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Evaluating Pollen Production of Plants (with Sample Calculations for Almonds)

American Bee Journal – April, 2001

by JOE TRAYNOR
Scientific Ag Co.
P.O. Box 2144
Bakersfield, CA 93303


Knowing how much pollen a plant species produced would be useful in ranking plants as a food source for both honey bees and wild bees. Beekeepers have a good general idea, as to whether a particular species is a good pollen source (or nectar source), but there is little quantitative information on the subject.


Past studies have estimated pollen produced per flower, but none have calculated pollen produced per acre. Obviously, the way to calculate pollen production per acre is to multiply the number of flowers per acre by the weight of pollen per flower. These two parameters are estimated below for almonds.

Flowers per acre
The flowers/acre of almonds will vary from year to year (a lighter flower set often follows a heavy almond crop and vice versa). Estimating the number of flowers per tree, then multiplying by trees/acre may be the best way. Trees/acre vary according to planting distance; the most common almond plantings are:

25′ X 25′ = 70 trees/acre
24′ X 24′ = 76 trees/acre
22′ X 24′ = 83 trees/acre

Dan Mayer estimated that an acre of apples produces 1 million flowers.(5) Almonds bloom more profusely than apples and 2 million flowers/acre for almonds is not an unreasonable guess. Two million flowers/acre would give the following flowers/tree for the above 3 planting distances: 28,500, 26,300 and 24,000.

Another method is to calculate flowers/acre from nuts/acre. About 3000 lbs. of nut meats per acre is considererd an excellent yield and to achieve such a high yield, a relatively high percent of flowers must be set. Kester & Griggs(4) obtained a 50% almond set (50% of the flowers set an almond) as did Hill, et al(3), although the latter study did not account for May (aka “June”) drop.

Assuming there are 350 nuts per pound (varieties vary in this regard; Nonpareil produces significantly larger nuts than Fritz) a 3000 lb/acre yield translates to 1,050,000 nuts/acre which in turn translates to 2,100,000 flowers per acre if a 50% set is assumed.

For this discussion, we will assume that an acre of almonds produces 2 million flowers (realizing that this figure can vary greatly from year to year).

Weight of pollen per flower
DeGrandi-Hoffman, et al(2) used sonication to remove pollen from almond flowers; Hill, et al(3) removed anthers from 100 flowers, dehisced the anthers at room temperature, then sieved and weighed the pollen. Both of these research teams came up with a range of around 0.7 to 1.2 mg of pollen per almond flower and both obtained year-to-year variations in pollen production. Oberle & Goertzen(6) also showed significant year-to-year variations in pollen production for a number of deciduous fruit species.

Traynor(7) used the method used by Oberle & Goertzen(6) to count pollen grains per flower: Put pollen (dehisced at room temperature from a flower’s anthers) in 2 ml of 10% calgon solution, shake and count the pollen grains in a small portion of the solution in the counting chamber of a hemacytometer (a simple glass slide used to count red blood cells; Spencer Bright Line, Clay Adams Cat. #@-2440/B).

Traynor obtained a range of 42,000 to 67,000 pollen grains per flower for eight commercial almond varieties and a figure of 44,000 pollen grains per flower for the Nonpareil variety, which comprises about 40% of California’s almond acreage. Traynor did not count the grains in a given weight of almond pollen. However, Bosch(1) determined that an almond pollen provision of Osmia cornuta (a mason bee), that weighed 16.5 mg., contained roughly 300,000 pollen grains. Pollen from one almond flower (44,000 grains for the Nonpareil variety, as calculated by Traynor) would weigh 2.4 mg, considerably higher than the 0.7 to 1.2 mg calculated by the DeGrandi-Hoffman and Hill teams. The “glue” that mason bees use to mold their provisions could have added enough weight to account for the difference.

Pollen per acre
Assuming 2 million almond flowers per acre, the figure of 1 mg of pollen per flower(2,3) translates to 4.4 lbs. of almond pollen per acre, while 2.4 mg/flower(1,7) translates to 10.7 lbs/acre. This is a wide range, but it is in line with the 5 to 8 lbs. of pollen per acre that commercial beekeepers report that they trap from almonds.

Summary
For well over 200 years, biological scientists have counted and classified things in order to achieve a better understanding of Nature. Thomas Jefferson and Charles Darwin were two of the more distinguished classifiers. Classifying plants by pollen production would be of practical benefit as well as of academic interest. From a practical standpoint, it would be helpful to know which wild plants provided the most sustenance for bees (both honey bees and wild bees) with the idea of making efforts to conserve such plants. Knowing the nutritional value of different pollens would also be helpful. Dr. Christine Peng (UC., Davis) and others have done some work on this, but overall information is limited.

Pollen production of plants that produce wind-borne pollen (e.g., Ragweed) is being determined from a public health standpoint(8), but pollen production data of plants producing insect-collected pollen are very limited. Perhaps the reason is that the subject falls squarely in the “no-man’s land” between the disciplines of plant science and apiculture, with either side leery of intruding on another’s territory. The pollen production calculations given above for almonds are simple and it is hoped that others will be encouraged to perform similar calculations on other plant species.

References
1. Bosch, J. 1994. The nesting behaviour of the mason bee, Osmia Cornuta (Latr), with special reference to its pollinating potential. Apidologie. 25: 4-93.

2. Degrandi-Hoffman, Gloria, Gerald Loper, Robbin Thorp and Dan Eisikowitch 1991. The influence of nectar and pollen availability and blossom density on the attractiveness of almond cultivars to honeybees. Acta Horticultureae 288, 6th Pollination Symposium.

3. Hill, S.J., D.W. Stephenson and B.K. Taylor 1985. Almond pollination studies: pollen production and viability and cross-pollination tests. Australian J. Exp. Agric. 25, 697-204.

4. Kester, Dale and W.H. Griggs 1959. Fruit setting in the almond: The effect of cross-pollinating various percentages of flowers and The pattern of flower and fruit drop. Proc. Amer. Soc. Hort. Sci. 74:206-213 and 214-219.

5. Mayer, Dan 1995. How to figure number of bee colonies needed. Good Fruit Grower, April 1.

6. Oberle, G.D. and K.L. Goertzen 1952. A method for evaluating pollen production of fruit species. Proc. Amer. Soc. Hort. Sci. 59:263-265.

7. Traynor, J. 1981. Use of a fast and accurate method for evaluating pollen production of alfalfa and almond flowers. Amer. Bee J. 121 1:23-25.

8. Ziska, Lewis and F. Caulfield 2000. Rising CO2 and pollen production of the common ragweed, a known allergy-inducing species: Implications for public health. Australian J. of Plant Physiology. 27:893-893.