Helianthus sp. (Native Sunflowers)?

Question

I was watching some sunflowers on the side of the road today in southern California. It's about 104F (40C) and has been the past few days. I'm wondering; do these plants keep the percentage of sugar to water the same in their nectar under heat stress or do they let it rise and/or fall? That is, do they selectively keep water levels in blossoms above other plant parts? I noticed that the leaves are wilting, but the petals all seem to be fully turgid (at least what I could see from passing them on the road)

Answer 1

I really don't know, but I love sunflowers and envy you having them to look at, even roadside.  I have planted some in my garden and one year they were great, but since then, I can't get them to grow.

Answer 2

The light levels allow plants to manufacture their food, some of it those sugars you mentioned. Those are the light dependent reactions. The metabolism of those foods or what was called "dark reactions" are, contrary to it's label, happening all the time, day and night. This is respiration. Water is taken in especially during photosynthesis (light dependent), and as the liquid component of the soil solution which has the anions and cations of the needed nutrients. High light levels kick this up and when the soil solution is dryer for lack of soil moisture then the fine balance that keeps the plant hydrated and the leaf full, as it were, is at a lower pressure in the zylem and phloem as well as the osmotic pressure of the cells. Osmotic pressure will when the nutrient levels are high in concentration (and water low) actually pull water out of the cells hence the problems associated in the "burning" of a plant by overfertilization. Same thing. The stem stays firm because specialized cells have laid down a matrix of strong fibers (cellulose is one) that are not affected by moisture loss that the different softer tissues are prone to. Some plant stems, especially younger ones, will wilt when the fluids that fill the xylem of phloem are not at a pressure to support plant weight. These are all different depending on species and also age. Like a tree, the living part of the trunk is a layer of cells under the bark that covers the wood. Wood is not alive so the real tree life is the leaves and the thin membrane as I described in the trunk. Lack of water still leaves the tree standing but the unsupported leaf is wilted.