Total ascorbic acid and beta-carotene concentrations also generally increased as a result of supplemental foliar K applications however, the trends were not consistent among the K sources. The general trend of increased SSC, sugar, ascorbic acid and beta-carotene contents in fruit from of K treated plots is consistent with, but less dramatic than our previous observations with supplemental foliar K application in controlled environment studies (Lester et al., 2005). 3

4 External and internal fruit firmness differed significantly among treatments in Weslaco but not in Santa Ana (Table 3). While foliar K applications generally increased firmness, no significant differences were found among the K sources except for KNO 3 which tended to result in less firm fruit compared to controls. Fruit yields ranged from 136 to lb-boxes/ac based on a once-over harvest during peak fruit maturity and were not significantly affected by supplemental foliar K applications (Table 3). Individual fruit fresh weight and the percent dry weight of K-treated fruit did not differ significantly from that of control fruit. Pre-plant soil tests indicated moderate to very high levels of potassium (624 mg kg -1 at Weslaco and 479 mg kg -1 at Santa Ana) and other major elements. Lack of a yield response to supplemental foliar K is not surprising, giving the high pre-plant soil K levels. Studies of yield responses to K are mixed. Hartz et al. (2005) reported increased yields and fruit quality of fertigated tomatoes even when exchangeable soil K concentration was high; they found no effect of foliar applied K on yield and quality. Yield responses to foliar K applications in agronomic crops such as cotton are also inconsistent (Oosterhuis et al., 1994, Howard et al., 2000) perhaps due to confounding from factors related to soil processes and climatic conditions. Phytotoxicity problems were not observed with any of the foliar K sources and concentrations used in this study. The ph levels of spray solutions ranged from 6.5 to 7.7. Spray solutions of most K sources have an alkaline ph level that can cause leaf burns and this is more pronounced when applied under conditions of high temperature and/or low humidity (Swietlik and Faust, 1984; Tao et al., 1990). In the present study, all treatments were applied between 0500 and 0800 when air humidity (>80%), temperatures (< 25 C) and wind speeds (<1mph) were less favorable for leaf burn development. In summary, the current data generally support our previous controlled environment findings that supplementing soil K supply with foliar K applications during fruit development and maturation can improve muskmelon fruit quality by increasing firmness, sugar content, ascorbic acid and beta-carotene levels. The current data also provide additional evidence of differences among potential foliar K sources, with KNO 3 consistently emerging as a less desirable source of K during fruit development. These positive trends, as well as the variability among treatments underscore the need to further characterize the mechanisms of soil K uptake limitations and to define conditions for optimizing the benefits of supplemental foliar K nutrition. LITERATURE CITED Clarkson, D.T Ionic relations, p In: M. Walkins (ed.). Advanced plant physiology. John Wiley & Sons, N.Y. Engels, C. and H. Marschner Influence of the form of nitrogen supply on root root uptake and translocation of cations in the xylem exudate of maize (Zea mays L.). J. Exp. Bot. 44: Fisher, J.D., D. Hausen, and T.K. Hodges Correlation between ion fluxes and ion stimulated adenosine triphosphatase activity of plant roots. Plant Physiol. 46: Hartz, T.K., P.R. Johnstone, D.M. Francis and E.M. Miyao Processing tomato yield and fruit Quality improved with potassium fertigation. HortScience $): Howard, D.D., M.E. Essington, C.O. Gwathmey, and W.M. Percell Buffering of Foliar Potassium and Boron Solutions for No-tillage Cotton Production Journal of Cotton Science 4: Lester, G.E Whole Plant Applied Potassium: Effects on Cantaloupe Fruit Sugar Content and Related Human Wellness Compounds. Acta Hort. 682:

5 Lester, G.E., J.L. Jifon and D.J. Makus Supplemental Foliar Potassium Applications with and without surfactant can enhance netted muskmelon quality. HortSci. 41(3): Lester, G.E., J.L. Jifon and G. Rogers. (2005) Supplemental Foliar Potassium Applications during Muskmelon (Cucumis melo L.) Fruit Development can Improve Fruit Quality, Ascorbic Acid and Beta-Carotene Contents. J. Amer. Soc. Hort. Sci. 130: Marschner, H Functions of mineral nutrients: macronutirents, p In: H. Marschner (ed.). Mineral nutrition of higher plants 2 nd Edition. Academic Press, N.Y. Mengel, K. and E.A. Kirkby Potassium in crop production. Adv. Agron. 33: Mortvedt, J.J Calculating Salt Index. Fluid Journal. Spring Oosterhuis, D.M., Foliar fertilization of cotton with potassium. p In L.S. Murphy (ed.) Foliar fertilization of soybeans and cotton. PPI/FAR Spec. Publ. Potash & Phosphate Institute and Foundation for Agronomic Research, Norcross, GA. Oosterhuis, D.M., D.W. Albers, W.H. Baker, C.H. Burmester, J.T. Cothren, M.W. Ebelhar et al A summary of a three-year Beltwide study of soil and foliar fertilization with potassium nitrate in cotton. p In P. Dugger and D. Richter (ed.) 1994 Proc. Beltwide Cotton Conf., San Diego, CA. 5 8 Jan Natl. Cotton Council of America, Memphis, TN. Schönherr, J., Luber, M Cuticular penetration of potassium salts: effects of humidity, anions and temperature. Plant and Soil 236: Swietlik, D. and M. Faust Foliar nutrition of fruit crops, p In: J. Janick (ed.). Horticultural Reviews Vol. 6. Avi Pub. Co. Westport, Conn. Tao, Q.N., P. Fang, L.H. Guan, and M.Z. Chen Effects of N, P, K on yield and flavour components of watermelon. China Watermelon and Melon 3: Wyn Jones, R.G., C.J. Brady, and J. Speirs Ionic and osmotic relations in plant cells, p In: D.L. Laidman and R.G. Wyn Jones (eds.). Recent advances in the biochemistry of cereals. D. L. Laidman and R.G. Wyn Jones (Eds.) Academic Press, London. 5

6 Table 1. Influence of foliar potassium (K) sources (potassium chloride KCl, potassium nitrate - KNO 3, Monopotassium phosphate MKP, potassium sulfate - K 2 SO 4, potassium thiosulfate - KTS, and Potassium Metalosate - KM) on tissue K concentrations of field-grown muskmelon ( Cruiser ) fruit in south Texas. Weekly foliar K applications were made between fruit set and fruit maturity during the spring growing season in 2006 (Weslaco) and 2005 (Santa Ana). Treatment Leaf Petiole Stem Fruit Mesocarp (mgk gdw -1 ) (mgk gdw -1 ) (mgk gdw -1 ) (mgk gdw -1 ) Weslaco Control 11.9 cd z 48.2 d 42.9 d 23.6 b KCl 11.6 d 55.2 bc 49.3 c 26.0 a KNO d 47.6 d 41.6 d 23.3 b MKP 13.2 bc 51.6 cd 46.7 cd 28.9 a K 2 SO a 50.2 cd 64.0 a 25.8 ab KTS 13.9 ab 64.2 a 55.4 b 28.8 a KM 14.7 a 57.8 b 48.1 c 26.5 a Santa Ana Control 13.8 c 48.0 c 46.7 ab 18.6 b KCl 16.0 abc 54.3 b 43.8 b 23.0 a K 2 SO bc 54.4 b 51.0 a 18.9 b KTS 18.3 a 60.1 a 45.0 b 19.8 b KM 17.0 ab 49.2 c 37.8 c 20.2 ab Z Means with the same letter, within a column and location are not significantly different at Duncan s MRT 95% probability level (n=10). 6

7 Table 2. Influence of foliar potassium (K) sources (potassium chloride KCl, potassium nitrate - KNO 3, Monopotassium phosphate MKP, potassium sulfate - K 2 SO 4, potassium thiosulfate - KTS, and Potassium Metalosate - KM) on soluble solids content (SSC), sucrose, glucose, fructose, total sugar concentrations and human wellness compounds (ascorbic acid and betacarotene) of field-grown muskmelon ( Cruiser ) fruit. Weekly foliar K applications were made between fruit set and fruit maturity during the spring growing season in 2006 (Weslaco) and 2005 (Santa Ana). K Source SSC Sucrose Glucose Fructose Total Ascorbic acid β-carotene (%) mg gdw -1 (mg 100g -1 fw) µg gdw -1 Weslaco Control 9.2 b Z b a b c 17.4 b b KCl 10.0 ab a a b abc 22.9 a ab KNO ab a a b bc 21.8 ab ab MKP 10.6 a a a a a 21.3 ab a K 2 SO a ab a b bc 22.8 a ab KTS 10.7 a a a ab ab 21.3 ab a KM 10.3 a ab a ab abc 21.2 ab a Santa Ana Control ab b b b KCl a a a b K 2 SO b ab ab a KTS ab ab ab ab KM ab ab ab ab Z Means with the same letter, within a column and location, are not significantly different at Duncan s MRT 95% probability level (n=10). 7

8 Table 3. Influence of foliar potassium (K) sources (potassium chloride KCl, potassium nitrate - KNO 3, monopotassium phosphate MKP, potassium sulfate - K 2 SO 4, potassium thiosulfate - KTS, and Potassium Metalosate - KM) on yield, fresh weight, dry matter content and firmness of field-grown muskmelon ( Cruiser ) fruit. Weekly foliar K applications were made between fruit set and fruit maturity during the spring growing season in 2006 (Weslaco) and 2005 (Santa Ana). Treatment Yield Fruit Weight Dry Weight Fruit Firmness External Internal 50-lbBoxes/acre (g) (%) (N) (N) Weslaco Control z b 10.2 bc KCl b 10.6 abc KNO b 9.8 c MKP b 11.2 abc K 2 SO a 12.8 ab KTS ab 13.1 a KM b 11.2 abc Santa Ana Control KCl K 2 SO KTS KM Z Means with the same letter, within a column and location are not significantly different at Duncan s MRT 95% probability level (n=10). 8