Establishing Pollination Requirements in Japanese Plum by Phenological Monitoring, Hand Pollinations, Fluorescence Microscopy and Molecular Genotyping

This protocol is becoming increasingly important due to the high number of new self-incompatible Japanese plum cultivars with unknown pollination requirements. The main advantage of this technique is that it allows for the determination of the pollination requirements of cultivars by combining two complimentary approaches in field and laboratory conditions. This approach can also be applied to determine pollination requirements and other self-incompatible fruit tree species like apricot or sweet cherry.

For pollen extraction, collect flower buds at balloon stage according to stage 57 on the BBCH scale. Remove the anthers using a plastic mesh and place them on paper at room temperature for 24 hours until anther dehiscence. Sieve the pollen grains through a fine mesh and conserve them in a 10 millimeter glass tube with a cap at four degrees Celsius until use.

When 10 to 20%of the flowers are open, select and label several branches, leaving 1, 000 to 1, 500 flowers per pollination treatment. On the next day, when more flowers are open, pollinate each flower using a paintbrush with pollen from the same cultivar for self-pollination and pollen from other cultivars as cross-pollination control. Pollinate every day until all flowers are open.

Record weekly counts of flowers and developing fruits from anthesis to harvest to characterize the fruit drop pattern and quantify the final fruit set in each pollination treatment. Collect 50 to 100 flowers at balloon stage according to stage 57 on the BBCH scale. Emasculate 30 flowers per treatment in the laboratory for self and cross-pollination.

Make a fresh cut on the base of each flower pedicle under water and place it on a piece of wet florist foam. Use one piece of foam for each pollination treatment. After 24 hours, hand pollinate each pistil with the pollen collected previously using a fine paintbrush.

Pollinate one set of pistils with pollen from the same cultivar and the other set with pollen from another compatible cultivar as control. Fix these pollinated pistils in a fixative solution of ethanol and acetic acid for at least 24 hours at four degrees Celsius. Spread the pollen of each cultivar previously used as a pollen donor in the pollinations in two Petri dishes and incubate them at 25 degrees Celsius for 24 hours.

To observe the pollen grains, add two to three drops of aniline blue solution to each Petri dish and observe them under a UV epifluorescence microscope after five minutes. Count viable and non-viable pollen grains in the three fields per Petri dish with each field containing about 100 to 200 pollen grains in two Petri dishes for each cultivar. To observe the pollen tubes, squash softened pistils in a drop of 1%aniline blue solution under a cover glass on a slide.

Observe the pollen tube growth along the style under a microscope with UV epifluorescence using exciter filter BP340-390 and barrier filter LP425. Collect three to four young leaves from each cultivar in the field and extract DNA from the leaves. Set up a master mix for each pair of primers in 1.5 milliliter micro tubes.

Add all reagents and mix thoroughly. Aliquot 16 microliters of this master mix into each 200 microliter PCR tube containing four microliters of DNA template or control. Mix them gently, close the caps and centrifuge at 2, 000 times G for 30 seconds.

Place the tube in the thermocycler and run the PCR program as mentioned in the text manuscript. For estimating the fragment size by gel electrophoresis, load the first and last wells with two microliters of the DNA ladder and three microliters of the PCR product in the other wells. Close the chamber and run the gel at 100 volts for 30 minutes.

Observe the gel under UV light using a gel imaging system. Use the DNA ladder to estimate the size of the amplified fragments and compare it with the positive and negative controls. Monitor the phenology of different trees of each cultivar over different years of flowering.

Establish the length of the flowering period from the first to the last open flowers. Full bloom is considered when at least 50%of flowers are at stage F according to stage 65 on the BBCH. The pattern of fruit drop of hand pollinated flowers after emasculation or supplementary pollination showed that most flowers dropped two to three weeks after pollination, but 4%of self-pollinated flowers remained in the tree, indicating that this cultivar is self-compatible.

Viable pollen grains showed a pollen tube longer than its length after 24 hours in the culture medium and non-viable pollen grains did not germinate. During pollination, pollen grains were transferred to the stigma. Each germinating pollen grain produced a pollen tube, which grew through the pistil structures.

In self-incompatible cultivars, the pollen tube stopped growing in the upper third of the style. However, in self-compatible cultivars, the pollen tube could grow through the style, reach the ovary and fertilize an ovule. PCR analysis was carried out using primers from the conserved regions of sRNAs of sweet cherry and Japanese plum, making it possible to determine the size of S-alleles in each cultivar.

The length of the flowering period for a total of four years was calculated. Flowering in orchard conditions allowed the determination of which cultivars are coincident at flowering time each year. When attempting this protocol, make sure to use a higher number of flowers of Japanese plum for hand pollinations since the percentage of fruit set is very low compared to other Prunus species.

It is not possible to determine self-incompatibility by molecular approaches. Further work of S-allele sequencing is required to assess if some S-alleles are associated with self-compatibility. The combination of field and laboratory approaches has resulted in valuable information for the design of new commercial orchards and can be used in other fruit tree species.