**1. Introduction**

The tomato (*Solanum lycopersicum* L.) is a leading vegetable and one of the most important horticultural crops. The world production of tomatoes is second to only potatoes, with an estimated production of 160 million tons [1]. Tomato production problems with abiotic and biotic stressors as

results of intensive monoculture often create problems in tomato production. Tomato production losses caused by unfavorable growing conditions can be reduced by grafting onto specific rootstocks. Commercial vegetable grafting started at the beginning of the 20th century, with the primary intention to achieve tolerance to soil pathogens [2]. The substantial proportion of total tomato production in Europe and Asia currently include usage of grafted tomatoes [3]. In addition, the widespread use of grafting was expected to improve crop response to water, salt, nutrient deficiency and temperatures stresses and to improve fruit quality [4,5].

Water availability is decreasing worldwide—especially where agriculture uses between 50% and 90% of all water, such as semi-arid Mediterranean areas. In the context of climate change, improvement of irrigation practices is needed, by use of new sources (e.g., wastewater) or by application of new irrigation techniques [6]. Two main methods regarding the use of reduced irrigation are introduced, namely deficit irrigation (DI) and partial rootzone-drying (PRD) to improve water-use efficiency (WUE). These practices expose the plant to moderate drought stress, could increase abscisic acid (ABA) levels that leads to greater increase in WUE [7]. DI supplies less water to the entire rootzone than the amount lost by evapotranspiration, while PRD involves alternate wetting and drying of the root zones. PRD has been shown to improve DI and has resulted in substantial water savings, improved water-use efficiency and it is superior to DI in terms of yield maintenance in greenhouse or processing tomato [8,9].

Grafted plants showed better uptake of minerals and water than un-grafted plants due to vigorous root growth by the chosen rootstock [10]. Grafted tomato under deficit irrigation showed increased yield and WUE [11,12]. The tomato cultivar Boludo grafted on 144 tomato rootstocks showed higher shoot fresh weight under water deficit in 38% of combinations [13]. Tomato cultivars Belle and Clarabelle grafted on the rootstock He-man had similar vegetative growth and yield under PRD conditions in commercial greenhouse [14]. The breeding of commercial rootstocks like 'Maxifort' on the other hand was more directed to increase growth capacity and to alleviate soilborne diseases, instead of water economy [2], although it showed similar performance compared to drought tolerant cultivars [15]. Sink limitation is more pronounced in grafted tomato plants compared to non-grafted ones. One of the ways to avoid sink limitations is growing double-stemmed grafted plants. Grafting with two stems became the standard growing method for sustaining tomato production, as it decreased the costs per unit area by reducing the number of plants grown in a greenhouse by one-half [16,17].

The effects of grafting on tomato fruit quality are showing inconsistencies in of the results, mostly affected by the rootstock–scion combination [5]. Similarly, scarce reports showed that grafted plants under water stress differ in total soluble solids (TSS) and total acidity (TA) [11,14,18].

In the first study, we evaluated the effect of PRD on the growth, yield and quality of grafted tomato grown in a commercial greenhouse [14]. As proper evaluation of reduced irrigation methods apart PRD include DI, the two-year studies included the deficit irrigation treatments with similar amount of water applied as in PRD, but evenly to the whole root system. Since two stems are the standard practice for cultivation of grafted tomato, we also included stem number as a factor in our experiments. Finally, the purpose of our studies was to compare the responses of grafted tomato plants subjected to PRD or DI by evaluating vegetative growth, leaf gas-exchange parameters, yield traits, WUE and leaf and fruit mineral profile and fruit quality in a greenhouse located in a Mediterranean climate.
