Osmotic Pressure and Delicious Tomatoes.

[jonathanpassey]

Now I am being quite pretentious by starting this thread for two reasons:

1. I don't really understand Osmotic pressure. Nor am I a botanist. These are just things I vaguely remember from general ed courses years ago.

2. I have yet to ever harvest a delicious tomato that I grew myself (although my Italian Heirloom has a big one that is starting to look a little orangish green).

But, Here is my thought anyway:

In order to pull water out of the soil and pump it up into a tomato off the ground the dissolved solids in the solution at the top of the plant need to be concentrated enough so that the osmotic pressure is adequate to lift the water the vertical distance from the soil. (pressure in a fluid is entirely based on vertical distance)

So, the concentration of dissolved sugars (and etc.) must be higher the farther you get off the ground... Or the plant could never get the water up to those parts of the plant. And in order to get that water out of the vines and into the fruits the fruits would have to be as or more concentrated.

Guesses:

1. Tomatoes higher on the plant will be tastier than tomatoes lower on the plant. All other things being equal.
2. Trellising or caging or etc will improve the flavor of tomatoes since they will be farther from the ground.
3. Drier climates (lower humidity) will produce better tasting tomatoes. Since the plants will be unable to absorb water from the air and will have to rely solely on water from the soil.
4. The reason early tomatoes might taste more bland or watery than later tomatoes could be the combination of both increased vertical distance and reduced humidity later in the season.

Tell me what you think.

JP

I think plants and humans have mechanisms to regulate the dissolved solids concentrations from top to bottom. While human blood flow doesn't depend on osmotic pressure, cellular transfer of fluid and nutrients does. The results would be about the same. I can't imagine either my feet or my head receiving less or more nutrition based on location. Only an uneducated guess on my part.

Ted

[Cole_Robbie]

I think beneficial bacteria and fungi are the most important mechanisms of nutrient uptake, which is why good soil produces good tomatoes. Take for example worm castings. They look like dirt, but sell for $1/lb. People wouldn't pay that much money if they didn't work, and they do their job at a bacterial/fungal level. That's where the action is.

[RayR]

Osmosis is the passing of a solution through a semipermeable membrane from lower concentration to a higher concentration. Root cells keep a higher concentration of solutes in them than the soil water, so water and nutrient solutes pass into the cell.

The main mechanism of water being drawn up the stem into the leaves is caused by the hydrostatic pressure caused by transpiration of water vapor out of the leaves. Water molecules are bi-polar and therefore cohesive, so when water exits the leaves it pulls the molecules of water and the solutes behind up through the xylem.

Plants have mechanisms to move sugars and other compounds throughout the plant. Sugars are not at a higher concentration at the higher areas of the plant or the higher fruit. A good part of the sugars produced by photosynthesis are actually transported downward to the roots and out to attract and feed beneficial microorganisms that help the plant acquire nutrients and fight off pathogens and pests.

Thanks Ray,

I've always assumed positive pressure exists from the bottom of a plant upwards. My belief was based on something you know more about in New York than I do in Texas.

If you tap a maple tree in early spring before any leaves have formed, I assumed the sap would flow from the sap wood or cambium layer under positive pressure instead of negative pressure.

Ted

[Redbaron]

Originally Posted by RayR

Sugars are not at a higher concentration at the higher areas of the plant or the higher fruit. A good part of the sugars produced by photosynthesis are actually transported downward to the roots and out to attract and feed beneficial microorganisms that help the plant acquire nutrients and fight off pathogens and pests.

Actually sugars are at higher concentration higher up during the day. It is measurable. It is diurnal, with the root exudates primarily nocturnal.

But I don't think the mechanism is the osmotic pressure. Although it may be related in some way. The primary reason is sunlight creating photosynthesis. Lower branches will have higher sugar too, if they manage to get the same sun. (usually they don't)

[jonathanpassey]

Huh I read more about it since then and it looks like osmotic pressure is created mostly in the roots and the water sort of overflows up into the rest of the plant. Which is pretty awesome. Roots can push water up 50 vertical feet (the equivalent of 22 psi) (Edit: Wikipedia says only 7 meters which is 11 psi). That's quite the osmotic pump.

In large trees this isn't adequate so some trees maintain higher concentrations of sugars in the upper limbs during the summer. This is why I think I had the false impression from school; I remembered the trees.

Evaporation and hydro-static pressure mentioned above also help the water move up the stem. But it seems like there is an osmotic component there too: Water loss causes solutions to become more concentrated.

But either way, there wouldn't be any difference between the top parts and the bottom parts of the plant.

All in all, I was wrong and fruit at the top of the plant shouldn't taste better than fruit at the bottom. All other things being equal.

Super interesting stuff though.

[RayR]

Osmotic pressure is not enough since the xylem is made up of dead cells, so no cell membranes for osmosis to occur. Transpirational pull is the only way the transport of water and solutes can overcome the force of gravity.
I just found this video that explains how transport of water and solutes work in a plant.
http://www.youtube.com/watch?v=xGCnuXxbZGk

[jonathanpassey]

Hey RayR

I just watched that video and a few others. When I typed my response before my understanding was incomplete. I had just read this article on root pressure which says that roots can create excess water pressure from osmosis to push water up the xylem whether or not the plants are transpiring and I wrongly assumed it was the main force of transport rather than a secondary force. But apparently even if transpiration has slowed or ceased because of high humidity many plants can still get nutrients to their leaves.

Here is the interesting and pertinent quote:

The main contributor to the movement of water and mineral nutrients upward in vascular plants is considered to be the transpirational pull. However, sunflower plants grown in 100% relative humidity grew normally and accumulated the same amount of mineral nutrients as plants in normal humidity, which had a transpiration rate 10 to 15 times the plants in 100% humidity.[1] Thus, transpiration may not be as important in upward mineral nutrient transport in relatively short plants as often assumed.

Sorry I made my learning process public. Hopefully I am not too annoying. Transpiration pull is fascinating and I didn't know it existed until today, and, at my house, it probably accounts for most of water movement (my humidity is often around 15% in the heat of the afternoon).

JP

[RayR]

No Jonathan you are not being annoying.
This is all very interesting, a constant learning experience. I'm not discounting the importance of root pressure, it does play an important part in plants. Different species of plants have developed different adaptations to deal with things like their life cycle, their size and their natural environment.
A tree like a Sugar Maple isn't a tomato plant, it stores a tremendous amount of sap in it's massive roots over cold winters. It would have to have a great deal of root pressure to start moving that sap up to where its needed in the spring so it can leaf out and start transpiring. A tomato plant isn't even the same ball park as that.

[Lee]

Very interesting material here! Thanks for sharing.

Lee