Plant growth is closely related to photosynthesis rate and the assimilate partitioning. Photosynthesis rate can strongly depend on environmental and internal conditions including the source-sink balance. Sink activity is directly connected with the change in the sink/source ratio, providing one way in which various important cellular processes, including carbohydrate metabolism and the secondary metabolites synthesis and traort, are intrinsically regulated. Considering that the growth of tomato fruits suffers from competition from both other fruits in the same truss and fruits in other trusses, this study aimed to evaluate the effect of altered sink/source ratio on physiological changes in a truss that affect source strength, and fruit quality and quantity of two cultivars of tomato ( Solanum lycopersicum L.). The sink demand alteration treatments were: saving one fruit per truss (1F), two fruits per truss (2F), three fruits per truss (3F), and no fruit pruning (control). Each fruit cluster was pruned when the first fruit had reached a 1 cm diameter. The pericarp of the first fruit of the second cluster was collected at the fully ripe stage for fruit quality characteristics determination. For biochemical measurements, the leaves below and upper the third truss and pericarp of the first fruit on the third truss were collected (one fruit per replication) at three different developmental stages. i.e. end of the cell division (15 days after treatment; DAT), cell expansion (30 DAT), and the beginning of ripening (45 DAT). The results showed that traort the assimilates to the fruits was decreased by increasing sink demand and it was compensated by increased vegetative growth. According to the inhibited influence of the sink/source ratio on chlorophyll content and nitrate reductase activity, it seems that the sink/source ratio did not affect photosynthesis. The decreasing sink/source ratio increased the carbohydrate content of the leaves, but it seems that the carbohydrate would not accumulate to levels that are enough to induce the sugar repression of photosynthesis in the sink leaves. Decreasing sink/source ratio increased starch, sucrose, and hexose contents in the fruits and modulated the activity of the key enzymes and the expression of tomato hexose traorter genes ( LeHT s). Based on relative transcript levels, all examined LeHT s were overexpressed at the end of cell division and cell expansion stage of fruit development, but the strongest expression level observed at the onset of ripening was related to LeHT1 and LeHT2 . Given the concomitancy of cell wall invertase activity and LeHT s relative expression cell wall invertase activity seemed to be involved in the expression level of LeHT s. The increased trends of LeHT s expression by decreasing the sink/source ratio comfort the role of hexose traorters in determining the sink strength of tomato fruits. The •OH-scavenging ability of 1F and control was highest because of the highest amount of lycopene, flavonoids, and total phenolic compound content of these treatments. Overall, the obtained results showed that the decreased sink/source ratio increased dry matter partitioning into the fruits, and therefore, the content of phytochemicals. Decreasing sink demand increased the incidence of blossom end rot (BER) and relative fruit growth rate. The correlation between BER incidence and relative fruit growth rate showed that the fruit growth rate is an important factor in BER incidence. Endogenous auxin and cytokinin are key regulators controlling fruit growth rate and BER incidence. We suggest the slowing down of the relative growth rate by keeping more fruits per truss as an effective and cheap way to control BER incidence. Key Words Invertase, Blossom end rot, Phloem unloading, Fruit growth rate, Carbohydrate, fruit quality, Hexose traorters, Phytohormones.