Effects of brassinosteroids on walnut trees fruit set, nut weight and pollen germination

Homobrassinolide (HBR) is a plant growth regulator hormone that may be used to increase crop yields. To study the effects of HBR on the reproductive physiology of walnut cultivars, HBR was applied to ‘Serr’ and ‘Chandler’ walnut trees in Chile during full bloom. Parameters tested were fruit set, shelled nut weight and whole nut weight. Experimental treatments consisted of foliar sprays of solutions in concentrations of 1, 10, 50 ppm HBR and a treatment of 10 ppm HBR +10 ppm CPPU (a synthetic cytokinin). Foliar spray of water was used as the control. These solutions were then incorporated into a culture medium where their effects on pollen germination and pollen tube growth were evaluated in vitro. Fruit set was measured by the difference between the number of fruits harvested and the number of flowers sprayed during late flowering. Fruit set percentages and nut weights (shelled and unshelled) were both improved with all HBR treatments when compared to the control. Application of HBR to ‘Serr’ and ‘Chandler’ cultivars increased fruit set percentage from 32.5 to 45.9%, and 8.4 to 16.8%, respectively. The addition of CPPU did not result in a significant improvement in either fruit set or nut size. Regarding the in vitro experiments with pollen, the use of HBR 10 ppm significantly improved the elongation of pollen tubes, although germination was similar with all treatments.