The cohesion-tension theory, also known as the transpiration pull theory, was proposed by Henry Horatio Dixon and John Joly in 1894. This theory explains how water moves upward through the xylem of plants against gravity. It relies on the cohesive properties of water molecules, the adhesive forces between water and xylem walls, and the tension created by transpiration at the leaf surface, which pulls the water column upward.

Phloem loading is the active transport process of moving sugars, primarily sucrose, from source tissues into the sieve tube elements of the phloem. This mechanism facilitates the translocation of photosynthates to various sink organs, such as roots, tubers, and developing fruits, which require energy for growth and storage. This process is essential for maintaining the pressure gradient necessary for long-distance transport within the plant vascular system.

The mass flow hypothesis, proposed by Ernst Munch in 1930, is the accepted model for phloem translocation. It describes the movement of sugars and other organic materials from source tissues (like leaves) to sink tissues (like roots or fruits). This process is driven by a pressure gradient created by the active loading of solutes into the phloem, which induces osmotic water uptake and generates positive hydrostatic pressure.

Translocation of carbohydrates primarily occurs in the form of sucrose. In plants, sucrose serves as the major transport form for photo-assimilated carbon, providing both carbon skeletons and energy for plant organs that cannot perform photosynthesis. Because it must be transported over long distances through the phloem vessels, sucrose is an ideal transport molecule due to its stability and solubility, allowing it to pass through various cellular membranes efficiently.

Leaf water potential is a measure of the energy status of water within plant tissues. Under drought or water-deficit conditions, the water potential becomes more negative, meaning it decreases. This reduction in potential is a physiological response that helps the plant maintain a gradient to extract water from drying soil, though it eventually limits metabolic processes and growth.

Guttation is the exudation of water droplets from the margins or tips of leaves. This process occurs specifically through specialized pores called hydathodes, which are typically located at the ends of vascular bundles. Unlike transpiration, which occurs through stomata, guttation is driven by root pressure when soil moisture is high and transpiration rates are low, usually occurring at night or early morning.

Phloem is the specialized vascular tissue responsible for the translocation of photosynthates, primarily carbohydrates, from source organs like leaves to sink organs such as roots, fruits, and developing shoots. This process, known as translocation, occurs through sieve elements. Additionally, phloem contains parenchyma cells that assist in the storage of water, non-structural carbohydrates, and proteins, supporting the overall metabolic needs of the plant.

Phenyl mercuric acetate (PMA) acts as an antitranspirant. It functions by inducing the closure of stomata on the leaf surface. By reducing the size of the stomatal aperture, it limits the amount of water vapor that can escape from the plant, thereby reducing the overall rate of transpiration. This is often used in agricultural practices to help plants conserve water during periods of drought.

The opening and closing of stomata are primarily regulated by the turgor pressure and the resulting shape of the guard cells. When potassium ion concentrations increase within the guard cells, water enters via osmosis, causing the cells to become turgid and bow outward, which opens the stoma. Conversely, a decrease in potassium ions leads to water loss, causing the guard cells to become flaccid and close the pore, thereby regulating gas exchange and transpiration.

The swelling of dry seeds in water is caused by imbibition. This is a specialized form of diffusion where water is absorbed by solid colloids, leading to a significant increase in volume. The process is driven by a water potential gradient between the dry seed tissues and the surrounding water. As the seed coat and internal tissues absorb water, the seed expands, which is a critical step for initiating germination.