Pollen, the microscopic structure containing male genetic material, has a naturally short lifespan. Fortunately, pollen from male plants can be collected, preserved, and stored for extended periods. Saving pollen allows for controlled breeding programs, genetic preservation of desirable male traits, and the ability to overcome timing issues when parent plants flower at different times. This technique improves the efficiency of hybridization efforts and helps maintain genetic diversity.
Harvesting and Preparing Pollen for Storage
The process of saving pollen begins with careful harvesting. Male plants produce pollen sacs, or anthers, which must be monitored closely to identify the optimal time for collection, usually just as they begin to open and release their fine, powdery contents. The best time is typically about a week after the first male flowers start to open, though this timing varies between plant varieties.
One effective collection technique involves gently shaking the male flower clusters over a clean surface, such as parchment paper or a tray. Alternatively, cut whole branches bearing mature pollen sacs and place them over a collection surface in a dry, dark area for a day or two. Any debris, like flower fragments or leaves, must be removed from the collected powder using a sieve or tweezers, as plant material introduces unwanted moisture.
Reducing the pollen’s moisture content is the most important preparation step. Fresh pollen can contain 20–30% moisture, which must be lowered significantly for successful storage. The collected pollen should be spread thinly and left to air dry in a clean, dark, and low-humidity environment, ideally below 45% relative humidity, for at least 12 to 48 hours. This initial curing removes excess water before sealing the pollen for long-term preservation.
Principles of Long-Term Pollen Viability
Pollen grains remain metabolically active after release, causing them to degrade quickly if not stored properly. Viability, the ability of the pollen to successfully fertilize a female plant, is determined by its biological characteristics and environmental conditions. Pollen loses viability primarily because metabolic processes consume internal energy reserves, such as starches and sugars. High temperatures increase this metabolic rate, accelerating the consumption of reserves and leading to rapid decay.
The two main threats to stored pollen are high moisture and high temperature. Excess moisture can cause the pollen grain to prematurely germinate or promote the growth of mold and bacteria, rendering it useless. Many types of pollen are desiccation-tolerant, meaning they can be successfully dried to a low water content without losing their ability to function. This tolerance allows the pollen to enter a state of metabolic arrest, which is the key to long-term preservation.
Controlling the external environment by lowering both temperature and moisture dramatically slows the pollen’s internal metabolic activity. The goal is to reduce the water content to a level, often between 5% and 20% depending on the species, that prevents ice crystal formation during freezing and minimizes metabolic decay. Understanding this balance between desiccation and metabolic slowdown is fundamental to achieving successful long-term storage.
Methods for Successful Pollen Storage
Successful storage depends on maintaining the low-moisture state achieved during preparation and consistently regulating temperature. For short-term storage, which extends viability for a few days up to several weeks, simple refrigeration is the best practice. Pollen should be placed in an airtight container, such as a glass vial or small plastic tube, and stored at a consistent temperature between 0°C and 10°C (32°F and 50°F).
To manage humidity within the container, a desiccant should be included to absorb residual moisture. Food-grade silica gel packets or a few grains of dry, uncooked rice can serve this purpose effectively. The container must be fully sealed to prevent the introduction of outside humidity. This method is suitable for breeders who need to stagger pollination or synchronize parents that flower a few weeks apart.
For long-term preservation, extending viability for months or years, freezing is the preferred method. The prepared, dried pollen should be sealed in an airtight container and placed in a freezer at -18°C (0°F) or colder. Cryopreservation, using ultra-low temperatures like liquid nitrogen (-196°C), can virtually halt all metabolic activity and preserve pollen for decades.
Proper labeling with the date and source of the pollen is necessary regardless of the storage method. It is important to minimize temperature fluctuations, especially in a standard freezer, because thawing and refreezing cycles cause condensation inside the container. When a frozen container is retrieved, it must remain sealed until it has fully warmed to room temperature to prevent condensation from forming on the cold pollen powder.
Applying Stored Pollen for Breeding
The final stage is the application of the stored pollen to achieve fertilization. If the pollen has been frozen, the sealed container must warm slowly to room temperature before opening. This gradual warming prevents ambient moisture from condensing on the cold pollen grains, which would instantly reduce viability. The warming period can take an hour or more, depending on the container size and temperature difference.
Pollen is best applied when the female stigma, the receptive part of the flower, is at its most receptive stage. This period is often signaled by the stigma appearing moist, sticky, or fully open. Application methods are simple and often involve dusting the pollen directly onto the receptive stigma.
A small, fine-tipped paintbrush, a cotton swab, or a powder puff can be used to gently transfer the pollen from the container to the female flower parts. For larger-scale application, the pollen can be mixed with a non-hydrating diluent, like dry cooking flour or Lycopodium spores, to increase volume and ensure even distribution. Breeders may also use localized spraying or brushing to target specific flowers, minimizing the amount of pollen needed and preventing unwanted cross-pollination.