Yatu, a fruit character that stops fruit from looking ‘perfect’
Research & ExtensionEver wondered about that lump you sometimes see beside the stem on apples and frequently on pears?
Yatu is what the Chinese call the lump that pushes the stem out at an angle rather than having it straight up from the centre of the fruit (figure 1). At the moment this is not considered a defect but this may change as consumers become fussier and want perfect–looking fruit. Are you looking for the perfect apple or pear crop that does not have Yatu? Believe it or not some of the materials we apply at flowering have an impact on the appearance of Yatu.
Figure 1: Yatu (growth to one side of the stalk) on a Packham pear and a Gala apple.
Figure 2: Cutaway view of Yatu on Packham pear and Smyrna quince.
Hormonal implications
Notice on the cut pear and quince (figure 2) that Yatu is a result of additional growth within the flesh of the neck of the fruit on one side only, resulting in the vascular system being pushed slightly to one side and curving towards the top so the stem comes out at an angle. The substantial additional fruit flesh on the Yatu side of the fruit neck implies that there has been extra cell division and expansion in that region. If this is the case, then applied plant hormones that enhance cell division and expansion (cytokinins and gibberellins) could be expected to make Yatu larger, and materials such as prohexadione-calcium, paclobutrazol or abscisic acid (ABA) that reduce gibberellins would be expected to reduce the size of Yatu on fruit.
Chinese research
A recent research paper from China has reported on Yatu and the influence of applied hormones on the condition. It should be noted that this research is on Yali pear (Pyrus bretschneideri) and there is no guarantee that the same observations would apply to apples or European pears.
Method
The research paper looked at applying a gibberellin plus a cytokinin (GA3 + 6-BA) to enhance cell division and expansion and study the impact on Yatu development. In contrast, PAC – an anti-GA, ABA-promoting substance (probably paclobutrazol) – was applied to see if Yatu development could be inhibited. The plant hormones were applied 2 days before flowering and 10 days after hand pollination. At the time of application, the side of the fruit facing the centre of the panicle was marked (figure 3). Only the lowest flower in the panicle was studied as the researchers’ experience was that these flowers have a higher incidence of Yatu in the mature fruit. The remaining flowers in the panicle were removed to eliminate any fruit to fruit competition within the panicle.
Figure 3: The inner side of the inflorescence facing the centre of the panicle versus the outer side.
Results
This study found that, at maturity, Yatu developed almost exclusively on the outer side of the fruit, with very little developing on the inner side (figure 4). It also found that at 2 days before flowering and 10 days after pollination the outer side of the fruit had higher levels of GA3 and the cytokinin, zeatin, as well as slightly reduced levels of ABA. This implies that the small increase in GA3 and zeatin and the slightly reduced ABA content observed in the outer flesh of fruit 2 days before flowering and 10 days after flowering resulted in additional cell division and expansion causing the development of Yatu.
Figure 4: Location and incidence of Yatu on mature fruit and levels of GA3, zeatin (a cytokinin) and ABA 10 days after pollination. Letters indicate a reliable difference.
The application of GA3 + 6-BA at 2 days before flowering and again 10 days after hand pollination resulted in an increase in the number of fruit with an enlarged Yatu (figure 5). On the other hand, the application of PAC resulted in the complete elimination of the appearance of Yatu in mature fruit.
Figure 5: The impact of applied GA3 + 6-BA or PAC at 2 days before flowering and again at 10 days after pollination on incidence of small and large Yatu. Letters indicate a reliable difference. Red arrows highlight Yatu.
While the application of GA3 + 6-BA caused an increase of GA3 in the fruitlets, the data for PAC application is a little confused. This is because the PAC treatment not only increased the level of ABA in the fruitlets but also unexpectedly increased the level of GA3. Presumably the lack of Yatu in the PAC-treated fruit was either due to the elevated levels of ABA in the fruitlets or an inactivation of the GA3.
Research implications
These results indicate that products containing gibberellins, plus cytokinins which are applied at flowering for fruit thinning purposes, have potential to cause an increase in the size of Yatu encountered. It should be noted that in these trials GA3 was used while GA4+7 is used in fruit thinning products. Fortunately these products are not usually used to thin pears where Yatu is more common. GA is occasionally used on pears to improve fruit set however the decrease in fruit size and reduced return bloom have limited this practice so the impact on Yatu development has been minimal.
While anti-GA products are not registered for use on pears, prohexadione-calcium is registered for use on apples with an application timing around flowering. When this product is used on apples or pears it can be anticipated that there will be a reduction in the appearance of Yatu. Another product deserves mention here: abscisic acid (ABA). This plant hormone has become commercially available in recent years with a label use for advancing maturity and colouration of grapes. This product would potentially be useful for thinning apples and pears (see Further reading) and would potentially reduce the incidence of Yatu on pears.
Take home messages
- Yatu is the Chinese name for the lump beside the stem often observed in pears and occasionally in apples.
- With increased consumer demand for perfect-looking fruit, fruit with Yatu may become undesirable in the future.
- Some of the fruit thinning materials we currently use on apples may enhance Yatu development.
- The application of gibberellins for fruit set purposes on pears has potential to enhance Yatu development.
- The use of anti-GA sprays or ABA at flowering has potential to eliminate the appearance of Yatu.
Further reading
“The characteristic of Yatu morphogenesis and the efficacy of exogenous hormones on the development of Yatu during fruit development in ‘Yali’ pear (Pyrus bretschneideri Rehd.)”
“ABA chemical fruit thinning of Bartlett pears”
This article was first published in the Summer 2022 edition of AFG.
