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II. Overview of Nutrient Absorption by Roots

II. Overview of Nutrient Absorption by Roots

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ION ABSORPTION BY PLANT ROOTS



165



sibly is referred to as the apparent free space (AFS) (Hope and Stevens,

1952). It often represents 15-25% of the root volume (Butler, 1953;

Epstein, 1955; Briggs and Robertson, 1957). The AFS is thought to consist of the interconnected cell wall system (apoplast) up to the Casparian

strip of the endodermis. Thus, after a short time, the cortical cells and

the epidermal cells are bathed in a solution that is virtually identical to

the external solution.

The phase of slow ion absorption continues for several hours at a continually diminishing rate until eventually no apparent absorption occurs.

This steady-state condition is referred to as salt saturation, and the tissue

is now in a high-salt state as contrasted to the low-salt state at the beginning of the experiment. Before salt saturation, ion influx greatly exceeds

ion efflux; efflux may be almost nonexistent in low salt roots (Johansen

et al., 1970; Epstein, 1972a). At the salt-saturated state, ion influx continues but at a diminished rate (Pitman, 1969; Neirinckx and Bange,

1971; Cram and Laties, 1971), and the influx is balanced by an equal

rate of ion efflux (Pierce and Higinbotham, 1970; Cram and Laties,

1971).

Several features about the slow absorption phase are important. First,

it requires energy. If metabolism is impaired by lowering the temperature,

by imposing anaerobic conditions, or by adding a variety of metabolic poisons, ion absorption ceases. Under these conditions, the ion concentration

in the tissue may not even equal that in the external solution, indicating

that concentration-dependent diffusion is restricted. The reason for this

is that highly charged electrolytes cannot readily enter (or solubilize in)

the hydrophobic lipid of the cell membrane. Permeability coefficients for

inorganic ion entry into plant cells are in the order of

cm/sec (Scott

et al., 1968; Pierce and Higinbotham, 1970; Cram and Laties, 1971),

which is very low compared to the 10-*-10-' values commonly found for

nonelectrolytes (Collander, 1959). Another characteristic that illustrates

the energy dependency of ion absorption is that when metabolism is allowed to proceed the internal concentrations of both cations and anions

become greater than the external concentration. This phenomenon is frequently expressed as the concentration or accumulation ratio, i.e.,

c , , l d e / ~ o , , t , , d , = . Accumulation ratios of 10 to 100 are common, and

some values are as high as 10,000 (MacDonald et al., 1960). Although

an accumulation ratio significantly greater than 1 indicates energy expenditure was required to concentrate the ion, it does not necessarily mean that

the ion being studied was actively transported; active transport being defined as the movement of an ion against an electrochemical potential gradient (Ussing, 1949). Root cells maintain an electrical potential difference

across the cell membrane, and this gives rise to the distinction between



166



T. K. HODGES



energy-dependent transport and active transport, which will be discussed

in Section 111.

Studies in which both the electrical and concentration gradients have

been evaluated indicate that anions are actively transported across the

plasma membrane into the cytoplasm (Higinbotham et al., 1967; Pierce

and Higinbotham, 1970). Cations, with the possible exception of K , enter

cells passively, i.e., down the electrochemical gradient, and they then appear to be actively transported back across the plasma membrane from

the cytoplasm to the external solution (Higinbotham et al., 1967). Transport of K+ at the plasma membrane appears to be active inward in certain

instances and active outward in other situations (Etherton, 1963, 1967;

Scott et al., 1968; Pierce and Higinbotham, 1970). At the tonoplast only

K', Na+, and C1- have been investigated and the results, though needing

confirmation, suggest that K+and Na+ are actively pumped into the vacuole

while C1- may be passively distributed (Pierce and Higinbotham, 1970)

or actively transported (Cram, 1968a).

Another important characteristic of energy-dependent transport of ions

into roots is that as the external ion concentration is increased, the rate

of absorption increases in a biphasic manner. This apparent saturation of

the transport system at high external ion concentrations gave rise to the

ion-carrier concept (Osterhout, 1935; van den Honert, 1937). The

ion-carrier complex has been considered to be analogous to a substrate-enzyme complex (Epstein and Hagen, 1952), and Michaelis-Menten kinetic

analysis have been employed extensively to describe ion absorption data

(Fried and Broeshart, 1967; Epstein, 1973). It is now apparent, however,

that the kinetics of ion transport into root cells is very complex and that

Michaelis-Menten kinetics do not accurately describe the phenomena

(Epstein, 1966; Nissen, 1971; Leonard and Hodges, 1973). However, influx kinetics can be adequately described by assuming a single carrier consisting of several ion-binding sites which interact. This concept is based

on a model proposed by Koshland (1970) to account for enzyme kinetics

that deviate from Michaelis-Menten kinetics.

Energy-dependent transport is also selective. That is, the ion composition of plant extracts is totally different from the composition of the external solution (Collander, 1941, 1959). Calcium ions play an important role

in regulating the selectivity of ion transport (Jacobson et al., 1950; Epstein, 1961). Furthermore, the selectivity of transport is different at low

and high external ion concentrations (Rains and Epstein, 1967a,b).

It is proposed here that the basis for selective ion transport in plants is

the electrical field strengths (Eisenman, 1 962; Diamond and Wright,

1969) of the carrier binding sites, and the concentration-regulated speci-



ION ABSORPTION BY PLANT ROOTS



167



ficity is due to interactions of the binding sites, which change their field

strengths.

The actual energy source for ion absorption by roots has its origin in

aerobic respiration. Anaerobiosis (Hoagland and Broyer, 1936) and specific respiratory poisons (Ordin and Jacobson, 1955) inhibit ion absorption. Considerable evidence indicates that ATP drives cation transport

across plant cell membranes (Higinbotham, 1959; MacRobbie, 1970;

Fisher et al., 1970), and some evidence suggests that anion transport is

driven by some other product or aspect of aerobic respiration (Atkinson

and Polya, 1968; Cram, 1969a; Raven, 1969). ATP-driven cation transport at the plasma membrane appears to be mediated by an ATPase enzyme (Fisher et al., 1970; Hodges et al., 1972), and it is proposed here

that anion influx is brought about by an exchange reaction mediated by

an anion carrier. The internal anion driving the anion carrier could be

OH- that is generated by the ATPase and/or HC0,- that is generated by

aerobic respiration.



Ill.



Energy-Dependent and Active Ion Transport



A.



TERMINOLOGY



The terms energy-dependent and active transport are sometimes used

synonomously although they are quite different. Energy-dependent transport is broader and pertains to any transport that depends directly or indirectly on metabolism. Transport of this type generally results in accumulation ratios ( c , / c , ) significantly greater than 1. Active transport, on the

other hand, is defined as the movement of an ion against its electrochemical potential (Ussing, 1949; Dainty, 1962). Other definitions or treatments

of active transport have been proposed (Kedem, 1961); however, they

have not been employed for studies of ion transport in higher plants.

The basis for the difference between energy-dependent and active transport is that an electrical potential difference exists across membranes of

actively metabolizing cells. Charged solutes such as inorganic ions move

passively in response to this electrical field as well as to the concentration

(activity) gradients. The early studies of Lund (1928), Blinks (1935),

Osterhout (1935), and others (see Rosene and Lund, 1953) as well as

more recent studies (see Dainty, 1962; MacRobbie, 1971; Higinbotham,

1973) have clearly established that the cytoplasm is electrically negative

with respect to the external solution bathing the cells. The basis for this

membrane potential will be discussed subsequently, but its maintenance

depends on energy; the electrical potential difference falls when the cells



168



T. K. HODGES



are killed. Because the electrical potential is negative inside relative to outside, cations are drawn in and anions are repelled. Thus, cations may exist

in cells at much higher concentrations than outside, but when the concentration (activity) and electrical gradients (both are physical driving forces)

are both taken into account, the ion may possess the same electrochemical

potential on both sides of the membrane. In this situation, an ion can exist

at a higher concentration inside than outside due to the electrical driving

force, and this would be called an energy-dependent transport since energy

is necessary for maintaining the electrical potential. But, transport per se

would be passive since only physical driving forces acted on the ion. In

this situation, energy expenditure would be indirect.

Active transport is a special type of energy-dependent transport. Since

active transport is defined as the movement of an ion against its electrochemical gradient, this type of transport is an “uphill” process, and it must

be directly coupled to an energy releasing reaction.

Carriers, permeases, translocases, transporters, and porters are terms

used to describe substances which reside in membranes and aid the solute

in moving across the membrane, presumably through an association or

binding. In this discussion the term carrier is used since it is most common

in the literature on transport in plants. The two main characteristics of

carrier-mediated transport are saturating kinetics and specificity. Active

transport, as defined above, exhibits these characteristics and is therefore

believed to be carrier mediated.

Passive transport that is energy-dependent also frequently exhibits saturation kinetics and specificity and is therefore also thought to involve carriers. Such transport is frequently termed facilitated diffusion.Still another

type of carrier-mediated, but passive, transport phenomenon is exchangediffusion. In this type of transport, a carrier can only traverse the membrane when complexed with a specific ion. For example, when a radioactive ion is moved across the membrane from outside to inside by this type

of carrier and then released, the carrier will not return until it binds a

similar ion. There is a small chance the carrier will recombine with the

labeled ion; thus the return trip is likely to be with a similar, but nonradioactive ion. Thus, exchange-diffusion results in a bidirectional transport,

and no net transport occurs.

In summary, the term energy-dependent transport accurately describes

all transport that depends on metabolism, and it frequently leads to accumulation ratios significantly greater than 1, but the transport itself may be

either active (i.e., against the electrochemical gradient) or passive (down

the electrochemical gradient). Both types of transport may be carriermediated. Facilitated diffusion and exchange-diff usion are merely descriptive terms for carrier-mediated passive transport.



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