Boron (B) is an essential micronutrient for plant growth and development of sunflower (Helianthus annuus L.). Most sunflowers growing areas in Thailand are in coarse texture soils with low B availability. The objectives of this experiment were (1) to evaluate the response of sunflower to B applications and (2) to determine the optimum B levels for sunflower under field conditions in sandy loam soil. This experiment was conducted at Suranaree University of Technology, Nakhon Ratchasima, Thailand, in January 2011. Two varieties of sunflower, “S473” and “Pacific 77”, were grown in sandy loam soil with available B of 0.14 ppm. Five levels of B (0, 3.13, 6.25, 9.38 and 12.50 kg B ha-1) were applied as borax (Na2B4O7.10H2O). The data was recorded for leaf B concentration, B uptake, pollen viability and seed set. They were highly correlated between pollen viability and seed set. Pollen viability and seed set of both varieties responded to B application, but pollen viability was more responsive. In addition, variety “S473” responded to B application more than “Pacific 77”. Regression analysis showed that the levels of B application produced maximum pollen viability ranged between 5.6 and 11.3 kg B ha-1, but B application at higher rates tended to decrease pollen viability and seed set.
Key words: Helianthus annuus L., borax, pollen viability, seed set, 2,3,5-triphenyltetrazolium chloride.
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