Function in the Plant
Calcium is necessary for the proper growth and functioning of root tips and meristems. Calcium pectate is a component of the cell walls, increasing mechanical strength of the plant. Therefore, calcium tends to be stored in the leaf. It also activates many plant enzyme systems and neutralises organic acids in the plant. A large supply of calcium is needed for nodulation and nitrogen fixation by legumes. The roots of some legumes have a higher calcium demand during the stages of root infection by Rhizobia than during actual growth.
Because calcium performs an important part in the growth and functioning of meristematic tissue, symptoms of calcium deficiency always emerge in new growth. Poor root growth is usually the first sign of calcium deficiency in wheat. Calcium deficient roots often become stunted (especially the laterals), turn a dark colour and rot. Wheat leaves do not turn yellow if calcium deficient, old leaves particularly maintaining their dark green colour. A definite indication of a calcium deficient wheat plant is a necrotic spotting in the middle of the youngest leaf. This problem quickly spreads and the leaf collapses midway before unrolling. New tissue needs calcium pectate for cell wall formation. Thus, calcium deficiency causes gelatinous wheat leaf tips and growing points.
The first sign of calcium deficiency in lupins is a shortening of the lateral roots. Soon the tips of these laterals turn a brown colour, which extends some distance back from the tips. Signs of calcium deficiency on plant tops soon appear. New leaves that are not fully expanded show the ends of their leaflets remaining tightly closed as though they are stuck together. These needle-like ends (one-third to one-half the length of the leaflet) do not turn necrotic for some time although they give a look of water stress to the whole leaf. As seen in many other plant species suffering from calcium deficiency, petioles of new leaves bend and finally collapse, although the leaves may show no more than a few chlorotic areas. Some bending of the main stem can occur, indicating the role of calcium in stem wall construction. Young growth soon develops necrotic tipping of the unopened leaflets followed by the collapsing of the petioles. Eventually, the new growing tip decays before any elongation of the petioles can occur and old and middle leaves become mottled and chlorotic, die and shed.
Calcium deficiency severely hampers shoot and root growth in field peas. Symptoms appear on the second and third pairs of fully opened leaves subtending the tendrils. They include a chlorotic crescent with small pink spots at the base of the leaflet. The first symptoms on new growth appear on the young tendrils, which do not curl but instead wilt and collapse close to the junction of the main stem and the tendril. A common feature of calcium deficiency is the collapsing of petioles and leaves. New leaves do not fully open, curl along the margins and are sharply pointed at the tip. Old leaves remain healthy, dark green and fully formed, compared with the paleness of new leaves and new shoots.
For Faba Beans, a calcium deficiency affects the newest growth of the plant. The stem subtending the new growth and the petiole of the youngest emerged leaf blade will collapse when a calcium deficiency starts to take effect. These parts slowly turn black. The leaf edges at the bottom end of very young leaves will curl inwards and exhibit a purplish necrosis. This ‘pinching in’ of these leaf edges contrast with the dark-green apex sections in the open region of the leaf. When the leaves become fully developed, they also feature the purpling of the bottom end of the leaf and most notably the midrib and veins. All of these symptoms distinctly demonstrate the absence of movement of calcium from old to new parts of the plant.
Function in the Plant
Magnesium’s role as the central atom of the chlorophyll molecule represents its main purpose as a required chemical for photosynthesis. It also plays a substantial part in Phosphorus transport in the plant. Because of this, it accumulates in the seeds of plants rich in oil, because the oil is accompanied by an accumulation of lecithin, a fat that contains phosphate. Thus, the phosphate content of a crop can sometimes be increased to a higher level by adding a magnesium fertiliser instead of a phosphorus fertiliser. Magnesium also assists in phosphate metabolism, plant respiration, protein synthesis, and the activation of several enzyme systems in the plant.
The seeds of grain crops are relatively low in magnesium, while seeds from other plants are comparatively high. Fertiliser N, P and K are commonly applied to soils and, in doing so, calcium and sulphur are replenished. However, magnesium is not always considered in fertiliser management, even though it is viewed as a macronutrient.
An imbalance between calcium and magnesium in soils of low cation exchange capacity can exacerbate a magnesium deficiency. If the Ca:Mg ratio becomes too high in these soils, then plants may take up less magnesium. This may happen when a grower has used lime or gypsum on a soil relatively low in magnesium. Magnesium deficiency may also be exacerbated by high rates of potassium or high availability of ammonium-N.
New leaves of magnesium deficient wheat take on a faded appearance compared to the older leaves. These new leaves soon exhibit signs of abnormal yellowing and stay unopened causing a twisted look, similar to symptoms of water stress. If the deficiency is extreme, the entire length of the leaf will remain folded or rolled. After a while, numerous leaves, including the new shoot, may be folded or rolled. The yellowed new leaves will later develop blotches, and ultimately die, but the leaves will remain upright. Even the oldest leaves will show evidence of a mottled chlorosis and in some cases a reddish colouration around the outside of the leaf.
When magnesium deficiency symptoms first show up in lupins, no damage has yet occurred to the plant. Plants grown without any magnesium at all are a lighter green with some mild interveinal yellowing appearing on new and old leaves. The old-leaf symptom is very specific to magnesium. The presence of small bronze spots distributed randomly over the entire leaflet is a unique characteristic of magnesium deficiency in lupins. These spots expand only slightly and do not merge to form areas of necrosis. These old leaves slowly turn a dull greyish green. The new-leaf symptom on the other hand is vastly dissimilar. The thin and spiky leaflets arrange themselves into a cluster, the tips swiftly dying but with little distortion such as curling or twisting. The whole plant finally turns a dull green colour.
Magnesium deficiency in Faba Beans causes a wide array of symptoms that cover leaves of all ages, after an initial stunting of growth. A very early symptom is a rolling in of the youngest leaves, persisting for the duration of the deficiency. This is rapidly followed by middle-leaf symptoms, specifically an umbrella-like folding back of the leaflets, and the appearance of a series of purple spots scattered randomly over the leaf. Then a mottled chlorosis, minimal to begin with, gradually progresses into yellowing of the area in between veins. The area that is already affected on middle and older leaves turns a red colour, and after a while the young leaves follow suit. However, the young leaves appear duller and develop a pale necrosis around the edges.
The early symptoms in field peas are the presence of pink necrotic lesions between the midrib and the margins of the leaf, and the old leaves rolling over along the margins. The lesions can be in between or include the veins. No further deterioration of theses old to middle leaves occurs. Next new leaves change to a pale green colour and plant growth is soon reduced. Large areas of the leaf that have quickly turned white develop small pink necrotic spots. As the deficiency progresses, many new leaves lose all of their chlorophyll, are stunted and change to a narrow and pointed shape. Old leaves take on a speckled chlorosis with traces of pink areas. The edges of all leaves, especially at the tip, roll over considerably. The roots of these severely deficient plants, although not as vigorous as well-nourished plants, remain healthy with good lateral root and root hair extension.