Subtropicals

These are crops that, as their name indicates, grow in areas with tropical or subtropical climates. This type of fruit may be characterised as not tolerating cold conditions, which can cause damage. Below 4ºC. In Spain, subtropical crops account for 4% of all fruit crops. Among the most common fruits are avocados, mangoes, papaya,, custard apples, pineapples…

The main nutritional needs of subtropicals are:

Macronutrients in subtropicals

Nitrogen

This macronutrient is vital to proper plant growth and development, as it contributes to the construction of new tissues. Nitrogen forms part of enzymes, vitamins and chlorophyll. A lack of nitrogen causes the entire leaf to turn yellow and ultimately fall off the plant. Plant growth can be retarded and uneven.

Phosphorous

This element forms part of energy-rich bonds (ATP and ADP), which is necessary to carry out the respiratory and metabolic processes of the plant. It is crucial in the formation of RNA and DNA molecules. It is involved in the formation of reproductive organs, fruits and seeds. A lack of phosphorus causes smaller, brownish leaves. In old leaves, necrosis may appear at the edges. The branches may be weaker, with a lack of development in fruits and seeds.

Potassium

This basic element has an osmoregulation function, ensuring the efficient use of water, as it contributes to the formation, opening and closing of stomata. It activates more than 50 enzymatic systems involved in the metabolism of proteins and sugars. A lack of potassium causes a failure in the functioning of the stomata, reducing photosynthetic capacity. Old leaves will experience intervenal chlorosis, displaying a bright yellow colour. The fruits will take on a wrinkled appearance and their resistance to attack by insects will be diminished.

Calcium

This is the principal element involved in plant structures. Calcium accelerates the ripening of the fruit, improving its colour and texture, and any shortage of it may cause disorders in post-harvest fruit. It also controls the permeability of tissues and membranes. A lack of calcium causes reduced root development and consequently reduced plant growth. The leaves will take on a dark green colour. The shoots may die if the deficiency is very severe.

Magnesium

This element is crucial in photosynthesis as it is part of the chlorophyll molecule. It assists fructification by promoting synthesis of reserve fats. It protects the plant against drought, cold and attack of pathogens. A lack of magnesium will cause very small, brittle leaves, with the edges bent upwards. The tissues may dry up and die, exposing the plant to attack by fungi.

Sulphur

This nutrient participates in the synthesis of oils and is part of proteins and the amino acid cystine. A lack of sulphur causes yellowish green leaves and thin, weak stems in subtropical crops.

Micronutrients in subtropicals

Zinc

It is also involved in the synthesis of auxins, a vegetable hormone that promotes plant growth. A lack of zinc leads to a decrease in the size of the leaves, which will display necrosis, and the shoots will be reduced. The number of fruits per plant will diminish.

Copper

This element provides rigidity to tissues by inducing lignin synthesis. It optimises water transport within the plant and increases the plant’s resistance to fungi by promoting synthesis of phytoalexins. A lack of copper causes whitish chlorosis in the leaves and bland-tasting fruit pulp.

Iron

This is involved in the synthesis of chlorophyll and participates in the processes of respiration and redox within the cell. A lack of iron causes chlorosis in the youngest leaves, the edges of which may be folded upwards in subtropical crops.

Manganese

This promotes proper photosynthetic activity, stimulating the ripening and fertility of buds. It also increases the resistance of the plant to attacks by fungi. A lack of manganese causes chlorosis in the leaves, detectable as green veins with light green spaces between them. Plants will take on a thin, weak appearance.