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.