Frequency of Aborted Pollen Grains and Microcytes in Guayule, Parthenium Argentatum Gray¹

Abstract

U SUALLY considerably less than so% of the commercial seed of guayule is viable. Hence, any information pertaining to the causes for this comparatively low· percentage of viable seed is of value in breeding this crop. For that reason, studies on the percentage of aborted pollen grains and frequency of microcytes (for definition of term see Belling and Blakeslee (r) 3 ) were made for collections of guayule, Parthenium argentatum Gray. Since the collections reproducing apomictically are pseudogamous (s), the information derived from such studies should facilitate the improvement of these forms also. MATERIAL AND METHODSThe collections employed in this study were grown from seed obtained from natural populations growing in Mexico and the Trans-Pecos Area of Texas. The Mexico collections were made by Powers and those from the Trans-Pecos Region by Powers and Federer. The commercial varieties were grown from seed taken from the same lots as those used in planting the commercial fields. The empty pollen grains and those showing definite disintegration of the cytoplasm were classed as aborted. Pollen grains having diameters less than 13.6 microns were classed as microcytes. In the case of aborted pollen the frequency recorded was the number in 100 f~ll-size pollen grains per plant and hence is the per.centage also; whereas, the m1crocytes recorded are the number among these same IOO full-size pollen grains and therefore are not a percentage.In staining the pollen for classification as to aborted pollen grains and microcytes, Watkin's cotton blue-lacto-phenol method as given by Lee (3) was used.The seed was planted in the greenhouse February I I, I943, "spotted out" March 8, and transplanted into the field at Salinas, Calif., on June 8 of the same year. After "spotting out" there were 72 plants per standard greenhouse flat. The design of the experiment was the same for greenhouse and field and was a 7 X 7 lattice squared with K + I replicates. Hence, there were 49 variates and 8 replicates. Four of the 49 variates were duplicates. The individual plot was 4 plants by 3 plants, making a total of 12 plants per plot. One hundred pollen grains were examined per plant and the pollen was collected on the basis of replications so as to control any differences due to time of collection. The plants in the field were spaced 4 feet apart in the rows and the rows were 4 feet apart.The analysis of variance was used in analyzing the data for testing significance of differences between means. Since the smallest average percentage of aborted pollen for any collection was not below IS and for most collections was I8 or above, and since the between-plot variances for the different collections were homogeneous, transformation of the data was not necessary. Because the majority of the values for microcytes were below ro, the transformation, VX + o.s, was used in analyzing the data for testing the significance of differences between means for number of microcytes among 100 pollen grains. X is the observed number. In calculating the variances ...