2017 ASHS Annual Conference

Apple rootstocks have a substantial influence on yield, fruit quality and profitability of a modern apple orchard. Most of the rootstock selection traits used to identify a superior rootstock are measured indirectly on grafted scions. Selection could be more efficient if we could peek at the underground root machinery directly instead of relying exclusively on scion measurements. For example, in Geneva® apple rootstocks we have discovered a segregating fine root trait which may be associated with nutrient uptake, hormone (cytokinin) production, water uptake, etc. However, this trait is very difficult to observe in vivo and measurement of roots dug out from soil is very difficult and prone to error due to variable soil properties. Timing and length of growth is another example of a difficult to phenotype underground trait that is associated with whole tree source-sink metabolism and resulting yield potential. Mini-rhizotrons have been around since the 1990’s, and while the Geneva breeding program has used one of those systems to study apple roots, the system’s field of view has prevented high throughput phenotyping of root systems, requiring major investment in time to analyze thousands of pictures each capturing an area smaller than 3 cm² within clear viewing tubes which have 2500 cm² in total viewing surface. This task has been drastically simplified by the adoption of larger format (425 cm²) root scanning equipment (CI-602, CID Bio-Science) and the building of high density rhizotron containers featuring banks of 18 clear tubes fitted into plastic fruit bins able to accommodate several small apple trees. The task of measuring root growth, morphology, gene expression, nutrient uptake and physiology has also been somewhat simplified by growing the roots of small apple trees in aeroponics systems (dark containers that spray a mist of nutrient solution onto the roots while suspended in air). This system allows for nutrient pulse treatments and harvesting roots for analysis of metabolic compounds and gene expression by RNA sequencing – something that was very difficult to do in soil media conditions. We discuss current research and results leveraging these new root peeking systems.