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Advanced knowledge

State of water in the plant

The water content of a leaf or other plant organ is measured as the relative water content (R) which is the water content stated as a percentage of the maximum water content that the tissue is capable of holding.

R = 100 (M_f - M_d) / (M_t - M_d)

When M_f is the mass of the plant material fresh from the plant, M_t is the mass when the material is fully hydrated by being placed in water in the dark until no further water can be absorbed(such a leaf is said to be fully turgid) and Md is the mass after drying by removing all water in an oven at a reference temperature, often 80¢X C.This index of tissue hydration generally more useful than the water content stated as a percentage of the dry mass, as the latter is more sensitive to the varying amount of structural tissue, and the transient nature of storage materials such as starch. R was originally devised for reporting the water content of leaves, but can also be used to report the water content of woody tissues.

The state of water in the plant is measured as the water potential(£Z),which is the difference in free energy (Jmol^-1 or Jm^-3) between the water under consideration and that of pure water at sea level. It is the work that would be required to move water from where it is, to the pure state at the sea level. The water potential tells us about the tendency of the water to move one direction or another. Water always moves from high potential to low potential. For historical reasons, the units used are those of pressure, pascals (Pa), which are dimensionally the same as Jm^-3.Water potential of pure water at sea level is arbitrarily set to zero, and the water potential in plant leaves are nearly always negative, often by as much as -1 or -2MPa.In leaves, the water potential tends to be reduced by the presence of solutes, and increased by the force of the cell walls tending to squeeze the water from the cells. The cellulose walls are not rigid but elastic, and they exert their greatest pressure on the protoplast when the tissue is fully hydrated, and a declining pressure as water is lost from the system. Total water potential £Zt is the sum of the solute potential £Zs and the pressure potential £Zp brought about by the wall pressure : £Z_t=£Z_s+£Z_p

The relationship between the water potential and the water content is very important. As the leaf loses water, the cells reduce in volume and the solutes become more concentrated (£Z_s declines).At the same time, the pressure exerted by the walls declines( £Z_p declines).

The relationship between the water potential and the water content differs markedly between species, and may influence the ecological range of the species. For example, a tomato plant (an example of a mesophyte, a plant unable to grow in dry places) may show a small decline in water potential for a particular decline in relative water content, but an acacia (a xerophyte, normally growing in dry places) shows a relatively large decline whilst still maintaining a positive turgor. Thus, the xerophyte is more able to extract water from the soil, by virtue of its highly negative water potential, and thus is well suited for survival in dry soils.

Water potentials are routinely measured using a pressure chamber. Leaves are cut from the plant with a sharp blade and placed inside a pressurized vessel with their cut petioles protruding. On cutting, the water meniscus retreats into the cut end of the xylem and the cut surface appears very dry when viewed with a hand-lens. Pressure is applied by adding nitrogen gas to the chamber, squeezing the leaf until water begins to exude from the cut surface of the petiole. This pressure (the balancing pressure)is equal and opposite to the water potential. An alternative technique using a thermocouple psychrometer gives very similar readings, and both support the classical view (the cohesion theory of water transport) that water in the stem is under considerable tension when the plant is actively transpiring.

The range of water potentials usually found in plant varies on a diurnal cycle. Immediately before dawn, vascular plants are in a relatively hydrated state, and typically £Z_t falls to a minimal value soon Afternoon. The relationship between transpiration rate and water potential is usually almost linear. Minimum water potentials recorded in vascular plants vary from-1.0MPa in wetland herbs to-6.0MPa in some desert shrubs.