2017 ASHS Annual Conference

The wild blueberry relative or sparkleberry (Vaccinium arboreum)is being used as a rootstock in Southern Highbush (SH) blueberry production to mitigate the intolerance to high pH soils. Additionally, previous experiments showed that the rootstock leads to better performance than whole blueberry plants under drought conditions. We developed the present work to find if the better adaptation of the rootstock to water stress conditions has a positive impact on photosynthesis rate in blueberry production when grafted. We collected data during the summer of 2015 and 2016 in the San Joaquin Valley in California, using sparkleberry as a rootstock and three SH Blueberry varieties: Jewel, Star and Emerald as scions. There were no differences found in photosynthesis rate among varieties, however, shoots from the rootstock had significant differences when compared to SH blueberry. The average in photosynthesis rate was 12.87 µmol CO₂ m^-2s^-1 for sparkleberry and 8.89 µmol CO₂m^-2s^-1 for the southern highbush blueberry species. What it is important to highlight is that when SH blueberry was grafted onto sparkleberry, the photosynthesis rate increased to reach a value of 11.27 µmol CO₂ m^-2s^-1, which might be an evidence that constrained transpiration leads to lower photosynthesis and possibly yield. To understand the magnitude of the constraint and if the blueberry and sparkleberry plants were photosynthesizing at their optimum or if they had restrictions due to hot weather conditions, we performed CO₂ curves. With this, we found that even though the grafting has a positive impact, these plants are only operating at around 50% of their photosynthetic capacity even under an atmospheric concentration of CO₂of 400 ppm. This could be a suggestion that the restrictions in transpiration might been constraining photosynthesis in Californian weather conditions such as the ones during the experiment, with 28 to 35°C air temperature and a VPD of 2.1 to 4.8 kPa In conclusion, the photosynthesis rate in SH blueberry can be enhanced when grafted onto sparkleberry, and this better performance seems to be related to the access to water by the rootstock in deeper soil. However, this better performance is under the optimum photosynthesis rates that could be reached by SH blueberry plants in CO₂ atmospheric conditions.