We investigated the effects of substrate type, substrate volume, fruit thinning, and intrarow spacing on root mass and yield of long-term hydroponic tomatoes (Solanum lycopersicum L.) to quantitatively evaluate the relationship between root mass and yield. Four substrates (Rockwool [RW], RW + organic, zeolite + cocopeat [ZC], and organic bags) were used in the experiments. Two sub-treatments examined the effect of heavy vs. light fruit thinning. No correlation was observed between total root

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... weight (DW) and fruit fresh weight (FW) or DW. The substrate had no significant effect on fruit DW. Root DW had no significant effect on fruit DW or vegetative DW. Root DW decreased with increasing fruit load. A minimum root DW of about 15 g was sufficient to maintain fruit DW. Increasing the distance between the bed centers from 1.6 to 2.1 m while keeping the planting density at 2.5 plants m -2 , helped maintain yield without changing root DW. Thus, the number of beds could be reduced from four to three, leading to an increase in light transmittance to leaves in the middle and lower leaf layers. The higher the SPAD value (leaf color index) and nitrogen content of leaves, the longer their photosynthetic ability can be maintained. Thus, it appers that root mass has little influence on yield under suitable environmental control of greenhouse-grown tomatoes. Furthermore, approximately 15 g root DW per plant was sufficient to obtain fruit DW of more than 600 g per plant in Midi tomato.