This thesis (Light on phloem transport – an MRI approach) aims to answer the question whether phloem transport can be a limiting factor for photosynthesis efficiency (and ultimately causing a bottleneck towards achieving higher yields). To answer this key question, we manipulated the source: sink ratio within tomato (Solanum lycopersicum L.) while measuring phloem transport with magnetic resonance imaging (MRI) flowmetry. Additionally we compared phloem flow characteristics of two potato plants (Solanum tuberosum L.) which differed in source : sink ratio. In Chapter 2, the source strength was manipulated by varying the light intensity. An increase in phloem sap volume flow under higher light intensities was observed. However, under all light intensities applied, the phloem flow velocity was found to be constant (as has previously been suggested in other studies) although a clear diurnal pattern was observed. This finding does not fit in current models to describe the mechanism of phloem transport and a different mechanism must be at play. The results of this chapter demonstrate that increased levels of photo-assimilates are transported in sieve tubes, which are activated when needed by the plant. This is the first study which shows that plants activate individual sieve tubes when more photo-assimilates are available, yet maintain constant velocity. Those observations were in a tomato plant with pruned fruit trusses (i.e., in a simplified system). In Chapter 3, we investigated whether tomato plants still exhibit constant phloem flow velocity (with a diurnal pattern) under normal conditions, i.e., with strong sinks (tomato fruits) still attached. This was tested for both a long and short photoperiod by measuring flow characteristics with MRI flowmetry. We simultaneously monitored other plant processes like xylem flow rates with a heat balance sensor, net photosynthesis with gas exchange and stem diameter changes with a linear motion potentiometer. With this integrated approach, we revealed a correlation between night phloem volume flow, dark respiration and stem growth. We also conclusively showed that phloem volume flow performs a diurnal pattern under a variety of source-sink ratios which appears to be a normal behaviour for tomato plants growing under moderately-high light conditions. In chapters 2 and 3 we learned that under higher source strength a greater amount of phloem sap is transported, but the changes in flow were not accompanied by changes in velocity. To further our understanding of the mechanisms driving phloem transport, it is of interest to know how the sucrose concentration in phloem sap relates to phloem flow. In Chapter 4 we used an average T2 relaxation time in the phloem vascular tissue region to reveal the plant’s phloem carbon status under source manipulation. In this chapter we demonstrated that T2 relaxation time, when measured in parallel with phloem flow, can provide additional information about phloem region carbon status, i.e., changes in the T2 relaxation time are correlated with changes in sucrose concentration in the whole phloem region. When studying phloem transport in plants with magnetic resonance imaging (MRI) flowmetry, plants which are relatively easy to manipulate (e.g. fruit pruning) like tomato have so far been used. However, tomato plants (used in all three previous chapters) have relatively low sink strength beneath the MRI measurement site. A potentially preferable approach is to work with plants with strong sinks beneath the measurement site. In Chapter 5 we studied potato as a potentially better test subject for MRI flowmetry as it possesses strong sink below the MRI measurement site (i.e., developing tubers). For that purpose we used two potato plants (cv. Desiree) both with several developing tubers. One of the plants overexpressed the StSWEET gene (35S:StSWEET) which appears to have altered its source : sink ratio. As a result, the 35S:StSWEET plant transported 60% more phloem sap than Desiree WT. Strikingly, the average phloem flow velocity in both plants was the same and the greater amount of transported phloem sap in the 35S:StSWEET plant was accommodated by more sieve tubes than in Desiree WT. This finding agrees with the hypothesis about the conserved nature of phloem flow velocity, where volume flow is regulated by the number of active sieve tubes (Chapter 2 and 3). In this chapter we also demonstrate that a potato plant with developing tubers represents a good subject to study phloem transport with MRI flowmetry. We concluded that under optimal conditions (which are commonly met in greenhouses) phloem transport is likely to reach its maximum capacity and therefore photosynthesis could be limited by the export and transport of photo-assimilates because of the finite number of sieve tubes and constant flow velocity.