Yujin Hao
Shandong Agriculture University, China

Varieties and content of organic acid were one of important influencing factors on fruit flavor and fruit processing. Malic acid, as a main organic acid, occupied over 80% of organic acid in apple fruits. Otherwise, malic acid was involved in several metabolic pathways and played an important role in defense against stresses.
Phosphorus is a necessary nutrient element in plant development. The content of phosphorus was higher in soil, but most, in the form of insoluble phosphate, were not absorbed by plants. Organic acids, such as malic acid etc, were much higher compatibility to divalent and trivalent cations and replaced phosphate groups to release phosphorus for plant absorption. Fe in calcareous soils, usually in the form of hydroxides and polymers, was low solubility, so plants often appeared the symptom of Fe deficiency. Organic acid secreted from roots can increase the content of soluble Fe surroundings by chelation to meet the Fe demands of plants.
Aluminium is the most abundant metal in the Earth’s crust. Most Al are found in the form of Al(OH)3 or Al(OH)4- in soil, but Al3+ in acid soil poison to plants and affect plant development. However, organic acid, such as citric acid and malic acid can relieve injury of aluminium salt in plants.
Malic acid also affects the response to stresses by energy release and redox balance in tricarboxylic acid cycle, and regulating stomatal opening. Moreover, malic acid can change polar distribution of auxin transporter proteins by regulating PH in cell, and then modulate the auxin transportation and improve the growth condition of roots under stresses so as to increase the resistance to stresses,. In summary, malic acid is involved in not only fruit quality but also plant development and resistance in fruit crops.