Ecophysiological assessment of the effect of different apple rootstocks on scion performance

Scions perform below expectation on some apple rootstocks under South African conditions compared to results achieved in other production regions.  It is thought that this poor performance may be related to the poor adaptability of these rootstocks to local conditions.

The aim of this research is to study the effect of different dwarfing, semi-vigorous and non-dwarfing rootstocks on net photo-assimilation, water use efficiency and water status of ‘Rosy Glow’ grafted onto rootstocks that have recently become available.

Various ecophysiological measurements were conducted (predawn and stem water potential, net photosynthesis, stomatal conductance, leaf area index), as well as a complete leaf mineral analysis. However due to the word limitation all methodology is discussed in more detail later in the document.

It was clear that by 2013 and 2014 rootstocks had not reached their maximum influence on scion growth as dwarfing and non-dwarfing rootstock trees had very similar vigour. The differences between dwarfing and non-dwarfing rootstocks in terms of scion vigour were becoming more apparent in 2015. Although there were some significant differences in a few ecophysiological and morphological parameters measured, it was difficult to use these to explain why rootstocks are possibly underperforming under South African climatic conditions.

This study did however indicate that studies dealing with rootstock performance should be conducted on trees that have reached their full bearing potential if the aim is to understand why rootstocks are not performing under South African conditions. It was evident from this study that future research should focus on creating stress contrasts between rootstocks to tease out how rootstocks differ with coping with sub-optimal or harsh climatic conditions. This should be coupled with more continuous data logging and/or more frequent data capturing on a per tree basis. The effect of drought stress on growth and yield of a scion grafted onto various dwarfing and non-dwarfing rootstocks would provide an important contrast. Induced water stress would allow us to investigate whether rootstocks cause a difference in the time it takes before soil water deficits are manifested in the scion. Also, whether the type of rootstock influences the rate of recovery in terms of water status and photosynthetic rates after irrigation resumes? Both could have a potential impact on yield and fruit quality. This should be broadened out into a study investigating the impact of the magnitude and duration of water stress on yield of an identical scion on different rootstocks. Focusing on stress induced research in the future would provide a greater chance of pinpointing the cause of under-performance of rootstocks under South African conditions.