Is osmosis the mechanism for water uptake?

Rick Donnan

Rick Donnan

I believe that osmosis is the mechanism for the uptake of water by plants in hydroponics. Is this correct and how does it work? 

Answer by RICK DONNAN

Answer
Yes, osmosis is the mechanism for water uptake by plant roots, but this applies to all plants, not just those growing in hydroponics. Where hydroponics has an advantage is that the grower has some control over the root zone solution strength (usually measured by EC—electrical conductivity). It is the root zone solution strength which has a major influence upon osmosis and hence water uptake.

Osmosis is the transfer of water through a membrane driven by a difference in concentration of the solutions on either side of the membrane. The membrane is known as a semi-permeable membrane because it allows the transmission of water, but not other molecules such as nutrient ions.

Osmosis is easiest explained by means of a simple scientific laboratory experiment. It is possible to manufacture a semi-permeable membrane and to install this down the centre of a glass container. On one side is placed a weak solution (for example a nutrient solution), and on the other side a stronger solution of the same composition. There is now a difference in osmotic pressure between one side of the membrane and the other.

Osmosis results in the system trying to achieve equilibrium, that is, to make the solution on both sides the same. In order to achieve this, water moves through the membrane from the weaker solution into the stronger solution. This is shown in Figure 1.

Figure 1

Water transfers away from the weak solution making it stronger, and into the strong solution making it weaker. Eventually the strength of the solutions on each side will become equal, and there will be a larger volume on the side which was originally the stronger. This is shown in Figure 2.

Figure 2

The greater the difference in concentration across the membrane the higher the osmotic pressure difference and the greater the water flow rate.

In plant roots the cells act as a semi-permeable membrane. With normal hydroponic root zone solutions, their strength will virtually always be lower than the solution within the roots. Consequently, the water flow will be from the root zone solution into the plant roots. (Occasionally a home gardener gets it wrong when they decide to fertilise a pet shrub with a large dose of fertiliser and just a little water—the fertiliser solution strength is then too high and consequently the water flows outward from the roots instead of inwards and the shrub speedily wilts and dies.)

With hydroponic solutions, the higher the strength (EC) of the root zone solution the smaller the difference to the internal root solution and hence the slower the rate of water uptake. Similarly, the weaker the root zone solution the greater the concentration difference across the root cell membrane and the faster the rate of water uptake. This is shown in Figure 3, which compares two different scenarios—the water uptake rate with stronger and weaker root zone solutions.

Figure 3

The water uptake also has an impact upon the solution strengths within the plant and hence the overall strength of the plant. High water uptake from a very low EC root zone solution will result in a weak plant with low shelf or vase life and relatively tasteless leaves and fruit. Higher root zone ECs lead to longer shelf or vase life and tastier produce, but there is a practical limit to how far a grower can go before there will be a reduction in yield and possibly quality. Ω

PH&G July 2014 / Issue 145


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