Grafting, Irrigation, and Fertilization Effects on Tomato Plant Growth and Fruit Yield and Quality

The use of grafted vegetable plants is expanding in the U.S. partly because of reports that they withstand biotic and abiotic stresses more effectively than standard, ungrafted seedlings. However, it is still unclear whether this capacity extends across sites, growing conditions, and rootstock-scion combinations. Our objective was to test the effects of irrigation and fertilization regimes and grafting on tomato growth, and fruit yield and quality in conventionally managed field plots. Studies were conducted in 4 years as a split-plot design with irrigation (2012) or fertilization management (2013-2015) as the main plot and grafting as the subplot. Two irrigation treatments (standard, 50% of standard) were tested in 2012. Two fertilization treatments (pre-plant fertilization only, pre-plant fertilization plus standard fertigation) were tested in 2013 and 2014; and, four fertilization treatments (no pre-plant fertilization, pre-plant fertilization only, pre-plant fertilization plus standard fertigation, pre-plant fertilization with 150% of standard fertigation) were employed in 2015. Two commercial rootstocks (‘Maxifort’ and ‘Emperador’) and one experimental line (‘320’) were used in 2012 to 2014. Three commercial rootstocks (‘Maxifort’, ‘Estamino’ and ‘Supernatural’) were included in 2015. ‘BHN589’ was used as the scion and ungrafted control each year. Ripe fruits were harvested weekly 7, 7, 5, and 4 times in 2012, 2013, 2014, and 2015, respectively. Total and marketable fruit weight and number were measured, and average marketable fruit weight and marketable yield percentage were calculated. A subset of fruits was analyzed for fruit quality including Brix, pH, and titratable acidity (TA). In 2013 and 2014, plant growth was also monitored. Above-ground vegetative biomass, leaf area and N content, and truss number were greater in grafted than ungrafted plants in 2013, but the opposite was true in 2014. Regardless, yield trends were consistent across all years, with grafted plants having greater total seasonal yield than ungrafted plants. Neither irrigation nor fertilization management affected yield in 2012 and 2013, but fertilization affected yield in 2014 and 2015. Brix was higher in fruits from ungrafted versus grafted plants across all years. Fruit pH and TA were greater at the higher fertilization rate in 2014 and 2015. The grafting-irrigation/fertilization interaction was not significant for most variables. We conclude that plant performance is influenced by grafting combinations and fertilization regimes. Grafted plants have a higher yield potential than ungrafted ones under both high and low rates of irrigation/fertilization.