How do plants take up nutrients?

From an indian grower:

Please refer your answer to the reader’s question in the February 2017 PH&G issue.

You have mentioned that in osmosis only water moves from lower concentration to higher concentration and salts do not get through the membrane. If this is so, then how do nutrients reach plants? I am a grower and a regular reader of your magazine. Thanks a lot.

Answer

Thank you for your email.

This is a very useful question.

I had overlooked that someone not having detailed knowledge of plant mechanisms might reasonably expect that nutrient ions just go with the water flow. Particularly in the case of uptake by the roots, this is not so.

The mechanisms for water uptake and nutrient uptake are quite different. The mechanisms for moving water and ions through the plant are also different, although there is much more interaction here, ion uptake mechanism The uptake of nutrient ions by the roots is an entirely different process to water uptake.

Ions are not taken up by osmosis

On the root cell walls are sites, which directly take up nutrient ions, a different site for each nutrient. These are called ‘transporters’ for the anions and ‘channels’ for the cations. For most ions, these sites occur along the entire root, but for some ions, especially calcium, the sites are just behind the root tip. These uptake sites respond to signals from the plant, such as ‘get more potassium’.

In hydroponics, around the roots is the root zone solution. The strength of this root zone solution will influence how much nutrient is taken up. The stronger the solution the easier it is for individual nutrient ions to get to their specific sites. Conversely, the weaker the solution there will be less opportunity for ions to reach their sites. Referring back to my February answer on osmosis, you will notice that the water uptake and nutrient ion uptake mechanisms reinforce one another. That is, strengthening the root zone solution the will reduce the water uptake by osmosis and also increase the ion uptake. Conversely, weakening the root zone solution will increase the water uptake by osmosis and reduce the ion uptake.

To maximise the uptake of individual nutrient ions, the ratio of the ions in the root zone solution must be optimised. The Dutch horticultural research stations have published recommended optimum root zone solution analyses for a wide range of crops in hydroponics. These optimised recommendation take account of the relative rate of uptake of individual nutrient ions. That is, for example, the relative strength is boosted of those nutrients, which are slower to be taken up.

If the root zone solution analysis gets well out balance, the nutrient uptake will be adversely influenced. For example, if the concentration of a particular nutrient ion is well below the recommended level, its ions won’t get enough opportunity to reach their sites. This effect is compounded by the other ions increasingly blocking access to the sites. The end result leads to a nutrient deficiency of that ion.

It is interesting and important to note that plant yields will actually be significantly reduced by deficiencies (and toxicities) well before visual symptoms of the problem can be detected.

ion transport within plants

Ion movement inside the plant is different again. Once inside the root the ions have a mixture of mechanisms, that is, they can. move both through and between cells and also with the water flow in the xylem. The xylem can be simply described as a leaky pipe system, which takes the water from the roots to the leaves, mainly for transpiration. Typically about 90% of the water uptake by hydroponic plants is for transpiration (to keep the plants cool). The remainder is used for photosynthesis and as part of the plant structure. Most plants typically grown in hydroponics are composed of more than 90 per cent water.

The other leaky pipe system in the plant is the phloem. This takes water from the leaves, which carries the products of photosynthesis and distributes them to the rest of the plant, including the roots. These products are known as ‘assimilates’, which are initially sugars, but then converted into a huge range of plant chemicals, such as proteins, celluloses starches, etc, etc. Together with assimilates, the phloem water also carries many ions, helping to distribute them around the plant. Some of these products and ions are exuded from the roots and possibly change the pH of the root zone solution.

Calcium is unique in that its ions only move in the water stream in the xylem. Once calcium ions have found a place in the plant structure (a major function is strengthening cell walls) that is where they stay and can’t move elsewhere even if badly needed.

Potassium is another interesting ion, which has major roles in activating about 60 different enzymes used by the plant and also helps to control the opening and closing of the stomata in the leaves. However, potassium ions don’t become part of any plant structure. They remain only completely in solution within the plant.  RD

PH&G March 2017 / Issue 177


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