|MadSci Network: Botany|
External calcium ions (Ca++) are required to maintain cell membrane integrity. Without external calcium, cell membranes become "leaky" and lose ions they previously absorbed. Their ion uptake is also reduced. Other positively-charged ions (cations) can displace calcium ions from the negatively-charged cation exchange sites on the cell membrane. External calcium also protects roots from low pH injury which is due to hydrogen ions (H+) and injury from other cations, such as ammonium (NH4+) and sodium (Na+). Plant roots often excrete a lot of H+ to balance out any excess postively-charged ions they have absorbed. Ca++ is a very important ion in many plant cellular processes including cytoplasmic streaming and cell division. The latter should not be surprising because cell walls contain lots of calcium pectates. Calcium is a phloem-immobile mineral nutrient, meaning once absorbed by the plant and transported in xylem, it is not redistributed to other parts of the plant via the phloem. Mobile mineral nutrients, such as nitrogen and phosphorus, can be redistributed in plants via the phloem when there is a deficiency. Continuous uptake by the roots is not required for phloem-mobile mineral nutrients. Calcium deficiency symptoms occur first in growing points, such as shoot tips and root tips, which have meristems where cell division occurs. So one reason calcium is required in the external solution is that the plant must constantly absorb calcium to keep growing. It cannot redistribute calcium already deposited in the plant to the growing points. The first website found that "calcium ion influx through plasma membrane channels is required for normal root hair growth." The Ca++ level in the cell cytosol is also very important in signal tranduction in plants. Calcium is considered a secondary messenger in many plant processes (see second website). External calcium is also required fro pollen tube germination and growth. References Calcium and plant roots A Second Messenger: Calcium Ions Calcium Nutrition of Cotton Epstein, E. 1972. Mineral Nutrition of Plants: Principles and Perspectives. New York: Wiley. Cation Exchange
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