Yield Response of Watermelon and Muskmelon to L-Tryptophan Applied to Soil

This study was conducted to determine the influence of L-tryptophan (LTRP) applied to soil on the yield of two watermelon (Citrullus Lanatus L.) and one muskmelon (Cucumis melo L.) cultivars. Experiments were established in which L-TRP was applied to seedlings as a soil drench in the glasshouse ranging from 6 × 10 to 60 mg·kg soil; the seedlings were transplanted into the field 2 weeks later. The application of L-TRP enhanced the cumulative weight of the melons 58% (’Royal Sweet’ watermelon), 80% (‘Royal Windsor’ watermelon), and 42% (‘Top Score’ muskmelon) over their respective controls: the average weight per fruit was increased up to 43% for watermelon and 36% for muskmelon. No increase in yield was observed when LTRP was applied to the seedlings directly in the field, but only when added in contained systems 2 weeks before transplanting into the field. The increase in yield was due to a physiological response rather ‘than a nutritional effect because of the low L-TRP concentrations applied. Table 1. Cumulative yield of ‘Royal Windsor’ watermelon in response to L-tryptophan during three harvests. Fruit L-Tryptophan (L-TRP), also known as ß3-indolylalanine, is considered an essential amino acid to man, animals, and some bacteria (Lactobacillus arabinosus, L. casei, Streptococcus faecalis, and Leuconostoc mesenteroids) (Meister, 1965). Plants are capable of synthesizing TRP from 3-phosphoshikimic acid and subsequently chorismic acid and anthranilic acid (Lehninger, 1975). Several microorganisms release secondary metabolites upon the use of TRP. The release of auxins has been reported as a result of TRP metabolism in soil (Frankenberger and Brunner, 1983). Thimann (1935) was the first to demonstrate that TRP is a precursor of the plant hormone indole-3-acetic acid (IAA) when working with cultures of Rhizopus suinus. We recently demonstrated the conversion of L-TRP to IAA by a fluorescent pseudomonas isolated from a soilroot interface by ion suppression-high performance liquid chromatography (HPLC)-UV spectrometry (Frankenberger and Poth, 1987a). The isolation and detection of IAA as a secondary metabolize of L-TRP was also demonstrated from Pisolithus tinctorius (an ectomycorrhizal fungus) by thin-layer chromatography, HPLC, an enzyme-linked immunosorbent (monoclinal antibody) assay, and unequivocally identified by gas chromatography-mass spectrometry (Frankenberger and Poth, 1987b). Received for publication 23 Apr. 1990. We thank UlrichKarlson and Karin Nieto for their assistance in the field and P.A. Hammer (Associate Editor for Statistics, Hortscience) for his advice on the statistical analysis of the data. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. HORTSCIENCE , VO L. 26(l), JANUARY 1991 Tracer studies have demonstrated that TRP is metabolized to IAA in higher plants. Exogenously supplied 2-14C-DL-tryptophan incubated in watermelon tissue was incorporated into IAA (Dannenburg and Liverman, 1957). Dual-labeled experiments with 14C-labeled indole and 3H-serine added to pea seedlings confirmed that the indole was incorporated into IAA via TRP (Erdmann and Schiewer, 1971). Further work in our laboratory has established that the microbial pathway of IAA production with TRP serving as a precursor involves a transaminase reaction with the formation of indole-3-pyruvic acid as an intermediate (Frankenberger and Poth, 1988). Recently, we have reported that the application of TRP to soil affects the yield of radish, most likely due to TRP-derived auxins (Frankenberger et al., 1990). The objective of this study was to determine the effects of L-TRP applied as a soil additive on the yield of watermelon and muskmelon in the field. This study was conducted at the Moreno zFigures in parenthesis indicate percent increase ove *Means significantly different from control at P = Valley Field Station, Univ. of California, in Summer 1987 on a Hanford sandy loam (coarse-loamy, mixed thermic Typic Xerorthent). Soil analyses indicated the following chemical and physical properties: pH 5.9; total N, 730 ppm; total P, 530 ppm; composition, sand 55%, silt 29%, and clay 16%. On 29 Apr. 1987, seeds of two watermelon cultivars, Royal Sweet and Royal Windsor, and a muskmelon, ‘Top Score’, were planted in flats of washed silica sand in a glasshouse to obtain uniform size. Temperatures ranged from 28C during the day to ≈19 C at night. Ten days later, the seedlings were transplanted into cells containing the field soil (50 g each). The seedlings were treated with Hoagland’s mineral nutrient solution (one-half strength). On 19 May 1987, L-TRP (Sigma, St. Louis) (15 ml solution) was added as a soil drench at concentrations ranging from 10-9 to 10-3 M (6 × 1 0-5 t o 60 mg·kg-1 soil). A control was treated with deionized water. On 2 June 1987 the seedlings were transplanted into the field. The experimental layout was a completely randomized design with 6 × 6 m spacing for the watermelon and 3 x 3 m for the muskmelon with 15 replicates per treatment. Irrigation water was applied as furrow irrigation. The field was broadcast with ammonium nitrate (100 kg·ha-1) before planting. The yield was expressed in number of melons per plant and the cumulative weight of fruit per plant. The harvest schedule for the watermelon was as follows: 28 Sept., 26 Oct., and 20 Nov. (harvests 1, 2, and 3, respectively). Muskmelons were picked weekly starting from 14 Sept. throughout 28 Oct. with the data pooled into three harvests. Ripe watermelons were picked based on the dried curly tendril on the leaf near the stem. The muskmelon was considered mature when the fruit changed from green to yellow (tan) and an abscission layer formed between the stem and fruit. The data in each harvest were subjected to an analysis of variance and means of L-TRP treatments were compared with the control according to Dunnett’s test. Regression analyses were performed on the treatments excluding the control (Steel and Torrie, 1980). Fruit size (kg/fruit) was determined by dividing the mean fruit yield per plant by the mean number of fruit per plant for each