Rooting for red colour : Apr / May 2021
How rootstock selection affects blush in pears
Blushed or red pears made up about a third of cartons of pears passed for export from SA in 2020. Blushed Forelle dominates this segment. It was the second most exported pear and represented a quarter of cartons exported in 2019. Blushed pears have the potential to command top dollar in the market – but only if they show the required intensity and extent of red colour development. By Anna Mouton
Red colour development in pears has not received as much scrutiny as in apples. It has been known for more than 50 years that apple trees on dwarfing rootstocks tend to produce redder fruit. More recent work has confirmed that this is also true for pears. Some scientists believe that the improved red colour is simply due to better light exposure in smaller trees. Others have pointed out that apples from trees on dwarfing rootstocks are redder regardless of their position in the canopy.
So do dwarfing rootstocks offer a colour benefit in blushed pears? Or does it just come down to light management? Read on to find out.
Comparing pears to quince
Researchers from the Department of Horticultural Science at Stellenbosch University and from the Horticultural Division at the ARC Infruitec-Nietvoorbij teamed up to examine red colour in Forelle pears grown on different rootstocks. The study was funded by Hortgro.
The trial orchard was in Villiersdorp in the Western Cape. Fruit was assessed when the trees were in seventh and eighth leaf after planting. Pears were only sampled from positions receiving full sun. This removed light exposure as a variable that could affect fruit colour. In the first year, fruit was sampled one week prior to optimal maturity and in the second year, fruit was sampled two weeks after optimal maturity.
The researchers analysed background as well as red colour. Blush percentage was also recorded. Maturity measures included flesh firmness and sugar content.
The six rootstocks that were compared are summarised in table 1. OHxF 97 had similar vigour to QA and QC 51 at the site used in this specific trial.
Quince finishes first
The results of the colour analysis are summarised in figure 1. A lower number for hue angle indicates a redder colour, while a higher number for chroma indicates less interference from the background with the blush colour. The percentage blush is a measure of the area of peel that is red.
Pears harvested from trees on quince rootstocks consistently showed redder and more vivid colour – lower hue angle and greater chroma values – than pears from trees on pear rootstocks. Of the pear rootstocks, the less vigorous – at this site – OHxF 97 performed the best, producing fruit intermediate in colour intensity between the quince and the BP rootstocks.
Pears from trees on quince rootstocks as well as trees on OHxF 97 had a greater percentage blush than pears from trees on BP rootstocks.
Quince rootstocks are dwarfing by nature whereas pear rootstocks are semi-dwarfi ng or vigorous – therefore, the effects of vigour and species are somewhat entangled. However, BP1 and BA 29 impart similar vigour, and yet fruit from BP1 had less vivid colour and a smaller area of blush in both seasons. This fi nding lends support to the hypothesis that the differences in colour are not only due to differences in rootstock vigour.
Pears from trees on quince rootstocks consistently showed redder and more vivid colour.
The story behind the story
The researchers measured the levels of red pigments – anthocyanins – in the blushed areas of their experimental pears. Although there seemed to be a trend of higher levels in more blushed pears, the differences were not statistically signifi cant. Surprisingly, signifi cant differences were found instead for green pigment – chlorophyll – and yellow pigments – carotenoids.
Pears produced on quince rootstocks had lower peel concentrations of chlorophyll and higher peel concentrations of carotenoids. They also had a yellower background colour when visually compared to a colour chart.
The researchers speculated that the pears produced on quince rootstocks seemed yellower not only because of higher carotenoid levels, but also because of lower chlorophyll concentrations. Lower chlorophyll concentrations would also improve the visibility of anthocyanins and help accentuate red blush.
Chlorophyll concentrations often fall as fruit mature and chlorophyll is degraded. But fl esh fi rmness – an indicator of maturity – did not correlate with background colour in this study. In addition, the leaves of trees on quince rootstocks appeared to be less green than those of trees on the BP rootstocks.
Work done on apples by other scientists found that different rootstocks will result in different patterns of gene expression in the same scion cultivar. Gala was grafted onto either M.7 EMLA or M.9 T337. The scions on M.9 T337 expressed more genes related to photosynthesis, protein production and cell division, whereas those on M.7 EMLA expressed more stressrelated genes. This correlated with the physiological differences between the two graft combinations.
It is intriguing to consider whether quince and pear rootstocks differentially affect the expression of genes related to pigment production in pear fruit. Whatever the reason for the distinction, the take-home message is that BP rootstocks are best avoided for Forelle, and possibly also for other blushed cultivars. Stick to quince instead.
Jensen, P.J., Rytter, J., Detwiler, E.A., Travis, J.W. and McNellis, T.W. 2003. Rootstock effects on gene expression patterns in apple tree scions. Plant Molecular Biology. 493:493-511.
Roberts, S.C., Steyn, W.J. and North, M.S. 2008. Effect of rootstock on red colour of bi-coloured ‘Forelle’ pears. Acta Hortic. 800:625-631.