Boron requirement for reproductive development in wheat

Abstract

The effect of boron (B) deficiency on reproductive development was examined in a wheat line SW41 at 4 B levels in a field experiment. The soil B levels (designated B0, B1, B2, B3) ranged from 0.1 to 0.2 mg hot water soluble B kg-1. No effect on vegetative growth was detected at any of the B levels, but the number of grains per ear increased with B, from 7 in B0 to 21 in B3; and Grain Set Index (GSI) also increased from 22 to 60%. At ear emergence, B content (mg B kg-1 DW) of the ear ranged from 2.2 to 3.1, and of the flag leaf from 4.1 to 4.7, but these bore slight to no relationship to the number of grams per ear or GSI. Higher B concentrations were found in the carpel, and higher still in the anthers. More significantly, grain set was closely correlated with B in the carpel (R2 = 0.86) and anthers (R2 = 0.77). The significant correlation between GSI and carpel and anther B was confirmed with a larger set of data. With 7 mg B kg-1 DW in the anthers and 5 mg B kg-1 DW in the carpel, the number of grains per ear was doubled when cross pollination with B deficiency tolerant Fang 60 growing near by was allowed. In ears with 9 mg B kg-1 in the anthers and 8 mg B kg-1 in the carpel the effect of cross pollination was smaller, but still significant. The critical deficiency concentration (CDC) of B for the anthers was determined at 10 mg B kg-1 and for the carpel at 8 mg B kg-1. Histological examination of SW41 anthers revealed that microsporogenesis in B-deficient plants proceeded normally up to at least the uninucleate, vacuolate stage. However, pollen maturation was severely affected resulting in shrivelled pollen grains. The programmed development of the tapetum and endothecial layers appeared normal in anthers where partial or complete sterility occurred. It is concluded that reproductive development in wheat requires more B than vegetative growth. The B requirement of the male reproductive organ, the anthers, was also greater than that of the carpel. Since the B requirement in the carpel for successful fertilization and grain set probably also reflects B requirement for pollen germination, it is suggested that B requirement for female gamete development may be lower than the apparent requirement of the carpel.