57TECHNOLOGY OCT | NOV 2013

SUMMARYPollination must first take place before fruit can develop. Most of the Japanese plum cul-tivars are self-incompatible. For adequate pollination to take place it is very important to plant suitable cross pollinators in the cor-rect ratio of cultivar to pollinator to ensure an economically viable fruit set. Data from the ARC’s Bien Donné Research Farm as well as commercial farms are presented to indi-cate which cross-pollinators and ratios are required for most commercially grown Japa-nese plum cultivars planted in early and late maturing areas. At least 10% of the blossoms of the trees must be open before bee hives are moved into the orchard. Most pesticides are toxic to bees and should not be sprayed in the orchard during flowering time. The effec-tive pollination period (EPP) influences fruit production and is a useful parameter to ana-lyse and detect factors limiting fruit set.

INTRODUCTIONThe release of a new Japanese plum variety to the fruit industry raises questions regarding the pollination requirements of the variety. Is it self-pollinating or does it require a cross-pol-linator to help ensure a good fruit set of quality fruit? If the variety needs a cross-pollinator, which one will be the most suitable? In what ratio must trees of the variety and the cross-pollinator be planted? These questions, the bi-ology of fruit set, as well as the role of honey bees in the pollination process of Japanese plum cultivars, will be discussed in this article.

FLOWERSFlowers of the Japanese plum (Prunus sali-cina) variety are each about 2.5 cm wide and consists of five petals, several stamens with anthers (male part of the flower), a one com-partment ovary (two ovules inside) with a

style and a two-lobed stigma (female part of the flower). Only one of the two ovules usu-ally develops. Stamens are longer, shorter, or the same length as the stigma depending on the variety. The stigma is receptive as soon as the flower opens and the flower stays fully open for three to five days. Flowers in a tree and between trees in an orchard can flower over a period of time depending on the cli-mate and environmental conditions. Anthers usually release pollen only after the flower is fully open. The flowers will soon drop from the tree if they are not pollinated. Apart from the absence of sufficient bee activities during the EPP, unfavourable weather conditions like low temperatures, rainfall and strong wind, have an adverse effect on set and opti-mum fruit production.

COMPATIBILITYIn order for fruit to develop, pollination must first take place during the flowering season. Pollination is the transfer of pollen

from the anther of a flower to the stigma of the same flower, or other flowers on the same tree, or flowers on a tree of a different variety resulting in fertilisation. Some plum cultivars are pollinated with their own pol-len and are therefore classified as self-com-patible (self-pollinating). For the release of new plum cultivars, the detection of the S-alleles (genes that prevent self-pollination) has become important in order to determine the most suitable cross-pollinator for a par-ticular cultivar in a specific climatic zone. Most of the Japanese plum cultivars require viable pollen from a different Japanese plum variety to be pollinated (cross-pollination) and are therefore self-incompatible. Cross pollinator(s) for a specific plum variety may differ in the flowering period between areas due to environmental differences, affected by the specific chilling and heat require-ments of cultivars. A self-incompatible vari-ety and its cross-pollinator(s) should flower at more or less the same time in order for

Pollination Requirements of Japanese Plum (Prunus salicina) Cultivars JANNIE DE KLERK & CHRIS SMITH

Figure 1: Artificial pollination of two plum trees in an enclosed structure.

58TEGNOLOGIE OKT | NOV 2013

Figure 3: Excessive fruit on the plum trees after pollination are harvested, counted and weighed.

cross-pollination to take place and to ensure good fruit set. The ideal situation would be for the donor variety to start blossoming prior to the receiving variety in order to pollinate the early blossoms which normally provide the better fruit.

PLANTING RATIOWithout adequate pollination, the fruit set of a plum variety may be insignificant or even absent, resulting in financial losses to the producer. This is, however, not necessarily an indication of the yield potential of the variety

and in most cases can be rectified by plant-ing the correct cross pollinator for a specific area in a recommended ratio. The two ques-tions in respect of a self-incompatible Japa-nese plum variety are: which cross-pollinator must be used and in what ratio must trees of the variety and the cross-pollinator be planted. It is essential that producers realise the importance of planting not only suitable cross pollinators, but also planting the cor-rect ratio of cultivar to pollinator to ensure an economically viable fruit set. This can also differ geographically as previously indicated.

To determine the planting ratio for a new cultivar, the total fruit set (number of fruit thinned to commercial standard plus the number of fruit harvested) under controlled

Figure 2: Development of fruit on the plum tree after successful pollination.

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59TECHNOLOGY OCT | NOV 2013

conditions as well as the percentage fruit thinned, must be calculated. This calculation is usually done by artificial cross- or self-pol-lination of two trees by honey bees by isolat-ing the two trees from foreign pollen in an enclosed structure (Figure 1, 2 and 3).

Cross- and self-pollination trials are routine-

Cultivar Cross-Pollinators for Cross-Pollinators for Ratio ** Early Maturing Areas (<600 ICU*) Late Maturing Areas (>600 ICU*)

African DelightTM Pioneer Pioneer 1:10

African Rose™ Self-compatible Self-compatible -

Angeleno Sapphire, Harry Pickstone, Pioneer, Fortune Santa Rosa, Souvenir, Larry Anne, Laetitia, Gaviota 1:8

Casselman Self-compatible Self-compatible -

Early Santa Rosa Self-compatible Self-compatible -

Flavorking Sapphire, Harry Pickstone, Santa Rosa Laetitia, Songold 1:12

Flavorich Harry Pickstone Sapphire, Fortune 1:12

Flavor Queen Harry Pickstone Sapphire, Ruby Red 1:12

Fortune Harry Pickstone, Angeleno, Souvenir 2 Souvenir 2, Santa Rosa, Angeleno, Lady Red 1:12

Gavearli Santa Rosa Santa Rosa 1:12

Gaviota Santa Rosa Santa Rosa 1:12

Golden Kiss Sapphire, Angeleno Sapphire, Angeleno 1:12

Harry Pickstone Self-compatible Self-compatible -

Lady Red Laetitia, Songold Songold, Laetitia 1:12

Lady West Sapphire, Laetitia Laetitia 1:12

Laetitia Songold, Lady Red Songold, Casselman 1:12

Larry Anne Songold, Laetitia Songold, Angeleno, Santa Rosa 1:12

Pioneer Self-compatible Self-compatible -

Reubennel Self-compatible Self-compatible -

Purple Majesty Harry Pickstone, Sun Kiss Sun Kiss, Laetitia 1:12

Ruby Red Sapphire, Souvenir, Laetitia Songold, Laetitia 1:12

Ruby Star Self-compatible Self-compatible -

Santa Rosa Self-compatible Self-compatible -

Sapphire Harry Pickstone, Fortune, Angeleno Harry Pickstone, Souvenir, Sun Kiss 1:12

Songold Laetitia, Ruby Red, Lady Red Laetitia, Ruby Red 1:12

Southern Belle Songold, Laetitia Songold, Laetitia -

Souvenir Sapphire, Harry Pickstone Santa Rosa 1:12

Sun Breeze Sapphire, Laetitia, Ruby Red Laetitia, Ruby Red, Sun Kiss 1:12

Sun Kiss Harry Pickstone, Sundew, Sapphire Laetitia, Sundew, Sapphire 1:12

Sundew Harry Pickstone, Sundew, Sapphire Laetitia, Sun Kiss 1:12

Sunset Songold Larry Anne 1:12

Sun Supreme™ Sun Breeze, Sun Kiss, Golden Kiss, Songold Golden Kiss, Laetitia, Reubennnel 1:12

Sensation Self-compatible Self-compatible -

* ICU = Infruitec Chilling Units. ** = Cross-pollinator to Cultivar ratio.

Table 1. Update of the general cross-pollination chart regarding the different commercial Japanese plum cultivars planted in South Africa.

ly conducted annually at Bien Donné Research Farm (Simondium area) in the Western Cape on all promising newly bred ARC Japanese plum cultivars and the results are summarized in Table 1. This tabulation of pollination data is an ongo-ing process which is supported by field observa-tions in the industry on farm level.

POLLINATION BY HONEY BEESHoney bees are an important vector for pollinating plum trees in South Africa. The fact that honey bees are not attracted to the blossoms of certain plum cultivars creates a major problem for producers. Bees prefer to forage in the flowers of other plant species

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before turning to the flowers of plum trees. If possible, any competing flowering plants in the plum orchard should be eliminated in or-der for the bees to concentrate on the flowers of the plum trees. At least 10% of the blos-soms of the trees must be open before bees are moved into the orchard. This is to keep the bees working in the flowers of the plum trees and not lured away by more attractive flowers of plants nearby. Plum flowers are the most receptive to pollination as soon as they have opened. Under cool, moist condi-tions, plum flowers may not be the most re-ceptive until 24 hours after opening.

Bees work best when temperatures are above 18ºC, wind speed is lower than 38 km/h and if there is more than 60% full sunlight.

Cool weather, rain or wind may prevent bees from leaving their hives, and reduces pollen tube growth which in turn reduces fertili-zation and fruit set. Unfavourable weather conditions during the pollination period are a problem that producers have to contend with. Most pesticides are toxic to bees and should not be sprayed in the orchard during flowering time.

EFFECTIVE POLLINATION PERIODThe effective pollination period (EPP) con-cept was developed by Williams (1965) and is defined as the number of days during which pollination is effective in producing fruit (in-terval of time during which fertilization can occur). This is determined by the longevity of

the ovules minus the time lag between polli-nation and fertilization. EPP plays a clear role controlling cropping in fruit trees and is a useful parameter to analyse and detect factors limiting fruit set. It is influenced by three main processes: stigmatic receptivity, pollen tube growth rate and ovule longevity. These three processes are in turn influenced by tempera-ture, flower quality and chemical treatments.

S ALLELEThe S-allele or self-sterility genes are genes that prevent self-pollination in flowering plants. This is a genetic phenomenon breed-ers have to deal with in order to secure a good pollination ratio for all new cultivars. S-alleles control the growth CONTINUED ON PAGE 62

61TECHNOLOGY OCT | NOV 2013

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62TEGNOLOGIE OKT | NOV 2013

of the pollen tube, thus causing male sterility and governing compat-ibility relationships. Molecular methods to detect these S-alleles are becoming increas-ingly available. This would be helpful to pre-dict compatibility relationships in Japanese plum cultivars and needs further investiga-tion.

CONCLUSIONFor the fruit farmer, it is important that there should be the optimum number of fruit on the tree at harvest time. Besides other factors, optimal fruit production is obtained with the correct pruning method as well as proper thinning of flowers and/or fruit.

To obtain a good fruit production with most of the Japanese plum cultivars, suffi-cient pollination in addition to above men-tioned factors, must occur. Therefore it is im-portant to plant the correct cross-pollinator in the correct ratio to the cultivar. Ideal con-ditions for the bees should prevail to ensure that it can work optimally and that adequate pollination can take place.

There are a number of factors that influ-ence the production of Japanese plums. If one of these factors is absent or not optimal, fruit production will be adversely affected.

ACKNOWLEDGEMENTSSincere thanks to SASPA for partial funding of the research project, Mike Allsopp of the Plant Protection Research Institute of the ARC for essential advice, as well as supply-ing the bees and beekeeping equipment, and Pieter Quixley, farm manager at Bien Donné Research Farm (ARC Infruitec-Nietvoorbij) for handling the bees.

REFERENCESWilliams, R.R. (1965). The effect of summer nitro-gen applications on the quality of apple blossoms. J. Hort. Sci. 40: 31-41.

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