RESPIRATION RATE IN PLANTS
263
ordination, may also account in part for an apparent accumulation without an expenditure of energy.
The view is now gaining ascendency that the absorption of salt is
achieved by the same mechanism whether the absorption is against a
chemical potential gradient or not (Ussing, 1949; in Clarke and Nachmansohn, 1954). That is to say, the ions of inorganic salts do not diffuse
freely either into, or out of, that part of the cell wherein accumulation
takes place—whether in the cytoplasm or cell vacuole. This concept,
however, does not exclude the existence of diffusion exchange, as postulated by Ussing (1949), and which, in distinction to free diffusion, is
thought to involve the combination of one member of an ion pair with
some component of the cell membrane. By random rotational movement,
the membrane component delivers the combined ion to the inside of the
cell, where it may be exchanged for an internal ion. Diffusion exchange
is thought primarily to exchange cations. In any event, it fails to lead
to ion accumulation.
The relation of salt accumulation to respiration having been generally
established, attention has turned to the nature of this relationship. The
evidence has been increasing that in many plant tissues only a fraction
of the total respiration is of a type which may be linked to the accumulation of salt. In carrot (Robertson and Turner, 1945), wheat roots
(Lundegárdh, 1953a), and barley roots (Milthorpe and Robertson, 1948)
there is clearly a basal respiration, unrelated to the absorption of salts,
besides a respiratory component which arises upon exposure of the
tissue to salt and which appears to be related to the salt-absorbing
process. In these same tissues the distinction between the two types of
respiration is readily made, for the basal respiration is resistant to
cyanide, whereas the "salt respiration" is abolished by low levels of cyanide. In other tissues the distinction between basal respiration and salt
respiration may frequently be made even when the entire respiration
is sensitive to cyanide. There is reason to believe that salt respiration
is akin to the respiratory increments induced by aging (in disks of
tuberous organs), by auxin (in tissues wherein auxin induces growth),
and by dinitrophenol (see previous sections). Machlis (1944) and
Ordin and Jacobson (1955) have shown that in barley roots the ability
to accumulate salt is associated with the activity of the tricarboxylic acid
cycle. Following inhibition of a fraction of the respiration, and most of
the salt absorption, by either iodoacetate or malonate, the addition of
components of the tricarboxylic acid cycle was found to restore the saltaccumulating capacity of the roots. The underlying principle which
unites all these apparently diverse types of respiratory stimulation is
that each of them represents the release of some restraint upon respira-
263
ordination, may also account in part for an apparent accumulation without an expenditure of energy.
The view is now gaining ascendency that the absorption of salt is
achieved by the same mechanism whether the absorption is against a
chemical potential gradient or not (Ussing, 1949; in Clarke and Nachmansohn, 1954). That is to say, the ions of inorganic salts do not diffuse
freely either into, or out of, that part of the cell wherein accumulation
takes place—whether in the cytoplasm or cell vacuole. This concept,
however, does not exclude the existence of diffusion exchange, as postulated by Ussing (1949), and which, in distinction to free diffusion, is
thought to involve the combination of one member of an ion pair with
some component of the cell membrane. By random rotational movement,
the membrane component delivers the combined ion to the inside of the
cell, where it may be exchanged for an internal ion. Diffusion exchange
is thought primarily to exchange cations. In any event, it fails to lead
to ion accumulation.
The relation of salt accumulation to respiration having been generally
established, attention has turned to the nature of this relationship. The
evidence has been increasing that in many plant tissues only a fraction
of the total respiration is of a type which may be linked to the accumulation of salt. In carrot (Robertson and Turner, 1945), wheat roots
(Lundegárdh, 1953a), and barley roots (Milthorpe and Robertson, 1948)
there is clearly a basal respiration, unrelated to the absorption of salts,
besides a respiratory component which arises upon exposure of the
tissue to salt and which appears to be related to the salt-absorbing
process. In these same tissues the distinction between the two types of
respiration is readily made, for the basal respiration is resistant to
cyanide, whereas the "salt respiration" is abolished by low levels of cyanide. In other tissues the distinction between basal respiration and salt
respiration may frequently be made even when the entire respiration
is sensitive to cyanide. There is reason to believe that salt respiration
is akin to the respiratory increments induced by aging (in disks of
tuberous organs), by auxin (in tissues wherein auxin induces growth),
and by dinitrophenol (see previous sections). Machlis (1944) and
Ordin and Jacobson (1955) have shown that in barley roots the ability
to accumulate salt is associated with the activity of the tricarboxylic acid
cycle. Following inhibition of a fraction of the respiration, and most of
the salt absorption, by either iodoacetate or malonate, the addition of
components of the tricarboxylic acid cycle was found to restore the saltaccumulating capacity of the roots. The underlying principle which
unites all these apparently diverse types of respiratory stimulation is
that each of them represents the release of some restraint upon respira-