Is the Taste of Strawberry fruits (Fragaria x ananassa Duch.) affected by Ammonium and Nitrate Nutrition?
Strawberries are one of the most popular fruits. Since only tasteful fruits are attractive for the consumers the improvement of strawberry quality has been brought into focus over the last few years. The contributors to strawberry flavor are sugars, organic acids, and various volatile substances, making up the odor. Fructose, glucose, and sucrose represent 99% of the sugars. About 97% of the organic acids are citric acid and malic acid. In these studies we investigate the effect of the macronutrient nitrogen on the concentration of organic acids in strawberry plants and the fruits. Plant metabolism is differentially affected if nitrogen is taken up as NH4+ or NO3-. A high NH4+ uptake leads to a decreased pH in the cytoplasm and has to be stabilized by a proton excretion and by decarboxylation of organic acids by malic enzyme. In contrast, during NO3- reduction OH- is produced causing an increased cytosolic pH. This activates the enzyme PEP-caboxylase, which leads to an enhanced synthesis of oxaloacetate that is reduced to malate. Malate can be directly translocated into the vacuoles or incorporated into the cytoplasmic pool of the organic acids of the citric-acid cycle. The results of hydrophonic and soil experiments indicate that the N-form has an effect on the vegetative plant organs, but it does not affect the quality of strawberry fruits.