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Desire Djidonou and Daniel I. Leskovar

The effect of different nitrogen (N) concentrations on growth changes, leaf N concentration and accumulation patterns, N nutrition index (NNI), fresh weight yield, and N use efficiency (NUE) was determined for lettuce grown over three consecutive seasons (fall, winter, and spring) in a recirculating hydroponic system, under unheated and naturally lit hoop house in Uvalde, TX. The lettuce cultivars Buttercrunch, Dragoon, and Sparx were grown at six N concentrations, initially 100, 150, 200, 250, 300, and 400 mg·L−1 using a nutrient film technique (NFT). Leaf number, accumulated dry weight (DW) and N, and leaf area index (LAI) followed a logistic trend over time, characterized by a slow increase during early growth followed by a linear increase to a maximum. By contrast, plant total N concentrations were the highest at early stage and decreased slightly over time. Effect of season and cultivar on these growth traits was more pronounced than that of the N concentrations. Averaged across cultivar and N concentrations, DW in spring was 73% and 34% greater than that in fall and winter, respectively. At each sampling date, there were linear, quadratic, or cubic effects of N concentrations on each of these variables. The cultivar Sparx was the most productive, with 63% and 32% higher fresh weight yield in fall, 145% and 114% in spring, than ‘Buttercrunch’ and ‘Dragoon’, respectively. Increasing nutrient solution N concentrations from 100 to 400 mg·L−1 increased the yield from 5.9 to 6.7 kg·m−2 in fall, 8.1 to 10.7 kg·m−2 in winter, and 10.3 to 12.6 kg·m−2 in spring. The NUE was the highest at the lowest N concentration (100 mg·L−1) and decreased with increasing N concentrations. The NNI during mid- to late-growth stages was near or greater than one, even at the lowest N. These results demonstrated that N concentrations of 100–150 mg·L−1 maximized the growth and yield of hydroponically grown lettuce.

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Desire Djidonou, Zhifeng Gao, and Xin Zhao

In addition to controlling soilborne diseases, grafting with selected rootstocks has the potential to enhance growth and yields in tomato (Solanum lycopersicum) production. However, information is rather limited regarding its economic viability in different production systems in the United States. The objective of this study was to compare the costs and returns of grafted vs. nongrafted fresh-market tomato production under common management practices in fumigated fields in northern Florida. The field trials were conducted in Live Oak, FL, during Spring 2010 and 2011. ‘Florida 47’ tomato was grafted onto two interspecific hybrid tomato rootstocks: ‘Beaufort’ and ‘Multifort’. Grafted and nongrafted ‘Florida 47’ plants were grown on fumigated raised beds with polyethylene mulch and drip irrigation using recommended commercial production practices for nutrient and pest management. The estimated costs of grafted and nongrafted transplants were $0.67 and $0.15 per plant, respectively, resulting in an additional cost of $3020.16 per acre for using grafted transplants as compared with nongrafted plants. Grafting also led to higher costs of harvesting and marketing tomato fruit as a result of yield improvement (1890 to 2166 25-lb cartons per acre for grafted plant vs. 1457 to 1526 25-lb cartons per acre for nongrafted plant). Partial budget analyses showed that using grafted transplants increased tomato production costs by $4488.03–$5189.76 per acre depending on the rootstock and growing season. However, compared with nongrafted tomato, the net farm return of grafted tomato production was increased by $253.32–$2458.24 per acre based on the tomato shipping point prices. Sensitivity analysis further demonstrated that grafting would be more profitable as the costs of grafted transplants decreased and the market tomato prices increased. These results indicated that although grafting increased the total cost of production, the increase in marketable fruit yield generated significant gross returns to offset costs associated with the use of grafted tomato transplants. Nevertheless, further research is warranted to provide more production budget and net return data about the economic feasibility of grafted tomato production based on a wide range of commercial growing conditions in Florida.

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Desire Djidonou, Xin Zhao, Karen E. Koch, and Lincoln Zotarelli

Growth and yield typically increase when tomato plants are grafted to selected interspecific hybrid rootstocks from which distinctive root system morphologies are envisioned to aid nutrient uptake. We assessed these relationships using a range of exogenous nitrogen (N) supplies under field production conditions. This study analyzed the impact of N on growth, root distribution, N uptake, and N use of determinate ‘Florida 47’ tomato plants grafted onto vigorous, interspecific, hybrid tomato rootstocks ‘Multifort’ and ‘Beaufort’. Six N rates, 56, 112, 168, 224, 280, and 336 kg·ha−1, were applied to sandy soil in Live Oak, FL, during Spring 2010 and 2011. During both years, the leaf area index, aboveground biomass, and N accumulation (leaf blade, petiole, stem, and fruit) responded quadratically to the increase in N fertilizer rates. Averaged over the two seasons, the aboveground biomass, N accumulation, N use efficiency (NUE), and N uptake efficiency (NUpE) were ≈29%, 31%, 30%, and 33% greater in grafted plants than in nongrafted controls, respectively. More prominent increases occurred in the root length density (RLD) in the uppermost 15 cm of soil; for grafted plants, RLD values in this upper 15-cm layer were significantly greater than those of nongrafted plants during both years with an average increase of 69% over the two seasons. Across all the grafted and nongrafted plants, the RLD decreased along the soil profile, with ≈60% of the total RLD concentrated in the uppermost 0 to 15 cm of the soil layer. These results demonstrated a clear association between enhanced RLD, especially in the upper 15 cm of soil, and improvements in tomato plant growth, N uptake, and N accumulation with grafting onto vigorous rootstocks.

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Desire Djidonou, Xin Zhao, Jeffrey K. Brecht, and Kim M. Cordasco

Grafting is considered to be a unique component in sustainable vegetable production. In addition to its usefulness for managing soil-borne diseases, it has been suggested that grafting with vigorous rootstocks can improve crop growth and yield. The objective of this greenhouse study was to assess the effects of different interspecific hybrid tomato rootstocks (Solanum lycopersicum × Solanum habrochaites) on yield, growth, nutrient accumulation, and fruit composition of tomato (S. lycopersicum). Using the determinate tomato cultivar Florida 47 as the scion, plants were grafted onto four interspecific rootstock cultivars including Beaufort, Maxifort, Multifort, and RST-04-105. Overall, the use of rootstocks resulted in total and marketable fruit increase 53% and 66% higher than non-grafted and self-grafted scion plants, respectively. The increase in marketable yield by ‘Beaufort’, ‘Maxifort’, and ‘Multifort’ was largely attributed to an increased number of fruit per plant, whereas higher average fruit weight contributed to the yield increase in plants grafted onto ‘RST-04-105’. Self-grafting of ‘Florida 47’ resulted in similar yield as the non-grafted scion control. Analyses of plant growth parameters demonstrated significant enhancement of total leaf area at first fruit harvest in plants grafted onto interspecific rootstocks as compared with the non-grafted and self-grafted scion controls. In addition to plant growth and yield improvement, enhanced accumulation of nitrogen, potassium, and calcium was also observed in grafted plants. The enhancement in mineral nutrient accumulation was largely related to increased biomass accumulation rather than higher nutrient concentration (on a dry weight basis). The overall accumulation of phosphorus was not influenced markedly by the rootstocks used. In general, grafting with the interspecific rootstocks maintained fruit soluble solids content (SSC) and total titratable acidity (TTA), concentrations of vitamin C, carotenoids, and total phenolics at levels comparable with non-grafted plants, whereas harvest date showed a more pronounced effect on fruit composition.

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Desire Djidonou, Amarat H. Simonne, Karen E. Koch, Jeffrey K. Brecht, and Xin Zhao

In this study, the effects of grafting with interspecific hybrid rootstocks on field-grown tomato fruit quality were evaluated over a 2-year period. Fruit quality attributes from determinate ‘Florida 47’ tomato plants grafted onto either ‘Beaufort’ or ‘Multifort’ rootstocks were compared with those from non- and self-grafted controls. Grafted plants had higher fruit yields than non- and self-grafted plants, and increased production of marketable fruit by ≈41%. The increased yield was accompanied by few major differences in nutritional quality attributes measured for these fruit. Although grafting with the interspecific rootstocks led to consistently small, but significant increases of fruit moisture (≈0.6%), flavor attributes such as total titratable acidity (TTA) and the ratio of soluble solids content (SSC) to TTA were not significantly altered. Among the antioxidants evaluated, ascorbic acid concentration was reduced by 22% in fruit from grafted plants, but significant effects were not evident for either total phenolics or antioxidant capacity as assayed by oxygen radical absorbance capacity (ORAC). Levels of carotenoids (lycopene, β-carotene, and lutein) were similar in fruit from grafted plants with hybrid rootstocks compared with non- and self-grafted controls. Overall, the seasonal differences outweighed the grafting effects on fruit quality attributes. This study showed that grafting with interspecific hybrid rootstocks could be an effective horticultural technique for enhancing fruit yield of tomato plants. Despite the modest reduction in ascorbic acid content associated with the use of these rootstocks, grafting did not cause major negative impacts on fruit composition or nutritional quality of fresh-market tomatoes.

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Desire Djidonou, Xin Zhao, Eric H. Simonne, Karen E. Koch, and John E. Erickson

In addition to managing soilborne diseases, grafting with vigorous rootstocks has been shown to improve yield in tomato (Solanum lycopersicum L.) production. However, the influence of different levels of nitrogen (N) and irrigation supplies on grafted tomato plants has not been fully examined in comparison with non-grafted plants, especially under field conditions. The objective of this two-year study was to determine the effects of different irrigation regimes and N rates on yield, irrigation water use efficiency (iWUE), and N use efficiency (NUE) of grafted tomato plants grown with drip irrigation in sandy soils of north Florida. The determinate tomato cultivar Florida 47 was grafted onto two interspecific hybrid rootstocks, ‘Beaufort’ and ‘Multifort’ (S. lycopersicum × S. habrochaites S. Knapp & D.M. Spooner). Non-grafted ‘Florida 47’ was used as a control. Plants were grown in a fumigated field under 12 combinations of two drip irrigation regimes (50% and 100% of commonly used irrigation regime) and six N rates (56, 112, 168, 224, 280, and 336 kg·ha−1). The field experiments were arranged in a split-plot design with four replications. The whole plots consisted of the irrigation regime and N rate combination treatments, whereas the subplots represented the two grafting treatments and the non-grafted plants. Self-grafted ‘Florida 47’ was also included in the 100% irrigation and 224 kg N/ha fertilization treatment as a control. In 2010, the 50% irrigation regime resulted in higher total and marketable yields than the 100% irrigation regime. Tomato yield was significantly influenced by N rates, but similar yields were achieved at 168 kg·ha−1 and above. Plants grafted onto ‘Beaufort’ and ‘Multifort’ showed an average increase of 27% and 30% in total and marketable fruit yields, respectively, relative to non-grafted plants. In 2011, fruit yields were affected by a significant irrigation by N rate interaction. Grafting significantly increased tomato yields, whereas grafted plants showed greater potential for yield improvement with increasing N rates compared with non-grafted plants. Self-grafting did not affect tomato yields. More fruit per plant and higher average fruit weight as a result of grafting were observed in both years. Grafting with the two rootstocks significantly improved the irrigation water and N use efficiency in tomato production. Results from this study suggested the need for developing irrigation and N fertilization recommendations for grafted tomato production in sandy soils.