Pollen: Biology Biochemistry Management (1974)
R.G. Stanley, H.F. Linskens

pp. 46-48

Factors Controlling Pollen Availability

Pollen quantities produced by individual plants are endogenously established and environmentally modified.

Genetic Controls. The most important factors determining the time of pollen development and dehiscence are inherited. Genotypic variation and blocks to natural hybridization are often established when pollen dispersal does not coincide with stigma receptivity of plants within pollinating distance (STANLEY and KIRBY, 1973). The time of pollen shedding in pine species and many other plants has been carefully followed and related to geographic distribution and elevation (DUFFIELD, 1953; SCAMONI, 1955). Pollen shedding in Pinus radiata during a single season in Australian plantations at one elevation occurred over a period of six weeks; the bulk of the pollen was dispersed during the second to fourth week (FIELDING, 1957). P. pinaster in Europe sheds pollen over a two- or three-week period (ILLY and SOPENA, 1963).

Dehiscence time depends in part on meteorological conditions which change from day to day. Under identical conditions every species has its specific time of anther dehiscence (OGDEN et al., 1969). Collecting insects are well adapted and quickly aware of this time. As an example, presentation time when pollen was available on a clear, sunny day is given in Table 4-2 (KLEBER, 1935). The relation of time of flowering to pollen collection by bees is also illustrated in Fig. 7-7.

In most plants pollen quality, as measured by percent germination in vitro, remains about the same throughout the period of natural dehiscence. Pollen isolated at the beginning and the end of the three weeks dehiscence period of a single cultivar of Arachis hypogaea showed no difference in viability (DE BEER, 1963). But pollen from potato plants showed optimum germination and viability at mid-flowering period (ERVANDYAN, 1964). In this latter study, the in vitro tests and the same sucrose concentration throughout the growing season. Variation frequently occurs in the external sugar concentration required to obtain optimum germination of pollen from the same plan at different times of the year. The results with potato plants can be interpreted to indicate, not that the mid-point of the pollen dispersal period gives pollen of maximum viability as tested in vitro, but that pollen growth requirements and endogenous nutrient status change during the plant's development period. Determinations of relative pollen viability most often require testing over a range of conditions.

Moving a pine tree from its normal geographical range to another latitude, at the same elevation, thus modifying the photoperiod, has relatively little effect on the time of pollen development and shedding (WRIGHT, 1962). This suggests pines are relatively day-neutral with strong genetic control of flowering time (MIROV, 1967). The same relative day-neutral response is not observed in all plants; many do not produce pollen when moved to shorter or longer photo periods than that in which they evolved. While in most plants dehiscence occurs in the day, in Lagenaria vulgaris and some other species, anthers dehisce in the evening or at night (PERCIVAL, 1950; VASIL, 1958).

External Factors.  a) Temperature and moisture are the primary elements affecting pollen development on mature plants. If one is collecting pollen from a given plant species and misses pollen dehiscence at one elevation or planting, it may be possible to find a similar genotype at a higher elevation or colder location where  the pollen has not yet been shed. MILLETT (1944) compared two genetically similar plantings of Pinus radiata separated by 1,800 feet of elevation in Australia. Pollen at the higher elevations started to shed after dehiscence at low elevations was completed. MILLETT (1944) also discussed the effect of an extended period of low temperature or high moisture in delaying dehiscence. Variations in temperature or moisture can shorten the interval of pollen dispersion of these trees from seven weeks to two weeks. The maximum dissemination of pollen in an oak plantation occurred on days when meteorological conditions were such that average day temperatures were not lower than 15° C, relative humidity was between 30 and 80% and wind velocity was 2 m/sec above the canopy in sun (ROMASHOY, 1957).

Low or high temperatures during the development period can adversely affect the quantity and germination response of mature pollen. CHIRA (1963) attributed low pollen production and fertility in Pinus sylvestris to low temperatures during development. Low temperature resulted in pollen abortion and inhibition of stamen development in Pennisetum clandestrium, although stigma development continued normally at the same low temperature (YOUNGER, 1961). This suggests that in this case the cool weather increased plant sterility by blocking pollen development, and that different flower organs, and in particular the stamens and pollen, are more sensitive to temperature during development than other parts of the flower. DE BEER (1963) observed that in peanuts, Arachis hypogaea, pollen on plants growing at a constant 33° C was only 10% viable, compared to 40% viability for pollen developed in greenhouses under a normal day-night temperature cycle. Rice plants grown with a root temperature of 28° C yielded pollen with higher viability than plants grown with roots at 23° C or 33° C (YAMADA and HASEGAWA,1959).

Moisture and nutritional status of the plant can also affect pollen viability and abundance. DOMANSKI (1959) found that barley plants, subjected to water stress, or grown in sub-optimal phosphorus levels, developed less pollen with decreased viability. Boron deficiency in the media or soil can also reduce viable pollen production.

Low light intensities or non-normal photoperiods during microsporogenesis can reduce sugar pools in leaves and anthers as indicated by chemical analyses, and a visible shift in the quantity of flavonoid pigments in anthers. In Ornithogalum caudatum, shading of leaves during pollen development resulted in a decreased quantity of viable pollen (GOSS, 1971). Temperature also influences substrate metabolism in maturing anthers and the amounts of pollen maturing (KIYOSAWA,1962).

b) Collecting time. When pollen was isolated from peanuts at different periods throughout the day it was observed that in the morning 35% of the pollen germinated, but 10 hrs later only 3% was viable. Similar observations of percent germination differences, although less dramatic, were made on Acer, Crataegus and Tilia trees near Leningrad. Pollen isolated in the early morning germinated better than that collected at other times of the day (KAUROV, 1957). While the reasons for such deviations have not been ascertained, they probably are related to metabolic transitions and moisture stresses. It may be meaningful, in this regard, to follow translocation of sugars in developing pollen.

p 92-95

Factors Influencing Pollen Collection

Available flora is only important after spring as a limiting factor to pollen collection. Temperature is the most important single factor (Fig. 7-5). No collecting activity can be observed by bees at temperatures below 10° C. However, above 10° C a correlation exists between temperature and the amount of pollen collected (LOUVEAUX, 1958a, b; 1959). Tropical bees collect pollen all year around (IBRAHIM and SELIM, 1962). Following the initial critical 10° C temperature, presence or absence of brood is next in importance in limiting pollen collecting activity. Above the critical temperature, light intensity may also be a limiting factor. In the temperature range of 13-21° C, light intensity seems to be most critical. Maximum bee activity occurs after solar radiation exceeds 3,600 calories per square centimeter per hr. On clear days bee flights begin at lower temperatures than on cloudy days.

Each colony shows its own particular behavior with regard to available flora. Some colonies follow complicated pollen collecting programs; others, simple ones. Bees prefer a source of pollen used previously. As expected, there is a correlation between flowering time and the main pollen species collected. This results in graphs which relate flowering time to the main collecting period of a pollen by bees, irrespective of the month of flowering (Fig. 7-6 b). Analysis of pollen loads from heterostylic plants, e.g. Fagopyrum, Lythrum and Primula show that bees may visit both flower forms on one flight (DAVYDOVA, 1954). Bees can be selected and bred which have a preference for one flower (MARTIN and MCGREGOR 1973).

The level of activity of bees, and when they discover a pollen source play large parts in establishing the collecting pattern. These factors are the basis of constancy in pollen collecting by bees (BETTS, 1935) and of the famous fragrance training of beekeepers by which an apiarist learns to detect the flower sources from the honey fragrance. The physical condition of the individual pollen grains and the nutritional value can also determine the selection of pollen by bees (LOUVEAUX, 1955; 1958a; b; 1959). Nitrogen content of pollen has been suggested as important in establishing preferences for certain pollen species. Oils, instead of nectar, are the bee attractants of some flowers. In Calceolaria the main oil component is a glyceride of acetoxypalmitic acid (VOGEL, 1971).

Specific chemicals in pollen may serve as attractants for bees. HUGEL (1962) isolated a mixture of steroids, in particular 24-methylene cholesterol (Chapter 10), which was suggested as the bee attractant. Since bees apparently cannot synthesize this sterol, which constitutes 72% of the sterol fraction in bee larvae (BATTAGLINI et al., 1970), pollen would provide a logical source. A free fatty acid, octadecatrienoic acid, and an ester of the flavone pigment lutein have been isolated from mixed bee collected pollen and found to be specific attractants to honey bees (LEPAGE and BOCH, 1968; HOPKINS et al., 1969; STARRATT and BOCH, 1971). It would be fortuitous if the primary chemical attractant for bees was localized in pollen, not just in nectar odor or flower color, as is generally assumed. DOULL and STANDIFER (1970) suggested that some volatile chemicals from pollen act as activating "release" stimulants. Such compounds stimulate the hypopharyngeal glands in nurse bees resulting in further uptake of pollen to feed the larvae. Pollens differ in their attractiveness and capacity to supply nutrients. In all cases tested the bees selected mixtures of pollen plus sugar solutions over sugar solutions alone (DOULL and STANDIFER, 1970; STANDIFER et al., 1973). Field applications of specific chemical attractants have many limitations. When the flower-and bee-derived chemicals, geraniol and nerolic acid were sprayed on flowers to improve pollination, the bees spent their time collecting the applied attractant from foliage instead of gathering pollen (ANDERSON and ATKINS, 1968).

Fig. 7-6b. Time of pollen availability by different flowers. (1-32 from PARKER, 1926; 33-40 from KLEBER, 1935)