Effects of boron (B) deficiency on reproductive development and grain set in wheat was studied in experiments in a sand culture in which grain set was increased by increasing B supply in the nutrient solution. Early vegetative response was also studied in a solution culture experiment with 5 μM B and without added B. Effects of B deficiency on the male and female part of the wheat flower were studied in a cross fertilization experiment involving B deficient and B sufficient wheat plants. An international trial (the Boron Probe Nursery) was conducted as a collaboration between Chiang Mai University, CIMMYT and National Agricultural Research Systems in various countries, to verify the B response in non-traditional, warm wheat-growing areas.

There was a wide genotypic variation in reproductive responses to B among the eight wheat genotypes studied. In sand culture with low B (0.2 μM), grain set index ranged from 9.5% in SW41 to 94.5% in Fang 60; with high B (10 μM) it was ≥90% in all genotypes. Early vegetative response to B was measured in the length of the youngest emerged blade at 12 days after sowing. Without added B the length of the leaf blade relative to that with 5 μM B ranged from 0.82 to 0.92. This indicates some variation in vegetative response to B among the genotpes. However, there was no relationship between vegetative and reproductive responses to B of the wheat genotypes.

Fertility of both the male and female part of the wheat flower appears to be affected by B deficiency. Ears from B deficient plants that were bagged to prevent cross fertilization set no grain. Cross pollination of B deficient female flowers with pollen from B sufficient plants resulted in only 28% grain set, compared with 94% percent from manual crossing of B sufficient pollen on B sufficient female.

Reponses to B application of SW41 and other sensitive genotypes at field sites of the first international Boron Probe Nursery (1990/91) confirmed that B deficiency can be a major cause of grain set failure in wheat in warm areas.