Enhancing nitrogen and water use efficiency in tomato production by using grafting technique

Final Report for GS10-097

Project Type: Graduate Student
Funds awarded in 2010: $10,000.00
Projected End Date: 12/31/2012
Grant Recipient: University of Florida
Region: Southern
State: Florida
Graduate Student:
Major Professor:
Dr. Xin Zhao
University of Florida
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Project Information

Summary:

A two-year field study demonstrated that tomato plants grafted onto vigorous interspecific hybrid rootstocks produced significantly higher total and marketable yields in comparison with non-grafted plants. Grafting also significantly improved irrigation water use efficiency and nitrogen use efficiency based on marketable fruit yields. Fruit quality assessment did not reveal any major consistent changes as a result of grafting with interspecific rootstocks. Although use of grafted transplants increased the total production and harvest costs, the yield improvement generated significant gross returns to offset these costs and led to higher net returns compared to the non-grafted tomato production.

Introduction

The purpose of this project was to evaluate the potential of grafting as an economically viable approach to optimizing irrigation and nitrogen (N) management in tomato production in Florida. Tomato is widely grown in the U.S., with Florida being a leading producer. Like other vegetable crops grown commercially, tomato production in Florida is intensively managed with relatively high inputs of fertilizers and irrigation water which may contribute to pollution of water resources due to nutrient leaching and runoff. Vegetable grafting may provide an environmental-friendly approach to optimizing water and nitrogen management. In effect, vegetable grafting has been shown to enhance yield and improve crop tolerance to abiotic stresses like drought, salinity, flooding, and low soil temperatures (Davis et al., 2008; Lee et al., 2010; Rivero et al., 2003). It is suggested that underlying mechanisms for increased plant vigor and yield in grafted plants may be attributed to increased water and nutrient uptake by using vigorous rootstocks. Nutrient and water uptake as well as plant hormone status can be influenced by rootstocks (Aloni et al., 2010; Lee et al., 2010). It was pointed out that fertilization programs should be modified accordingly to reflect the improvement of nutrient use when growing grafted tomato plants (Leonardi and Giuffrida, 2006).

Owing to its multifaceted benefits, grafting has been widely used in the production of solanaceous and cucurbit vegetables in Asia and Mediterranean countries for disease control, plant vigor, and yield increase (Davis et al., 2008; Lee, 1994; Lee and Oda, 2003). In the U.S., grafted plants have been employed in hydroponic tomato production, while grafting has just emerged as an alternative practice for field tomato cultivation (Kubota et al., 2008). The research projects funded previously by SARE (O’Connell, 2008; Rivard, 2006) were focused on heirloom tomatoes grown organically in North Carolina. These projects provided preliminary information on grafting performance to manage soilborne diseases and improve nutrient uptake in heirloom tomatoes; however, water use and its interaction with nutrient management was not addressed, particularly within a broad context of crop production. In addition, quality attributes of fruit from grafted tomato and the economic feasibility of the grafted tomato cropping systems were not well elucidated. While quality attributes of tomato fruit such as titratable acidity, soluble solid content, fruit color, fruit firmness did not show any significant difference between grafted and self-rooted tomato plants (Khah et al., 2006), grafting has been shown in some instances to increase levels of lycopene, ß-carotene, vitamin C, and antioxidant activity in tomato fruit (Davis et al., 2008; Dorais et al., 2008; Fernandez-Garcia et al., 2004). Furthermore, it is also critical to evaluate the economic feasibility of grafted vegetable systems. Efficient management strategies can only be fully integrated into agricultural management practices when farmers can also sustain long term profitability of their operations (Baggs et al., 2000). Given the increasing interest from growers, in-depth research is needed to demonstrate successful integration of the grafting technology into site-specific tomato production systems in Florida.

Project Objectives:

1. Examine the effects of grafting with vigorous rootstocks on plant growth, fruit yield, and water and N use efficiency in field production under different combinations of irrigation regimes and N fertilization rates;

2. Determine whether grafting can alter the nutritional quality attributes of tomato fruit;

3. Assess the economic feasibility of the grafted tomato production systems.

Research

Materials and methods:

GRAFTED TRANSPLANT PRODUCTION AND EXPERIMENTAL DESIGN. The field-grown, determinate tomato cultivar Florida 47 was used as the scion and grafted onto two commercially-available, interspecific hybrid rootstocks ‘Beaufort’ and ‘Multifort’. These two rootstocks are currently among the most widely used tomato rootstocks in the United States. Rootstock seeds were sown on 21 Feb., 2010 and 19 Feb., 2011, 2 days before the ‘Florida 47’ scion. Plants were splice-grafted on 16 Mar., 2010 and 20 Mar., 2011, when 4-5 true leaves were present. Grafted plants were immediately placed in a closed healing chamber equipped with two humidifiers and an auto-control air conditioning system for healing the grafts, where temperature was maintained at 25 ± 3°C and average relative humidity above 80%. Light and ventilation were introduced gradually after a dark period of 4 days. Twelve days after grafting, grafted plants including ‘Florida 47’ grafted onto ‘Beaufort’ (FL/BE) and ‘Florida 47’ grafted onto ‘Multifort’ (FL/MU) were completely healed and ready for transplanting to the field. Non-grafted ‘Florida 47’ (FL) transplants provided the control treatment.

The field experiments were conducted during the spring seasons of 2010 and 2011 at the Suwannee Valley Agricultural Extension Center in Live Oak, FL. The soil type was a Blanton-Foxworth-Alpin Complex sandy soil. In both years, field plots were disked and plowed, five weeks before transplanting, followed by soil fumigation using Telone C-35 at the rate of 196.4 L/ha. The field was fumigated to eliminate interference of soilborne pest factors. Mehlich-1 soil test results conducted prior to field preparation showed a high level of soil P and a low level of soil K. Three weeks before transplanting, 13N-1.7P-10.8K fertilizer was applied at a rate providing 56 kg N/ha, 7.3 kg P/ha, and 46.5 kg K/ha to all plots during bed preparation. Grafted and non-grafted plants were transplanted to raised beds with plastic mulch and drip irrigation on 29 Mar. 2010 and 1 Apr. 2011. Beds were 0.71 m wide and spaced 1.52 m apart (from middle to middle) with 0.46 m in-row spacing for open-field tomato production.

In both years, a split-plot design with four replications was used. The whole-plot treatments, i.e., 12 factorial combinations of two irrigation regimes and six N fertilization rates, were arranged in a randomized complete block design. The subplot treatments included the two grafting treatments FL/BE and FL/MU and the non-grafted ‘Florida 47’ (FL) as a control, all randomized within each whole plot. There were 12 plants for each treatment combination per replication in both 2010 and 2011. The two irrigation regimes included: 1) 100% irrigation regime based on the current University of Florida-Institute of Food and Agricultural Sciences (UF/IFAS) recommendation for field production of round tomatoes in sandy soils in Florida, i.e., 9354 L/ha/day/string (Olson et al., 2010), and 2) 50% irrigation regime corresponding to 4677 L ha/day/string. The stake-and-weave method was used for trellising the tomato plants. The term “string” is an expression used in field tomato production to denote the growth stage of staked tomato plants; typically for ‘Florida 47’, 3 sequential strings are necessary from transplanting through final harvest. The six N fertilization rates were 56, 112, 168, 224, 280, and 336 kg/ha which represented 25%, 50%, 75%, 100%, 125%, and 150%, respectively, of the currently recommended total N application rate of 224 kg/ha (a preplant application at 56 kg/ha included) for field production of irrigated, round tomatoes in sandy soils in Florida (Olson et al., 2010). Except for the 56 kg N/ha rate, which only included a preplant application of 13N-1.7P-10.8K, ammonium nitrate (34N-0P-0K, Mayo Fertilizer Inc, Mayo, FL) was injected weekly through the drip tape starting one week after transplanting (WAT) to provide the remaining amount of N for other fertilization rate treatments. The weekly injected amounts of N for each of these five N fertilization rates during 1 to 2 WAT, 3 to 4 WAT, 5 to 11 WAT, 12 WAT, and 13 WAT, respectively, were as follows: 1) 3.0, 4.0, 5.0, 4.0, and 3.0 kg/ha; 2) 6.1, 8.0, 10.0, 8.0, and 6.1 kg/ha; 3) 9.1, 12.0, 15.0, 12.0, and 9.1 kg/ha; 4) 12.1, 16.0, 19.9, 16.0, and 12.1 kg/ha; and 5) 15.1, 20.0, 24.9, 20.0, and 15.1 kg/ha. Potassium chloride (Dyna Flo 0N-0P-15K, Chemical Dynamics Inc, Plant City, FL) was also applied through fertigation to provide each treatment with amount of K needed after accounting for the preplant application based on the soil test. The weekly injected amounts of K during the growing season were as follows: 11.8, 9.3, 14.3, 9.3, and 6.7 kg/ha during 1 to 2 WAT, 3 to 4 WAT, 5 to 11 WAT, 12 WAT, and 13 WAT, respectively. Other cultural practices, including pest control, followed current recommendations for commercial field tomato production in Florida (Olson et al., 2010). In addition to the non-grafted ‘Florida 47’ controls, self-grafted scion plants (FL/FL) were added as a second set of controls. These were included in the 100% irrigation and N rate (224 kg/ha) plots in order to examine the effect of graft injury and initial growth reduction associated with the grafting process.

YIELD, IRRIGATION WATER USE EFFICIENCY, AND NITROGEN USE EFFICIENCY. Mature green tomato fruit and fruit at more advanced ripening stages were harvested from 10 plants in each treatment combination per replication. Fruit were picked 80 and 88 days after transplanting (DAT) in 2010, and 75, 85, and 92 DAT in 2011. They were then graded as extra-large, large, medium, and culls (small fruit and defective fruit). Fruit in each grade were counted and weighed. Total fruit yield, marketable fruit yield, average fruit weight, and average number of fruit per plant were calculated. Irrigation water use efficiency (iWUE) was estimated as the ratio of the marketable fruit yield to the amount of irrigation water applied during the production season. Nitrogen use efficiency (NUE) was estimated as the ratio of the marketable fruit yield to the amount of N supplied during the production season.

PLANT GROWTH AND NITROGEN ACCUMULATION. Aboveground biomass was destructively evaluated on one representative plant per treatment combination at final harvest. Each sampled plant was cut at ground base and separated into leaf blade, petiole, stem, and fruit. The fresh weight of each plant part was measured and then subsamples of each plant part were taken and weighed. All the sub-samples from each plant were dried in a forced-air drying oven at 60°C for 72 to 120 h until constant weight. Total aboveground biomass was then determined. Dried samples of leaf blade, petiole, stem, and fruit were analyzed for total Kjeldahl nitrogen (TKN) concentrations. Aboveground N accumulation was determined by multiplying dry weight of leaf blade, petiole, stem, and fruit by the corresponding N concentrations.

ROOT CHARACTERISTICS. At 93 DAT in 2010 and at 97 DAT in 2011, root samples were collected from the grafted and non-grafted plants to examine the N fertilization effect on root growth and distribution. The root analysis study was focused on three N application rates including 112, 224, and 336 kg N/ha at the 100% irrigation regime, following the root sampling method previously described in Zotarelli et al. (2009a). Briefly, roots were sampled by taking soil cores at four different depths: 0-15, 15-30, 30-60, and 60-90 cm, using a 5-cm diameter soil auger and at two different positions around the plant (i.e., at the plant base vs. at 15 cm distance from the plant) in the center of each treatment plot. Soil samples collected at each depth per treatment were stored at 4°C prior to processing in the lab. During the sample processing, each sample was weighed and washed with running water using a fine sieve to collect root and other debris. Washed and cleaned roots per soil core were scanned using a root scanning apparatus and analyzed with the image analysis software WinRhizo 2008a (Regent Instruments, Quebec, Canada) to determine the total root length. Root length density (RLD) for each soil depth was then estimated.

FRUIT QUALITY ATTRIBUTES. Tomato fruit from non-grafted and self-grafted ‘Florida 47’ and grafted ‘Florida 47’ with ‘Beaufort’ and ‘Multifort’ were sampled for quality assessment. Fruit were harvested at 80 and 75 DAT in 2010 and 2011, respectively, from the field plots with the N fertilization rate at 224 kg N/ha and the recommended irrigation regime (100%). The harvested fruit (8-10 for each treatment per replication) were stored at 20°C to monitor the fruit ripeness. When fruit reached full ripeness, they were sliced and homogenized and then stored at -30 or -80°C for measurements of pH, total titratable acidity (TTA), and soluble solids content (SSC) as well as concentrations of ascorbic acid (vitamin C) and carotenoids (lycopene, beta-carotene, and lutein) and total phenolic content.

STATISTICAL ANALYSES. Data from the 2010 and 2011 experiments were analyzed separately. Analysis of variance was conducted using the GLIMMIX procedure of SAS version 9.2 (SAS Institute, Cary, NC). Tukey’s test (? = 0.05) was used for multiple comparisons among different treatments.

Research results and discussion:

TOTAL AND MARKETABLE FRUIT YIELDS. Irrigation regime, N fertilization rate, and grafting all showed significant influence on tomato fruit yields in both 2010 and 2011. In the 2010 trial, the 50% irrigation regime resulted in higher total and marketable fruit yields compared to the 100% irrigation regime. Increases averaged 15% and 19%, respectively. Within the six N rates evaluated, total and marketable fruit yields were significantly improved when the N rate increased from 56 to 112 kg/ha and from 112 to 168 kg/ha. However, no significant differences were observed with N rates of 168 kg/ha and above. Grafting with the two rootstocks significantly improved the yields of ‘Florida 47’. Averaged over the two rootstocks, the increase of total and marketable fruit yields relative to those of non-grafted ‘Florida 47’ reached 27% and 30%, respectively.

For the 2011 trial, the effect of N rates on tomato yields was dependent on irrigation regime as reflected by the significant interaction of irrigation regime by the N rate. With the 50% irrigation regime, the total and marketable yields at 56 kg N/ha were significantly lower than those at higher N rates, but yields at 112 kg N/ha and above did not differ significantly. In contrast, under the 100% irrigation regime, significant yield increases were observed as the N rates increased. The total yield at 280 kg N/ha was significantly higher than those at lower N rates, but did not differ significantly from that at 336 kg N/ha. A similar trend was found for the marketable yield which reached the highest level at 280 kg N/ha while it did not show significant differences from those at 224 and 336 kg N/ha. Comparison of the yield response to the two irrigation regimes at each N rate did not show a clear pattern; however, total yields did not differ except at 56 and 280 kg N/ha while marketable yields were similar except at 280 and 336 kg N/ha. Similar to the 2010 experiment, total and marketable fruit yields of FL/BE and FL/MU were significantly higher than the non-grafted plants in the 2011 trial, and a significant grafting × N rate interaction was also observed for marketable fruit yield. Grafting increased the marketable yield of ‘Florida 47’ with each N rate at or above 112 kg N/ha, and the yield increase reached 46% at 224 kg N/ha. Grafting also altered the effects of N rates on marketable yield. For example, for non-grafted ‘Florida 47’, marketable yield at 112 kg N/ha was significantly higher than that at 56 kg N/ha, but it did not show any significant increase at higher N rates. In contrast, the marketable fruit yields of the grafted plants exhibited a significant increase at 280 kg N/ha compared with those at 56 and 112 kg N/ha. In this study, fruit yields were similar for self-grafted and non-grafted plants under the recommended irrigation regime and N rate.

In both 2010 and 2011 trials, FL/BE and FL/MU had significantly more marketable fruit per plant than did non-grafted plants. The number of marketable fruit per plant was also affected by a significant interaction between irrigation regime and N rate in both years. In 2010, under the 100% irrigation regime, the marketable fruit number per plant was significantly lower at 112 kg N/ha, and most reduced at 56 kg N/ha. Under the 50% irrigation regime, fruit number was lowest at 56 kg N/ha and was significantly increased at 112 kg N/ha, while increasing N rate beyond 112 kg/ha significantly increased the fruit number at 224 and 336 kg N/ha. In the 2011 trial, the marketable fruit number at different N rates under the two irrigation regimes followed the same response of marketable fruit yields.

Consistent with the higher marketable yield under the 50% irrigation regime in 2010, the average fruit weight was also significantly greater. Significant effects of grafting, N rate, and their interaction were also evident in average fruit weight. Grafting with the two rootstocks significantly increased the average fruit weight of ‘Florida 47’ at most N rates tested but not at 112 kg/ha. In the 2011 experiment, the impact of grafting on average fruit weight also reflected significant interaction effects associated with irrigation regime and N rate. Under the 50% irrigation regime, grafted plants (FL/BE and/or FL/MU) showed significantly greater average fruit weight than the non-grafted plants at each of the six N rates applied. In contrast, under the 100% irrigation regime, the average fruit weight did not differ between grafted and non-grafted ‘Florida 47’ at 56 kg N/ha. Under the recommended irrigation regime (100%) and N rate (224 kg/ha), the average marketable fruit weight was increased by grafting by appropriately 17% in contrast to the non-grafted plants.

Our results demonstrated that grafted plants tended to show greater potential for yield improvement with the increase of N than non-grafted plants particularly in the 2011 trial. Compared with non-grafted tomato plants, it is likely that grafted plants may require a higher level of N for maximizing yield performance.

IRRIGATION WATER USE EFFICIENCY. The iWUE relative to marketable fruit yields was affected by significant two-way interactions between grafting, irrigation regime, and N rate. In both years, a marked decline of iWUE was observed when the irrigation regime was increased from 50% to 100% irrespective of grafting and N rate treatments. In 2010, the highest value of iWUE was achieved at 224 and 280 kg N/ha within the 50% and 100% irrigation treatments, respectively, while it did not differ significantly between N rates at and above 168 kg/ha within each irrigation regime. In 2011, under the 50% irrigation regime, the iWUE at 112 kg N/ha was significantly higher than that at 56 kg N/ha but the N rates above 112 kg/ha did not lead to any significant increase of iWUE. Within the 100% irrigation regime, the iWUE reached the highest value at 280 kg N/ha but it did not differ significantly from values at 224 and 336 kg N/ha. Furthermore, iWUE of the grafted plants were significantly higher relative to that of non-grafted plants at both irrigation regimes in both years, although the difference tended to vary with the irrigation regime. In 2010, the averaged iWUE values of the grafted plants were greater than that of the non-grafted plants by 29% under the 50% irrigation regime and by 32% under the 100% irrigation regime. In 2011, the average increase in iWUE as a result of grafting was 54% and 36% under 50% and 100% irrigation regimes, respectively. In general, both grafted and non-grafted plants in each season exhibited a consistent increase of iWUE with the increasing N rate from 56 to 168 kg/ha. A further improvement of iWUE at 224 kg N/ha was observed in grafted plants but not in non-grafted ‘Florida 47’. Increasing the N rate from 224 to 336 kg/ha did not result in any pronounced change of iWUE. Furthermore, the performance of the two rootstocks used also seemed to differ at certain N rates. In both years, grafting significantly enhanced the iWUE at different N rates except that there was no significant difference between the iWUE of FL/MU and FL at 56 kg N/ha, while FL/BE and FL showed similar levels of iWUE at 168 kg N/ha in 2010.

NITROGEN USE EFFICIENCY. In the 2010 trial, the main effect of irrigation regime was significant, while the significant effect of N rate on NUE was dependent on grafting. NUE was 20% higher in the 50% irrigation regime compared to the 100% irrigation regime. Moreover, compared with the non-grafted ‘Florida 47’, grafted plants with the two rootstocks enhanced NUE significantly at each N rate except the 168 kg/ha treatment. On average, the increase in NUE due to grafting with the vigorous rootstocks were 81% and 23% with the 56 and 112 kg N/ha, respectively, while ranging from 26% to 38% when increasing the N rate from 224 to 336 kg N/ha. In addition, for both FL/BE and FL/MU, increasing the N rate from 56 to 336 kg/ha consistently decreased the NUE. However, the decrease was mainly significant between the two lower N rates (56 and 112 kg/ha) and the two higher N rates (280 and 336 kg/ha). With the non-grafted plants, except the N rate at 56 kg/ha, NUE decreased as the N rate increased from 112 to 336 kg/ha. Furthermore, in 2011, the main effect of grafting was significant, while the significant effect of N rate on NUE was related to the irrigation regime. In 2011, grafted plants with the two rootstocks increased NUE by 42% relative to that of non-grafted plants. Under both irrigation regimes regardless of the grafting treatment, the rate of 112 kg N/ha resulted in the highest NUE, while the lowest NUE was observed at 336 kg N/ha. NUE values at certain N rates were also influenced by the irrigation treatments. At 56 kg N/ha, the NUE was significantly higher under the 50% irrigation regime while the opposite was observed at each of the higher N rates ? 224 kg/ha.

PLANT GROWTH AND NITROGEN ACCUMULATION. Grafted plants with the two vigorous rootstocks showed significantly higher levels of aboveground biomass as compared with the non-grafted plants. On average, grafting increased the total aboveground biomass by approximately 16% and 27% in 2010 and 2011, respectively. N accumulation was also significantly higher in grafted plants as compared with the non-grafted control. Averaged over the N fertilization rates, the accumulated N in the grafted plants was increased by about 20% and 33% as compared with the non-grafted treatment in 2010 and 2011, respectively.

ROOT LENGTH DENSITY. During the two seasons, root length density (RLD) varied significantly with the N rate, grafting, soil depth, and sampling position. Except at the N rate of 112 kg/ha, grafting with the two rootstocks significantly increased the RLD of ‘Florida 47’ at 224 and 336 kg N/ha by 58 and 118%, respectively. The majority of the root system was present in the upper soil level with up to 65% of the root system within the 0-15 cm soil depth. In contrast, only about 7% of the roots were found at the 60-90 cm soil depth. A significant interaction effect of grafting by soil depth on RLD was also observed in both seasons. At the 0-15 cm soil depth, RLD of the grafted plants was significantly greater than that of the non-grafted plants by about 78% and 69% in 2010 and 2011, respectively. However, values of the RLD within 15-30 cm and 30-60 cm of the soil profile were similar between grafted and non-grafted treatments except that FL/MU demonstrated a significantly higher level of RLD than FL at 30-60 cm in 2011. Furthermore, at the 60-90 cm soil depth, RLD was significantly greater in grafted plants than non-grafted plants in 2010, whereas it did not differ between grafted and non-grafted treatments in 2011. In addition, the position at which the root was sampled also demonstrated a significant impact on RLD which was also determined by the soil depth. RLD measured at the plant base position (P1) was significantly higher than those at the position of 15 cm away from the plant base (P2), with the exception of 60-90 cm of the soil depth where RLD was similar between P1 and P2 in the 2011 trial. On average, RLD at P1 was greater than that at P2 by approximately 66%, while the reduction of RLD from P1 to P2 appeared to be more pronounced at the top 0-15 cm soil depth. Moreover, the decrease of RLD along the soil profile (0-90 cm) seemed to be more rapid at P1 than at P2.

FRUIT QUALITY ATTRIBUTES. Overall, grafting with the two rootstocks did not show a significant impact on tomato soluble solids contents (SSC), pH, total titratable acidity (TTA), and the SSC:TTA ratio. In contrast, greater variations of pH, TTA, and SSC:TTA with the production seasons were observed. Fruit pH and TTA were significantly higher in 2010 than in the 2011 season, whereas the opposite trend was found for the SSC:TTA values. With respect to the levels of vitamin C, carotenoids, and total phenolics, grafting did not demonstrate any detrimental effects, while the measurements differed significantly between the two seasons. Concentrations of vitamin C, beta-carotene, and lutein were significantly higher in tomato fruit harvested in 2010 than in 2011; however, the levels of lycopene and total phenolics were elevated in 2011 as compared with those in 2010.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

DISSERTATION

Djidonou, D. 2012. Improving fruit yield and nutrient management in tomato production using grafting. Ph.D. dissertation, University of Florida, Gainesville, FL.

REFEREED PUBLICATIONS

Djidonou, D., X. Zhao, E.H. Simonne, K.E. Koch, and J.E. Erickson. 2013. Yield, water, and nitrogen use efficiency in field-grown, grafted tomatoes. HortScience (accepted).

CONFERENCE PRESENTATIONS

Djidonou, D., K. Lopiano, X. Zhao, and E.H. Simonne. 2012. Estimating nitrogen fertilization requirement for grafted tomato grown in the field. American Society for Horticultural Science Annual Conference, Miami, FL.

Djidonou, D. and X. Zhao. 2011. Optimizing field grown tomato yield: Effects of grafting, irrigation and nitrogen inputs. American Society for Horticultural Science Annual Conference, Waikoloa, HI.

Project Outcomes

Project outcomes:

Several studies have been conducted in recent years in the U.S. regarding the benefits of using grafting for disease control in tomato production, whereas research-based information is scarce with respect to the influence of vigorous rootstocks on water and nutrient management under open field production especially in sand soils with low water and nutrient retention. This study demonstrated that use of grafted plants could significantly improve fruit yields in field production of drip-irrigated tomato in sandy soils in Florida. The increase of marketable yields resulted from both more fruit per plant and higher average fruit weight. Grafting with the two interspecific tomato hybrid rootstocks used here also led to significant enhancement in efficiency of water and N use. Overall, the use of rootstocks did not show any major negative impact on fruit quality. It is expected that appropriate rootstocks may be selected with the goal of minimizing water and N losses from production sites by allowing tomato plants to more efficiently take up and use water and N. It is very likely that grafted plants may differ from non-grafted plants in terms of the nutrient need for optimizing fruit yield. This project provided the first report on modification of plant root length density as a result of grafting with vigorous rootstocks. Findings from this project may foster domestic tomato rootstock breeding programs to serve sustainable vegetable production.

Economic Analysis

COSTS OF GRAFTED AND NON-GRAFTED TOMATO TRANSPLANT PRODUCTION. Costs of all the materials, supplies, and labor were estimated to calculate the production costs of non-grafted ‘Florida 47’ plants and grafted ‘Florida 47’ plants with ‘Beaufort’ or ‘Multifort’ rootstock. In this study, the cost of producing grafted ‘Florida 47’ transplants with either ‘Beaufort’ or ‘Multifort’ was estimated at $0.67/plant vs. $0.15/plant for the non-grafted transplants. As a result, with the planting density of 5808 tomato plants per acre, grafting with either ‘Beaufort’ or ‘Multifort’ rootstock added approximately $3,015.63 per acre to the total pre-harvest production costs in field production of ‘Florida 47’.

PARTIAL BUDGET ANALYSIS OF GRAFTED AND NON-GRAFTED TOMATO PRODUCTION. A base cost model for growing, harvesting, and marketing ‘Florida 47’ in a 1-acre field with a raised-bed polyethylene mulch production system was established prior to conducting the partial budget analysis, using an existing crop budget model for fresh market tomato production in the Manatee/Ruskin area developed by the University of Florida Center for Agribusiness (2009). Partial budget analysis is a standard economic analysis tool commonly used to determine the effects of a series of changes to certain operations of the farming production system on the change of economic returns. This economic analysis approach compares the negative and positive effects of applying a new treatment relative to a base or standard treatment. It can provide a good snapshot on the possible economic benefit associated with adopting the new production practice. In the context of this study, using grafted tomato transplants is considered as the new treatment while using non-grafted transplants is the standard practice. The typical components of the partial budget analysis as presented by Sydorovych et al. (2008) were adapted as follows:

1. Negative effects:
a. Added costs due to grafting;
b. Reduced returns due to grafting;
c. Total negative effects due to grafting, i.e., the summation of the added costs and reduced returns.

2. Positive effects:
a. Reduced costs due to grafting;
b. Added returns due to grafting;
c. Total positive effects due to grafting, i.e., the summation of reduced costs and added returns.

Total effects, i.e., net change in revenue, is calculated as the difference between total positive effects and total negative effects.

It was assumed that added costs of grafted tomato production relative to the non-grafted tomato production would be incurred if the costs of grafted transplants and the harvest costs of grafted treatment were higher than the transplant and harvest costs of non-grafted tomato production. Furthermore, added returns would be incurred if use of grafted plants resulted in higher yields as compared to the non-grafted plants, which could result in higher gross returns in grafted tomato production than non-grafted tomato production.

In 2010, the gross returns reached $22,158.09 and $20,697.35/acre for production of grafted ‘Florida 47’ with ‘Beaufort’ and ‘Multifort’, respectively. In contrast, the non-grafted tomato production was valued at $15,948.89/acre. Hence, the additional gross returns relative to non-grafting were $6,209.20 and $4,748.45/acre for ‘Beaufort’ and ‘Multifort’, respectively. In 2011, these additional gross returns were $7,646.66/acre and $7,318.25/acre for ‘Beaufort’ and ‘Multifort’, respectively. The gross returns were slightly higher in 2011 than in 2010 due to higher marketable fruit yields achieved in the 2011 season. In terms of the production costs, the negative effects (added costs) related to the use of grafted transplants involved the increased costs of transplants and the harvest. These total negative effects amounted to $4,937.24 and $4,485.17/acre with grafting ‘Florida 47’ onto ‘Beaufort’ and ‘Multifort’, respectively, in 2010; and $5,184.82 and $5,091.66/acre in 2011.

After accounting for grafting and harvest costs, the net returns of grafted tomato production with ‘Beaufort’ and ‘Multifort’ increased by $1,271.96 and $263.28/acre, respectively, as compared with non-grafted tomato production in the 2010 season. The increases of net returns as a result of using grafted transplants were $2,461.82 and $2,226.59/acre in 2011. This indicated that the increased crop value resulting from the significant improvement of marketable tomato fruit yield could offset the increased cost of grafted tomato production and even made it more profitable than non-grafted tomato production in fumigated soils.

Farmer Adoption

Although on-farm trials were not conducted as part of this project, research results have been disseminated to growers during farm visits. At present, grafting is used primarily as a tool for soil-borne disease control; however, increasing interests from growers are driving the needs for systematic research to investigate the full potential of grafting as an integral part of sustainable vegetable production. Some greenhouse tomato producers in Florida have been using grafting for extending fruit harvest. This study showed the great potential of grafting for yield improvement in field tomato production under low disease pressure. We hope to carry out on-farm trials in future studies to introduce grafting as an innovative management tool for enhancing fertilizer use efficiency and fruit yield and test the cost effectiveness of using grafted transplants in commercial tomato production systems.

Recommendations:

Areas needing additional study

More in-depth studies are still warranted to include more tomato scion cultivars and rootstocks in the commercial growing conditions to further examine the rootstock-scion interaction effects on plant growth and fruit yield beyond disease control. Nutrient requirement especially the N requirement for grafted tomato production under field conditions needs to be further explored. Different types of soils across different locations may be considered. In addition, future research should be conducted to monitor the movement of nutrients, especially nitrate in the soil profile in order to demonstrate the impact of grafting with vigorous rootstocks on reducing nitrate leaching out of the plant active root zone. More studies are also needed to identify specific traits of rootstocks in relation to the improvement of water and N use efficiency in grafted tomato plants.

More on-farm trials are still needed to help growers decide whether grafting can be integrated as an economic viable component into their exiting production systems. A complete set of scenarios for economic analysis needs to be considered to fully assess the costs and profits associated with using grafted plants for disease management, enhancement of irrigation water and fertilizer use efficiency, and yield improvement.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.