Pollen shedding and dispersal in the castor plant Ricinus communis L

Studies were conducted with the castor plant at Stillwater, Okla., to determine the time of day of pollen shedding, the relative amounts of pollen dispersed in the air at various distances from their source, and the number of days that the staminate florets shed pollen. To obtain data on the first two phases of pollen shedding, vaseline-coated microscope slides were so exposed in 1948 as to intercept air-borne pollen. During the pollen shedding period, pollen was shed daily from about 4:00 a.m. until 8:00 p.m.; however, heavy shedding usually lasted from only 7:30 until 11:30 a.m. The fluctuation in shedding which occurred in the afternoon apparently was due to variation in wind velocity. Pollen shedding usually began at a temperature near 75 degrees F and a relative humidity of 70%. It reached its peak between 80 degrees and 86 degrees F and at a humidity of 60%. A temperature of 60 degrees F appeared to delay shedding. Apparently shedding was normal on partly cloudy days. With prevailing southerly winds, most of the pollen was dispersed toward the north from the field under study. Based on the 387 pollen grains caught per square inch in the center of the castor plant field as 100%, approximately 21% as much pollen was caught 5 rods north of the field, 8% at 15 rods, 4% at 25 rods, 2% at 40 rods, and 1% at 60 rods. A rapid drop in the average number of pollen grains caught at all heights as the distance from the field increased indicates that castor pollen may be classed among the heavier pollen. Some vertical dispersion occurred. The chances of maintaining genetic identity are much greater when the seed is produced under an isolation of 60 rods as compared to 25 rods or less. Less isolation might suffice in areas where the wind velocities are lower than in Oklahoma. The staminate flowers on a raceme shed pollen for an average of 15.67 +/- 0.34 days in August, 1947, and 16.32 +/- 0.39 days in June, 1948. A correlation coefficient of -0.18 was found between the number of staminate florets and the length of time required for the raceme to complete pollen shedding. This would indicate that lines with different numbers of flowers per spike could be developed to shed pollen over a long or short period.