Winemaker’s Bookshelf By Gary Strachan The intricacies of intracellular pH control


Uptake of nutrients into plant cells would change their pH if some balancingmechanism were not present.


E


very winemaker is aware of the importance of pH control during winemaking. This book goes far beyond the typical implications of wine production pH control and adjustment. It deals with the mechanisms that maintain a constant pH in plant cells. Buffers, which resist changes to pH by reversibly disassociating cations, are familiar to winemakers. A weak acid, such as malic acid, in the presence of a strong base such as potassium, is one of the most familiar buffer systems of wine.


This type of buffer, along with others such as phosphate, are active in maintaining a tightly controlled intracellular pH, but buffers alone do not account for the tight control of plant cell pH. This book is now old enough that some of the unsolved problems have now surely been investigated, but it is an excellent introduction to the subject. A detailed account of the known mechanisms is presented in Rengel, Z. 2002, editor, Handbook of Plant Growth: pH as the Master Variable. Marcel Dekker. 433 pp. ISBN -0-8247- 0761.


Hydrogen ions are central to everything that happens in a plant cell. The uptake of nutrients would change cell pH if some balancing mechanism were not present.


Every major nutrient or trace element has a charge and would change cellular pH if there were no compensating mechanism. The pH of plant cell cytoplasm remains at an almost constant 7.2 pH in spite of all internal or external factors.


Various plant species are adapted to grow in soil pH ranging from very acidic, around pH 3, to very alkaline, up to pH 10, yet the cytoplasm remains close a constant 7.2 pH.


Most of the cellular processes of photosynthesis, respiration, and growth release hydrogen ions. The hydrogen


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ion is the smallest atom, a single proton. It can pass through membranes more readily than other larger atoms or ions. In addition to


hydrogen’s ability to diffuse, it can pass


through


cell membranes against a concentration gradient with the assistance of an energy requiring system, the proton pump. There are many variations of proton pump that occur throughout the plant kingdom. Even within a single species there may be ten or more variations of proton pump associated with different organelles such as the


cytoplasmic membrane, vacuole membrane, mitochondria, or endoplasmic reticulum.


The enzymes that enable all cellular processes are composed of amino acids, all of which contain side chains specific to each amino acid. Most side chains have a partial charge which can be reversibly changed when pH changes. In order to catalyze a specific metabolic reaction the enzyme must fold into an unique configuration, which is set by the partial charges


which bind one part of the enzyme to another to create an active catalytic site. This is typically referred to as a “lock and key” mechanism. If pH changes, the partial charges


change, the enzyme may unfold, and the catalysis at the active site may be inactivated.


Plant hormones, such as gibberellic acid or abscissic acid, can induce localized changes in pH. For example, growth at a shoot tip requires the protein shell that encloses the tip to be relaxed enough to enable growth that will extend the shoot length. Modification of pH enables the enclosing protein to be relaxed and stretched.


Gravimetric effects that allow a shoot to bend also employ a similar pH-mediated mechanism. Plant cells can increase up to 100 times their initial size. The proteins that affect the loosening of the hemicellulose cell wall structure are active only over a narrow range of cell pH.


Within typical plant cells are large vacuoles. Vacuoles are like a large balloon within the cell and often have a


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