Water and Its Relation to Fresh Produce 271
temperature fluctuation. Latent heat of vaporization refers to the energy needed to
separate molecules from the liquid phase into the gas phase at constant temperature,
and it is important because it helps plants to cool themselves by evaporating water
from leaf surfaces and other plant parts during transpiration. Specific heat of vapor-
ization is much higher in water, and a large amount of heat energy is required to
evaporate water because hydrogen bonds must be broken to permit water molecules
to dissociate from one another and enter the gaseous phase.
At any airwater interface, water molecules are more strongly bound to neighboring
water molecules than to the gaseous phase on the other side of the surface. On plant
surfaces, a condition known as surface tension is created, and this surface tension at
the evaporative surfaces of the leaves generates the physical forces that pull a stream
of water through the plant’s vascular system. Cohesion, which is the mutual attraction
between water molecules, is another property that arises from the extensive hydrogen
bonding of water molecules, whereas adhesion is the attraction of water to the solid
phase. Surface tension, cohesion, and adhesion are attributed to the capillary movement
of water and may be responsible for water movement in the xylem and for keeping the
leaf mesophyll moistened (Steinbeck, 1995; Taiz and Zeiger, 1998).
9.3 FACTORS RESPONSIBLE FOR WATER MOVEMENT
IN PLANT SYSTEMSEnergy is required to move water from one place to another, and the difference in
potential energy between two locations creates an energy gradient, resulting in the
movement of water from a high- to a low-energy location. Gradients due to free
energy/chemical potential and concentration are important in mobility of water and
solutes, with mobility occurring toward locations low in energy (Burton, 1982; Devlin
and Witham, 1983; Kays, 1997). In freshly harvested produce, as water evaporates from
the surface a gradient is created, and thus there is a tendency for more water to migrate
toward the surface to establish equilibrium. In addition, biochemical components that
tightly bind or have high affinity for water create a matrix effect that affects water
mobility. Hydrophilic compounds such as proteins, cellulose, and starch tightly bind
water and limit its movement. Increased temperature around the fresh produce results
in increased mobility of water due to increased free energy.
9.3.1 DIFFUSION, BULK/MASS FLOW, AND OSMOTIC MOVEMENT OF WATER
Water molecules in solution are in a dynamic state and while in motion collide with
one another, exchanging kinetic energy. Taiz and Zeiger (1998) define diffusion as
the movement of molecules by random thermal agitation, resulting in the random
but progressive movement of substances from regions of high energy to regions of
low free energy. This diffusion is considered rapid over short distances but extremely
slow over long distances, and it plays a significant role during loss of water vapor
from leaves because the diffusion coefficient in air is much greater than that in
aqueous solutions. Bulk or mass flow is the general movement of groups of water
molecules in response to a pressure gradient, and this is the main mechanism
responsible for long-distance movement of water in the plant through the xylem.