Preliminary Studies on Catkin Forcing and Pollen Storage of Corylus and Juglans

L. G. Cox, Cornell University

Methods of collecting and storing pollen are of great interest to those engaged in plant breeding. Very little reliable information is available for the various nut species compared with many other horticultural plants. The following preliminary experiments were conducted to obtain data on germination media, forcing methods, and storage conditions for Corylus and Juglans Sieboldiana pollen. The former was mostly from hybrid plants produced by crossing the Rush filbert (Corylus americana) with European varieties.

The optimum temperature and sugar concentration for germination of Corylus pollen.

The cut ends of Corylus branches with mature catkins collected March 1, 1942 were immersed in water and forced into shedding pollen in a room at a temperature of approximately 20° centigrade. The collected pollen was sifted upon the surface of a thin layer of sugar-agar in petri dishes.

Commercial cane sugar was used in preference to purified sucrose, because other studies have shown it to contain impurities which stimulate pollen germination. A range in sugar concentration from 5% to 55% by weight in 5% intervals was made up in distilled water containing 1.5% agar, heated to boiling and poured into the petri dishes.

The pollen was incubated at 10° C. and at 25° C. on the agar medium for 48 and 24 hours respectively prior to making the germination counts. Pollen was assumed to have germinated if the length of the pollen tube exceeded the diameter of the pollen grain.

At 25° C. germination was prompt and uniform with a maximum of 19.5% at 25% sugar concentration. At 10° C. the rate of germination was very slow and incomplete at the end of 48 hours with a maximum of 9% germination at 35% sugar concentration. For subsequent work a temperature of 25° C. and a sugar concentration of 25% by weight was taken as a standard.

The effect of temperature and humidity during forcing on the viability of the pollen

Pollen shed from catkins forced in a warm, dry room (about 75° F.), and in a cool, humid greenhouse (60° F.) gave pollen germinating 36% and 69% respectively, which indicated that the air temperature and humidity surrounding the developing catkins may have considerable effect on the viability of the maturing pollen.

The experiment was repeated by forcing the catkins at 10° C., 18-20° C., and 24-26° C., at two humidity levels. The low humidity level corresponded to the natural room humidity, about 25% and the higher level of nearly 100% was achieved by enclosing the branches with catkins in large sealed cans over a water surface. As soon as a majority of the catkins began to shed their pollen or to absciss their full developed anthers, the catkins were removed and dried on a sheet of smooth paper at room temperature until the pollen was shed. The pollen was then collected and stored at 4° C. until used. The results obtained are given in table 1.

Table 1. Percentage germination after 24 hours of Filbert pollen forced at different temperatures and humidities.

Temperature

10° C.18-20° C.24-26° C.

Low humidity80317

High humidity966012

Later experiments indicate that the pollen viability is greatly lowered if the catkins are removed from the higher humidities prior to the maturity of the anthers as indicated by their tendency to shed their pollen. Apparently the high humidity hinders the dehiscence of anthers and shedding of the pollen grains.

Effect of catkins extracts on pollen germination

The failure of pollen to germinate in the catkins at 100% humidity suggested the possibility that the catkin tissue might contain some substance which prevented germination of the mature pollen grains until after it was shed.

Two mature catkins plus remnants of their unshed pollen were ground in a mortar with a small amount of water in clear quartz sand. One cubic centimeter of the resulting turbid suspension was added to 10 cc. of warm fluid agar and mixed by rotating the petri dish.

Pollen which gave a 91% germination on the standard medium showed only 50% germination on this catkin extract. Germination was distinctly abnormal with short stubby pollen tubes, often with numerous nodular swellings. In general the pollen tube grew up into the air away from the surface of the agar, rather than down into it or parallel with the surface as in normal germination.

Storage of Corylus and Juglans Sieboldiana pollen

Sulphuric acid solutions to give humidities from 10% to 100% in 10% intervals were made up. The storage chambers consisted of Atlas one-pint, wide-mouth fruit jars. In the bottom of each was placed a small 1-oz. bottle containing 20 cc. of the sulphuric acid solution. The pollen was placed in small glass vials loosely stoppered with cotton.

Two lots of Corylus pollen of 80½ and 96½ initial viability respectively, and one lot of Juglans Sieboldiana pollen of well over 50% viability were used in the experiment. Storage temperatures of 0° 40° and 10° were used.

The Corylus pollen was placed in storage March 20, 1942, and the Juglans April 12, 1942. The pollen was taken out of storage November 28, 1942 and germinated on the standard agar-sugar medium at 25° C. for 24 hours. Results are given in table II.

Table II. The effect of storage temperature and humidity on percentage germination of Corylus and Juglans pollen

Kind of PollenTemperature CentigradeDegrees Per cent relative humidity

1020304050607080

Corylus10°0000000—

Juglans —0—0300—

Corylus4°00009.00—0

Juglans —0—0—000

Corylus0°3.01.04.58.50000

Juglans —0—12.0—12.000

This preliminary work indicates that Corylus pollen can best be stored at 0° C. at 30 to 40% relative humidity and Juglans pollen at 0° C. at 40 to 60% relative humidity.

Summary

1. The optimum sugar concentration for germination of Corylus pollen is around 25% by weight in 1.5 per cent agar at 25° C.

2. Forcing the catkins at a low temperature (4° C.) and at high relative humidity (80%) favors the development of a high percentage of viable pollen.

3. The catkins contain some substance which when added to the germination media inhibits pollen germination and causes abnormal types of germination.

4. Preliminary results on pollen storage indicate that Corylus americana pollen can be stored for eight months or more in a viable condition at 0° C. with a range of 30 to 40% relative humidity. Juglans Sieboldiana pollen can be stored at 0° C. at 40 to 60% relative humidity. Whether or not pollen stored for this length of time would be effective in plant breeding should be tested by actual trial. The supposition based upon studies with other pollens is that germination tests are a reliable indication of the effectiveness of pollen in fertilization.