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Uptake of Mineral Nutrients in the Apoplasm
The concentration of mineral elements within a plant is usually different from that of the external solution, with some minerals being found in much higher levels while others are present in much lower levels. Some minerals are excluded from the plant completely. This shows that plants have the ability not only to take up certain minerals, but to regulate this uptake so that the appropriate amount of each mineral is acquired. This section describes the system used by plants to facilitate this regulated uptake.

The first level of selection occurs in the apoplasm (the continuum of cell walls), which mineral ions can enter passively from the external solution, i.e. by mass flow or diffusion. The ions can then travel through this pathway via the spaces in between the interlinking cell wall molecules. Their movement here is affected by the interactions of some of the cell wall constituents with charged molecules, with some molecules being slowed in their journey and others hastened. This is explained below.

The cell wall is made up of cellulose, hemilose, and glycoproteins. Contained within the hemilose fraction is a proportion of pectins. Some of these pectins are made up of polygalacturonic acid. The COO- groups of these acids interact with any ions passing through the apoplasm, attracting and bonding with cations but repelling anions. The ability for a particular plant species to interact with ions in this way is termed its cation exchange capacity.

The reason these interactions are important is that although binding ions doesn’t actually allow the ions to enter the cell, it does bring them closer to the plasma membrane, and hence to the various transport systems that move ions from the apoplasm into the cell. This obviously improves the chances of cation uptake over anions. The significance in this system has been highlighted in experiments with zinc which was taken up at a rate several times higher when it was introduced in ion form (cation) than when it was chelated with another molecule and thus uncharged.

Selectivity is further enhanced by the fact that poly-valent ions are taken up preferentially over single charged ions. This is shown by the close correlation between the ratio of Ca2+ to K+ in a plant and its cation exchange capacity. It should be noted that in the larger pores of the apoplasm the strength of repulsion of anions becomes weak enough to allow their passage.

• Effects nutrients have on the development of plants

• Effects nutrients have on yield

• Factors that effect the rate of mineral uptake

• Explanation of the mineral uptake into the cells of plants

• Uptake of mineral nutrients in the apoplasm

• Transport of nutrients throughout the plant

• Nitrogen nutrition