Malic Acid and Crassulacean Acid Metabolism

Oct 19

Introduction

Malic acid was first discovered in 1785 and later named “acide malique” - derived from the Latin word for apple, malum. It is the main acid in many fruits, especially in sour apples.

It is present in grapes and in most wines and imparts a tart taste. It is also a component of artificial vinegar flavors, like in "salt and vinegar" flavored potato chips !

Importance in photosynthesis

All plants photosynthesize during the day because they need sunlight. To do this they open their pores (stomata) and allow carbon dioxide into the leaf cell. But that is very risky to do if you are living in the desert - you might lose a lot of water !!!

In hot and dry environments some plants evolved the ability to capture their carbon dioxide at night, when the temperatures are cooler and the humidity higher. We call them CAM plants – which stands for Crassulacean Acid Metabolism.

During the night, CAM plants open their stomata to take in carbon dioxide. This carbon dioxide is then converted into malic acid and stored in the plant's vacuoles. This process leads to an increase in the acidity of the plant tissue.

When the sun rises, CAM plants close their stomata to minimize water loss. They then move the stored malic acid to the cytoplasm. Here, malic acid is broken down, releasing carbon dioxide. This released CO2 is then used in photosynthesis to make sugar - even with the stomata closed !

Who does this ?

Cacti, Agave (tequila), pineapple, Tillandsia (Spanish moss), corn, sugarcane, succulents, sedums, Euphorbia, Hoya, Kalanchoe, Sansevieria and many epiphytic orchids. Think hot and dry environments with infrequent rain like deserts, dry savannahs and arid coastal areas.

Importance in plant metabolism

Malic acid plays multiple roles in a plant’s life. Not only is it used in photosynthesis, but also plant growth, development, and stress response. It helps plants cope with environmental stresses like drought and high temperatures.

It's also a component of rhizosphere exudates – root sweat ! The acid is released into the soil and helps attract beneficial soil bacteria which can improve nutrient absorption and overall plant health. It also enhances root activity and growth.

Malic acid is also linked to amino acid metabolism in plant cells. Here it is made into oxaloacetic acid and then into various amino acids, including glutamine and lysine. These in turn help plants reduce toxicity from heavy metals in the soil.