Results

Microclimate

The microclimate during the experiments was characterised by an air temperature (± SE) of 22.95 ± 0.29 °C, a relative humidity of 94.61 ± 0.97 % and an average vapour pressure deficit of 0.15 ± 0.03 kPa inside the exposure bag. Average transpiration rate during all experiments was 0.162 ± 0.014 mmol m−2 s−1, while photosynthetic rate averaged around 2.139 ± 0.143 μmol CO2 m-2 s−1.

PET images and 11C-tracer dynamics

An example of strawberry fruits inside the FOV of the PET scanner can be seen (Fig. 2A), accompanied by temporally integrated PET images of the same fruits under the light intensity of 200 µmol m-2s-1 at leaf surface (Fig. 2B). The images were integrated from 0 to 120 min, 120 to 150 min, and 150 to 180 min, respectively from top to bottom and left to right. For the first 120 min after the 11CO2 feeding, no11C-tracer was detected, while from then on,11C started entering the fruits and peduncle that were placed inside the FOV. After that, the tracer uptake at fruits and peduncle position increased over time. This was true for all of the performed scans (see further).
PET image analysis of each strawberry fruit and subsequent normalisation (with regard to photosynthetic activity) resulted in normalised TTCs. Specifically, Fig. 3 shows an example of one fruit studied in four different PET experiments (one for each PPFD). Averaged over all normalised TTCs from each PET measurement per PPFD (i.e. 50, 100, 200 and 400 µmol m-2 s-1), the11C-tracer in the fruits began to increase after 132 ± 2.76, 133 ± 4.15, 113 ± 2.34 and 117 ± 2.02 min (± SE) of11CO2 feeding, respectively. In addition, the total amount of recorded 11C-tracer in the fruits during the PET measurement was between 5 and 32 times smaller (depending on PPFD – data not shown) in large plants compared to small plants.

Dynamic analysis of photosynthate translocation

To investigate the performance of the source leaves during the experiments, real-time photosynthetic activity was measured by analysing the gas entering and leaving the exposure bag (grey circled in Fig. 4A). The expected performance of the corresponding leaves was assessed by obtaining light response curves (LRC) after taking the leaf out of the exposure bag (white circles in Fig. 4A). The LRC showed that photosynthetic rate rapidly increased as light intensity increased from 0 to 200 μmol m-2 s-1, while a gradual increase was observed from 200 to 400 μmol m-2s-1. The light saturation point was reached at 400 μmol m-2 s-1. Inside the exposure bag, photosynthetic rate was however lower compared with the LRC-derived photosynthetic rate, reaching light saturation at 100 μmol m-2 s-1 and resulted in a maximum photosynthetic rate that was merely one fourth of the LRC one.
Relationship between PPFD and average relative rate of photosynthate translocation is also shown in Fig. 4A (black circles). Relative photosynthate translocation rate was highest under the light intensity of 100 μmol m-2 s-1. It was 4.6, 1.1 and 3.0 times higher than those under the light intensity of 50, 200 and 400 µmol m-2 s-1, respectively. Fig. 4B shows the response of average relative translocation rate (black circles in Fig. 4A) on the net photosynthetic rate of the source leaf averaged per PPFD of each PET measurement (grey circles in Fig. 4A). A linear relation between net photosynthetic rate and relative photosynthate translocation rate would be expected but was not observed for any of the experimental strawberries.
Whereas no relationship existed with photosynthetic rate, a clear response of relative translocation rate into fruits and transpiration rate was observed per fruit type (Fig. 5). There was a strong correlation between transpiration rate of the source leaf and relative photosynthate translocation rate. This was especially true for the red-27mm-S, white-22mm-S and red-18mm-L having an R2value of 0.68, 0.62 and 0.83, respectively, which indicates that lower transpiration rates lead to a higher relative photosynthate translocation rate. With the green-18mm-S and red-27mm-L, a sufficient correlation between transpiration rate and relative photosynthate translocation rate into fruit was found with an R2value of 0.59 and 0.53, respectively.