Ethylene impact on grapevine pistil temperature and fruit set

Ethylene is known to stimulate plant respiration, and this later is always associated with heat generation. The grapevine fruit set is dependent upon pistil temperature, controlling the pollen germination and ovule fertilization. This led us to test whether ethylene would be able to impact fruit set often limited in cool climate conditions, particularly in the case of Vitis vinifera L. cv. Malbec, a variety prone to poor fruit set. With this cultivar, we ran trials in growth chambers using the ethylene gas, and field trials using ethephon as an ethylene precursor. At 14.5 {degrees}C ambient temperature, we observed that ethylene at 1 and 10 ppm increased the stigma and pistil temperature to 15 {degrees}C, compared to 14.7 {degrees}C in controls. This minor temperature rise was associated with an increase of fruit set from 10 to 25 %. The field trials, conducted in the South West of France, confirmed this trend. Indeed, spraying a concentration as low as 216 mg/ha of ethephon onto Malbec at full bloom at 16 {degrees}C in the shade, led to an increase of fruit set from 35 % to 45 %. These experiments suggest that spraying ultra-low concentrations of ethylene precursors onto the grapevines prone to poor fruit set, during cool mornings, could improve fruit set and reduce crop losses.