Plant nutrition, water relations and transport
Plant physiology connects leaf metabolism, root uptake and transport through an organism. Keep water movement, mineral-ion transport and sugar translocation distinct. Their routes and driving forces are related, but they are not interchangeable processes and do not all require the same direct energy input.
Photosynthetic tissues use light energy to support organic-carbon production. In an ordinary illuminated C3 leaf, chloroplast light reactions supply ATP and reducing power, while carbon fixation incorporates carbon dioxide into organic molecules. A mature exporting leaf can be a sugar source; a growing leaf, root or developing fruit can be a sink. The source or sink role can change with development and conditions, so it should not be assigned solely by the name of an organ.
Water moves across selectively permeable membranes down a water-potential gradient. Solute concentration and pressure both contribute. Root cells exchange water with the surrounding solution, while selective transport proteins regulate mineral-ion movement. Some ion uptake is energy-coupled; not every ion flux is necessarily against its electrochemical gradient or directly powered by ATP at the transport step.
In the cohesion-tension model, evaporation from leaf surfaces and diffusion of water vapor through stomata contribute to tension in the xylem water column. Cohesion among water molecules and the continuous pathway support movement from roots toward leaves. This is not explained by a single upward pump in the stem. The rate depends on environmental gradients, pathway properties and stomatal regulation.
Phloem translocation follows source-sink relations. Loading and unloading of solutes can establish pressure differences that drive bulk flow, with the details of loading varying among species. Different sieve tubes can serve sinks in different directions; saying all phloem transport is downward misses this organization. Xylem primarily carries water and mineral solutes, whereas phloem carries assimilates and other transported compounds.
Guard-cell turgor helps regulate the stomatal aperture. Closing stomata can reduce water loss but may also restrict carbon-dioxide entry, creating a tradeoff for carbon fixation. Plants differ in their stomatal timing and metabolic strategies, so a typical daylight example is not a rule that all stomata always open by day and close at night.
For a hypothetical dataset, equal leaf areas of 100 square centimeters lose 2 and 6 grams of water per hour under otherwise matched humid and dry air conditions. The normalized rates are 0.020 and 0.060 grams per square centimeter per hour, a threefold difference. This supports an effect of the supplied conditions only if other relevant variables and non-leaf evaporation were controlled; it is not a universal transpiration rate or an irrigation prescription.