Reducing plant density by the use of rootstocks in processing tomatoes

The use of scions grafted on rootstocks may be considered a sustainable strategy on suboptimal soil conditions (salinity, pathogens presence, etc.). However, the cost of a grafted plant, at least, is three times the value of a non-grafted plant. Previous trials indicated that plant canopy, fruits and flower clusters were increased 10-20% by grafting associated to greater root and vine vigor than non-grafted plants. Therefore, increasing the distance between plants in the planting row could make plants to grow more efficiently. The objective of this study is to evaluate the effect of different number of grafted plants per ha on yield, fruit quality, and phenological aspects. Trials were conducted in La Consulta, INTA Experimental Station in Mendoza, Argentina and in different farms including San Juan Province during the seasons 2017-2018 and 2018-2019. 'Protector' (H.M. Clause) was used as rootstock and 'HM 1892' (H.M. Clause) as scion. Drip irrigation was used and daily water replenishment was done by a single surface tape according to evapotranspiration tank type "A", adjusted by local Kc. During the two seasons from 12,000 to 18,000 grafted plants ha-1 by Japanese top-grafting method made in Proplanta S.A. nursery were compared in the field using common varieties as scions against a non-grafted check with 22,000 plants ha(-1) were studied. All parameters were evaluated by using linear regression test at a=0.05. The results obtained in different field trials showed no significant increments on total and commercial yield as well as Brix content on grafted plants from 12,000 to 18,000 plants ha(-1). In general, the use of suitable rootstocks lengthened the growing season about 5-7 days, compared to non-grafted plants. Only 12,000 grafted plants ha-1 had significant fruit size increment, around 5%, compared to 18,000 plants ha(-)1. Overripe, blossom end rot index and culls fruits as well as pH were not affected by plant density. Density trials indicated that more than 12,000 rootstocks ha(-1) probably won't be needed with appropriate rootstocks to achieve the same yield as 22,000 non-grafted plants ha(-1). Further studies, will analyze even less plants ha(-1) as well as the impact on field cash costs.