Source Year: 2001

The importance of stomatal uptake of foliar applied nutrients into leaves was tested in annatto(Bixa orellana) in April 1996 and in orange (Citrus sinensis) and rubber (Hevea brasiliensis with a crown of H. pauciflora) in June 1998, in Amazonia, Brazil. 15N labelled NaNO3 (10 mmol for orange and rubber or 250 Mmol for annatto) and K2HPO4 (100 mmol) were applied during the day (open stomata) or shortly after sunset (stomata closed). Leaves and twigs were analysed for 15N and phosphate at 3 h after nutrient application. A significantly higher uptake of 15 nitrate (about 5 times in orange leaves) was found for the day treatment compared to the treatment at night in all species. Relocation of N from leaves to twigs was neglible. Differences were not detected for phosphate due to the reduced analytical sensitivity compared with 15N measurements. Humidity has been thought to promote foliar uptake at night, but the results suggest that stomatal closure at night hinders ion uptake.