This is an Organic Seed Resource Guide article.

Understanding the Reproductive Cycle of Seed Crops

There are several key reproductive steps in the life cycle of a flowering plant that results in adequate seed set. Pollination and subsequent fertilization of the ovules in the fruit are crucial processes in producing viable seed. There are a number of environmental challenges that can disrupt these processes and result in poor quality and quantity of a seed crop. Learning the basic steps of the reproductive process is very important in learning how to improve both the quality and quantity of the seed that you produce.

Squash flower pollination. Photo credit: Micaela Colley, Organic Seed Alliance.

Pollination: Self-pollinated or Cross-pollinated

Pollination, the movement of pollen from the anthers to the stigma, is essential for seed set and therefore crucial in seed production. An important consideration for any seed grower to know is whether the seed crop species that they are producing is predominately self-pollinated or cross-pollinated. Self–pollinated species have evolved to have perfect flowers that remain closed throughout the pollination process. These perfect flowers (bearing both male and female sexual parts on each flower) have anthers which are borne close to the stigma, allowing easy transfer of pollen to the stigmatic surface. This short journey for pollen to move from anther to stigma of the same flower (thus self-pollinating) often requires some external movement like wind to stimulate good pollen coverage of the stigma. In some selfers (runner beans and favas are good examples) insect visitation, even when the insect is unable to open the flower, can substantially increase seed set through their movement. Any grower producing a tomato crop in the greenhouse knows that the plants need a physical shaking or stiff air flow in order to achieve optimum pollination and subsequent fruit set on their crop.

Cross-pollinated species require genetic mixing between individuals of a population in order to remain genetically sound. All cross-pollinated crop species rely on either wind or insects (and occasionally animals) for pollen movement between individuals. All cross-pollinated crop species have flowers that open before pollen shed and receptivity. It is crucial that all of these species have adequate pollen availability during the flowering period. This requires; 1) a large enough population of the crop flowering in unison, 2) adequate insect populations present and visiting the crop or wind/airflow that is sufficient to move enough pollen for optimum pollination, and 3) environmental conditions that are such that pollen remains viable from the time of pollen shed until it reaches the flowers of other individuals in the population.

Fertilization and Seed Formation

The next step in this process leads to the fertilization of the ovules which become the seeds. After the pollen lands on the stigma, the receptive tip of the female parts of the flower, it must germinate and form a pollen tube which grows down through the style to reach the ovary. Each pollen tube that successfully reaches the ovary delivers one male gamete to fertilize an egg cell and one to fertilize the polar nuclei in a single ovule resulting in the formation of one seed. This fertilization event requires favorable environmental conditions and for fruit with multiple ovules a number of independent fertilization events must occur to insure good seed set. Once the embryo and endosperm form as a result of fertilization the seed undergoes a period of rapid cell division and growth. In most seed crops this growth and maturation of seed occurs in 40 to 60 days.

Problems in Pollination

Upon release of the pollen from the anthers, the environmental conditions must be such that the pollen grain remains viable until it reaches the stigma of a flower of the same species. If it is too hot, the pollen may be denatured; if it is too dry, the pollen may desiccate and lose viability before reaching a receptive stigma. If it is too cool or rainy, the activity of pollinating insects can be reduced; honey bees are especially sensitive to these conditions and will not fly when it is too cool or wet. Rainy conditions can also impede the movement of pollen in wind-pollinated species as it can wet the pollen as anthers open and wash much of the pollen to the ground or make it immobile in wind pollinated species.

Another condition that can impede pollination for all self-pollinated species or wind-pollinated cross-pollinated species is to have little or no airflow at the time of pollen maturation and release. In self-pollinating plants, the anthers are always borne in close proximity to the stigma within the closed flowers common to all selfers. In some cases it is so close that just the act of dehiscence (the opening of the anther to release the pollen) will cause the pollen to fall onto the stigma with little or no prompting. However, in many cases this short journey requires some type of external movement to literally shake the pollen from the anthers onto the stigma. In many selfing species (tomatoes, peppers, common bean, peas) this is easily accomplished when the plant is grown outdoors by the movement of the plant in the wind. In some selfers (runner beans and favas are good examples) insect visitation, even when the insect is unable to open the flower, can substantially increase seed set through their movement. Any grower producing a tomato crop in the greenhouse knows that the plants need a physical shaking or stiff air flow in order to achieve optimum pollination and subsequent fruit set on their crop. When wind-pollinated crossers like corn, spinach, or beets are grown in the absence of normal wind and airflow during flowering, several days of unusually still air can hinder full seed set (or random mating across the population for the genetic mixing that is essential for crossers) due to low pollen flow in the air.

Problems in Fertilization

From the time that the pollen comes in contact with the stigma, there are a number of problems that can arise. If the ambient temperatures are too high the pollen can become denatured and if it’s too low then the pollen will just sit until the temperature rises, although the flower’s receptivity is short lived. If the relative humidity is too low at this stage then the stigma or the pollen can desiccate, preventing the germination of the pollen. Low relative humidity has been found to be the culprit in a poor seed set for these reasons in a number of instances in vegetable seed production in the arid western states. The next step in the process of fertilization, the pollen tube growing down through the style can also be derailed by unfavorable weather conditions. The pollen tube is essentially a free living miniature plant (the gametophyte generation) and requires temperatures similar to the mother plant to grow vigorously. The pollen tube’s life cycle is usually 24 hours or less and it must make the trip from stigma to ovule in this period or not be successful in fertilizing the ovule. If the ambient temperature during this short period of time is colder or hotter than temperatures favorable to normal growth of the species than the pollen tube will stop growing and fail restart when the temperature comes back into a favorable range for growth. This will result in no fertilization for that particular pollen tube. In a cool loving crop like spinach that produces luxuriant growth at 58 – 65F (15 – 18C) and virtually stops growth at 78F (25.5C), this means that when it gets hotter than 78F (25.5C) as spinach seed crops are flowering there can be serious damage done to the yield due to poor fertilization of the ovules. Alternately, a heat loving crop like tomatoes can suffer blossom drop producing fewer fruit with low seed yields when tomatoes are exposed to cold night time temperatures during flowering.

Figure 1. Pollinating mechanisms and systems in common vegetable crops.

Web Resources

• Bees and pollination [Online]. C. Britton. Ohio Agriculture Research and Development Center. The Ohio State University. Available at: http://www2.oardc.ohio-state.edu/agnic/bee/pollinat.htm (verified 4 Jan 2009).
• Evaluating honey bee colonies for pollination: a guide for growers and beekeepers [Online]. M. Burgett. 1984. Pacific Northwest Extension Publication 245. Available at: http://extension.oregonstate.edu/catalog/html/pnw/pnw245/ (verified 4 Jan 2009).
• Pollination [Online]. Honeybee Program and Entomology Department. University of Georgia. Available at: http://www.ent.uga.edu/bees/Pollination/Pollination.htm (verified 4 Jan 2009).
• Insect pollination of cultivated crop plants [Online]. S.E. McGregor. Originally published in 1976. USDA Handbook 496. The first and only continuously updated virtual beekeeping book. Available at: http://gears.tucson.ars.ag.gov/book/index.html (verified 4 Jan 2009).
  
  • Economics of plant pollination
  • Flowering and fruiting of plants
  • Hybrid vigor in plants and its relationship to insect pollination
  • Wild bees and wild bee culture
  • Wild flowers and crop pollination
  • Pesticides in relation to beekeeping and crop pollination
  • Pollination agreements and services
• The Pollination Home Page [Online]. Pollination information and images. Includes state-by-state listings of beekeepers with bees available for contract pollination. Available at: http://www.pollinator.com/index.htm (verified 4 Jan 2009).
• The Xerces Society Pollinator Conservation Program [Online]. Contains excellent downloadable or print information on how to provide habitat for native pollinators on your farm. Available at: http://www.xerces.org/pollinator-conservation/ (verified 4 Jan 2009).

1. Farming for bees: guidelines for providing native bee habitat on farms [Online]. M. Vaughan, M. Shepherd, C. Kremen and S. H. Black. 2007. Available at: http://www.xerces.org/guidelines-farming-for-bees/ (verified 4 Jan 2009).
2. Pacific Northwest plants for native bees [Online]. M. Shepherd. Undated. Available at: http://www.xerces.org/wp-content/uploads/2008/11/pnw_plants_bees_xerces_... (verified 4 Jan 2009).
3. California plants for native bees [Online]. M. Shepherd. Undated. Available at: http://www.xerces.org/wp-content/uploads/2008/11/california_plants_for_n... (verified 4 Jan 2009).
4. Plants for native bees in the upper Midwest [Online]. E. Mader and M. Shepherd. Undated. Available at: http://www.xerces.org/wp-content/uploads/2008/11/xerces_bee_plants_upper... (verified 4 Jan 2009).
5. Using farm bill programs for pollinator conservation (draft) [Online]. M. Vaughn and M. Skinner. 2008. Available at: http://www.xerces.org/wp-content/uploads/2008/11 /using_farm_bill_programs_xerces_society.pdf (verified 4 Jan 2009).
6. Xerces Society fact sheets on native bee pollination of specific agricultural crops:
   
   • Native bee pollination of watermelon [Online]. C. Kremen, N. Williams, S. Greenleaf, and R. Thorp. Undated. Available at: http://www.xerces.org/wp-content/uploads/2008/10/factsheet_watermelon_po... (verified 4 Jan 2009).
   • Native bee pollination of cherry tomatoes [Online]. C. Kremen, N. Williams, S. Greenleaf, and R. Thorp. Undated. Available at: http://www.xerces.org/wp-content/uploads/2008/10/factsheet_cherry_tomato... (verified 4 Jan 2009).
   • Native bee pollination of hybrid sunflowers [Online]. C. Kremen, N. Williams, S. Greenleaf, and R. Thorp. Undated. Available at: http://www.xerces.org/wp-content/uploads/2008/10/factsheet_sunflower_pol... (verified 4 Jan 2009).

Print Resources

• Crop pollination by bees. K.S. Delaplane and D.F. Mayer. 2000. CABI Press, New York, NY.
• Insect pollination of crops. J.B. Free. 1993. Academic Press, London, UK, and San Diego, CA.
• Pollinator conservation handbook. 2003. The Xerces Society. Portland, OR.

This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.

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