Aspects of calcium nutrition to limit plant physiological disorders
Problems with calcium (Ca) nutrition are often first noticed as disorders within rapidly developing, low-transpiring plant organs. More severe Ca deficiencies will be noticed in other plant organs and growth parameters as well. In order to prevent these disorders, such as blossom-end-rot (BER) in tomatoes, peppers and melons, tip-burn in lettuce, bitter pit in apples, internal browning (IB) in pineapples, twins in pineapples, internal brown fleck in potatoes and many other Ca-related disorders, it is important to understand the functions as well as all the conditions that may affect the uptake and translocation of Ca within the plant. Since Ca is needed to strengthen cell walls and to maintain membrane integrity, Ca deficiencies lead to the collapse of cells, resulting in tissue enzymatic browning, caused by polyphenol oxidase and peroxidase enzymes, as well as tissue susceptibility to secondary infections, such as Phytophthora spp., Erwinia spp. and Botrytis spp. Leaky membranes may also lead to chlorophyll losses or water-soaked areas. Since Ca ions are passively taken up by the roots with water and transported in the xylem with the transpiration stream, any factor affecting the uptake of water, such as climatic conditions, root functioning, salinity and so on, will affect the uptake of Ca. Several conditions may enhance the development of Ca-related disorders and may be grouped as: insufficient Ca uptake by the plant due to inadequate root-zone moisture, low available soil Ca and cation imbalances in the soil or fertigation solution, poor root growth and saline root zones; inadequate Ca distribution to low transpiring, rapidly developing plant organs due to poor xylem development, high transpiration rates in leaf canopies and low night-time root pressures; intraplant factors such as strong carbohydrate sinks, high growth rates and auxin and enzyme activities; and cultivar differences. Some practical aspects of Ca nutrition, with special reference to pineapple, are also discussed.