it provides commercial value of the fruits in general (Kallio et al. 2000). The relationship between sugars and sensory properties such as flavour or colour of strawberries have been extensively studied: stage of ripeness, age of plants, soil quality, and genotype of variety are known to affect the quantitative variations in sugar and acids in strawberries (Avigdori-Avidov 1986; Kafkas et al. 2007; Recamales et al. 2007; Basson et al. 2010; Maraei & Elsawy 2017).
The essential plant tissue that is involved in sugar transport within vascular plants, is phloem (De Schepper et al. 2013). Principally, it transfers the compounds made during photosynthesis, i.e. photosynthates, from source leaves to the sites of utilisation in growth (e.g. developing fruit, new shoots and roots) and storage (e.g. roots). A multitude of techniques have been designed to monitor the fate of assimilated sugars in phloem transport (Liesche 2019). In particular, isotopic techniques, based on tracing of both stable or unstable isotopes, have gained increased interest (Kiser et al. 2008; Epron et al. 2012; Hubeau & Steppe 2015; Hidaka et al.2019). Despite its potency, 13C- and14C-tissue enrichment analysis requires destruction and extraction of plant tissues, resulting in fragmentary data. To this end, 11C in combination with positron emission tomography (PET) is convenient because it allows non-invasive, real-time imaging of plant physiological function (Hubeau & Steppe 2015; Hidakaet al. 2019). Although the lower spatial resolution (c. 1 mm in this study) compared to other medical imaging techniques like computed tomography (CT – submillimetre), it is the method of choice for measurement of long-distance tracer transport in 4D (x,y,z,t). Furthermore, it is complementary with positron autoradiography, in which snapshots are created at much higher spatial resolution to assessin vivo tracer distribution (Mincke et al. 2018; Hubeauet al. 2019a, b).
Carbon-11 has a half-life of 20.4 min and has been used mostly for dynamic studies (Minchin & Thorpe 2003; Kiser et al. 2008), including studies on photosynthate translocation to fruits of tomato (Kawachi et al. 2011; Yamazaki et al. 2015), eggplant (Kikuchi et al. 2008) and strawberry (Hidaka et al. 2019), as well as wheat grains (Matsuhashi et al. 2006). In these studies, 11CO2 was fed to a source leaf while the fruits were monitored using PETIS (positron-emitting tracer imaging system). Eventually, continuous 2D images were generated visualizing the accumulation of 11C-labelled photoassimilates to and within the sink organs, at good spatial (c. 2 mm) and temporal resolution (c. min). The results of this methodology clearly showed the considerable advantages of PET-based imaging in increasing our understanding of many of the physiological processes involved in fruit development. Moreover, studies on eggplant and wheat ears revealed that tracer uptake was not uniform within fruits suggesting that photoassimilate translocation and accumulation are affected by environmental conditions like light or diurnal regulation (Matsuhashi et al. 2006; Kikuchi et al. 2008). Furthermore, Kawachi et al. (2011) found that sink strength of individual tomato fruits is not dependent on developmental stage since comparable tracer uptake profiles were detected for developing and more mature tomatoes. The study on strawberry fruits revealed that one source leaf has vascular systems connecting to multiple fruits on an inflorescence and supplies photosynthate to all of them (Hidaka et al. 2019). In contrast to the study on tomato, photosynthate accumulation among strawberry fruits on various positions on the same inflorescence differed. It was suggested that these differences in photosynthate translocation among fruits might be caused by variation in their relative sink activity levels. Despite the added value of these PET studies, the drivers for photosynthate translocation into fruits remain unclear.
The aim of the present work was to investigate the effect of light intensity (shed on the source leaf) on the photosynthate translocation to strawberry fruits. In analogy with the above-mentioned studies,11C was supplied to an individual source leaf as airborne 11CO2 while strawberry fruits were placed in the field-of-view of a small animal PET scanner, resulting in 3D images of photosynthate translocation to the fruits. The source leaf was immediately below the inflorescence, which is the leaf position where highest photosynthate partitioning was observed to the inflorescence (Hidaka et al. 2019). By altering the height of the LEDs above the source leaf, different photosynthetic photon flux densities (PPFD ) were created. Leaf transpiration rate, photosynthetic rate, relative humidity and air temperature were measured to capture the functioning and environmental conditions of the source leaf.