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This experiment aimed to investigate the status of tomato plants in terms of sink or source-limitation of 2 cultivars of greenhouse tomato (Solanum lycopersicum L.), i.e., 'Grandella' and 'Isabella' under the greenhouse conditions of Iran and to improve the yield and plant growth by manipulating the sink-source balance. To this end, 4 treatments were applied: leaves were not pruned and fruits were pruned to one per truss (1F/3L), leaves were not pruned and fruits were pruned to two per truss (2F/3L), leaves were not pruned and fruits were pruned to three per truss (3F/3L) and no leaf and fruit pruning (control). The results showed that truss pruning reduced the sink demand and consequently, increased the amount of available assimilate for the growth of the remaining fruits or vegetative parts. The negative correlation between the leaf area index and the net assimilation rate and no significant difference in the net assimilation rate between different sink/source ratios showed that the excess leaf area index does not contribute in increasing the assimilate production and hence, total yield. Total fruit weight, harvest index, and the ratio of the ripe fruits to the total fruit led to the highest yield for control plants. No changes in chlorophyll, protein content and nitrate reductase activity were the evidence for the fact that sink/source ratio do not affect light-harvesting and light-utilizing components of photosynthesis. Since the individual weight of fruits increased with decreasing fruit number per trusses, the growth of individual fruits in both cultivars was source-limited and truss pruning can decrease this limitation. Future studies should be carried out to determine the best level of sink/source ratio that in addition to producing an acceptable amount of yield, meets the needs of consumers in the current stressful world by increasing the antioxidant and nutriceutical content of fruits.

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