Pollination and Fertilization in Organic Seed Production

Organic Agriculture August 21, 2015 Print Friendly and PDF

eOrganic authors:

John Navazio, Organic Seed Alliance and Washington State University

Frank Morton, Wild Garden Seed

Micaela Colley, Organic Seed Alliance

Linda Brewer, Oregon State University

Alex Stone, Oregon State University

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 plant
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.

Crop Common Name

Crop Species

Primary Pollinating Mechanism(s)

Pollinating system

Wild Crossable Species in US

Onion
Allium cepa
insects
cross
N
Garlic
Allium sativum
insects
most are sterile
N
Garden Chives
Allium schoenoprasum
insects
cross
N
Garlic Chives
Allium tuberosum
insects
cross
N
Dill
Anethum graveolens
insects
cross
N
Celery
Apium graveolens
insects
cross
Y
Beet
Beta vulgaris
wind
cross
Y
Swiss Chard
Beta vulgaris
wind
cross
Y
Mustard
Brassica juncea
insects
cross
Y
Kale
Brassica napus
insects
cross
Y
Broccoli
Brassica oleracea
insects
cross
N
Brussels Sprouts
Brassica oleracea
insects
cross
N
Cabbage
Brassica oleracea
insects
cross
N
Cauliflower
Brassica oleracea
insects
cross
N
Collards
Brassica oleracea
insects
cross
N
Kale
Brassica oleracea
insects
cross
N
Chinese Cabbage
Brassica rapa
insects
cross
N
Mustard, Chinese
Brassica rapa
insects
cross
Y
Turnip
Brassica rapa
insects
cross
Y
Pepper
Capsicum annuum
self
self #3
N
Lambsquarters
Chenopodium album
wind
cross
Y
Escarole/ Endive
Cichorium endivia
self
self #2
N
Radicchio/ Belgian Endive
Cichorium intybus
insects
cross
Y
Watermelon
Citrullus lanatus
insects
cross
N
Cilantro
Coriandrum sativum
insects
cross
N
Armenian Cucumber
Cucumis melo
insects
cross
N
Cantaloupe Melon
Cucumis melo
insects
cross
N
Honeydew Melon
Cucumis melo
insects
cross
N
Musk Melon
Cucumis melo
insects
cross
N
Cucumber
Cucumis sativus
insects
cross
N
Pumpkin
Cucurbita pepo
insects
cross
Y
Winter Squash and Show Pumpkins
Cucurbita maxima
insects
cross
Y
Winter Squash
Cucurbita moshata
insects
cross
Y
Summer Squash and Fall Squash
Cucurbita pepo
insects
cross
Y
Carrot
Daucus carota
insects
cross
Y
Arugula
Eruca sativa
insects
cross
N
Fennel
Foeniculumvulgare
insects
cross
N
Lettuce
Lactuca sativa
self
self #1
Y
Gourds
Lagenaria siceraria
insects
cross
N
Tomato
Solanum esculentum
self
self #1/#2
 
N
Basil
Ocimum basilicum
insects
cross
N
Parsley
Petroselinium crispum
insects
cross
N
Lima Bean
Phaseolus lanatus
self
self #2
N
Common Bean
Phaseolus vulgaris
self
self #1
N
Pea
Pisum sativum
self
self #1
N
Radish
Raphanus sativus
insects
cross
Y
Turkish Eggplant
Solanum gilo
self
self #2
N
Eggplant
Solanum melongena
self
self #2
N
Spinach
Spinacea oleracea
wind
cross
N
Fava Bean
Vicia faba
self
self #2
N
Cowpea
Vigna unguiculata
self
self #2
N
Corn
Zea mays
wind
cross
N
 
 
 
 
 

Self #1: self pollinating species, outcrossing is usually < 1%

Self #2: self-pollinating species that often outcross between 2-5%

Self #3: self-pollinating species that may cross at rates > 5%

 
 
 
 
 

 

Web Resources

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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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.