How are nutrients transported from the roots and leaves to those areas of the plants
which require it?
How does foliar spraying work when most people think that plant nutrition is a one-way movement from the roots to the leaves?
Plants range in size and in some instances they can draw water and minerals through their vascular systems to elevations of hundreds of feet.
A vascular network runs through the leaf, providing the cell walls with water and removing the food products of photosynthesis, as well as nutrients, to other parts of the plant.
The vascular tissue system consists of two kinds of conducting tissues: the xylem,
responsible for conduction of water and dissolved mineral nutrients, and the phloem,
responsible for conduction of food.
The xylem also stores food and helps support the plant.
In simple terms the vascular system runs from the roots up and from the leaves down, providing two ways for nutrients to be distributed to areas of the plant.
Plant hormones, specialised chemical substances produced by plants, are the main internal factors controlling growth and development.
Hormones are produced in one part of a plant and transported to others, where they are effective in very small amounts. Depending on the target tissue, a given hormone may have different effects.
Thus, auxin, one of the most important plant hormones, is produced by growing stem tips and transported to other areas where it may either promote growth or inhibit it.
In stems, for example, auxin promotes cell elongation and the differentiation of vascular tissue, whereas in roots it inhibits growth in the main system but promotes the formation of adventitious roots. It also retards the abscission (dropping off) of flowers, fruits, and leaves.
Commercially, synthetic auxins are used to initiate adventitous roots from plant cuttings eg. in nurseries.
Weed control by another synthetic auxin, 2, 4-dichlorophenoxyacetic acid (2,4-D), is widespread as a selective herbicide against broadleaf weeds.
Foliar fertilisation increases chlorophyll production and synthesis in the cells of leaves most exposed to direct sunlight. This increase in cellular activity requires much more water by the leaf.
This increase in water 'flow' automatically brings more fertilising elements into the plant via the vascular system, because we are stimulating the entire 'pumping' system. This increase in uptake automatically brings more fertilising elements to the plant via the vascular system.
Plants contain a vast network of conduits, which consists of xylem and phloem tissues. These conducting tissues start in the roots and transect up through the stem, branching off into the branches and then branching even further into every leaf.
The suction that exists within the water-conducting cells arises from the evaporation of water molecules from the leaves. The loss of water from a leaf is comparable to placing suction to the end of a straw. If the vacuum or suction thus created is great enough, water will rise up through the straw.
The main driving force of water uptake and transport into a plant is transpiration of water from leaves. Transpiration is the process of water evaporation through specialized openings in the leaves, called stomata. This chain of water molecules extends all the way from the leaves down to the roots and even extends out from the roots into the soil.
Evaporation creates a negative water vapour pressure in the surrounding cells of the leaf. Once this happens, water is pulled into the leaf from the vascular tissue, the xylem, to replace the water that has transpired from the leaf. This pulling of water, will extend all the way down through the rest of the xylem column of the plant and into the xylem of the roots Finally, the negative water pressure that occurs in the roots will result in an increase of water uptake from the soil.