When the detached first leaves of green or etiolated oat
(Avena sativa cv. Victory) seedlings senesce in the dark, their
oxygen consumption shows a large increase, beginning after
24 hours and reaching a peak of up to 2.5 times the initial rate
by the 3rd day. This effect takes place while the chlorophyll of
green leaves, or the carotenoid of etiolated leaves, is steadily
decreasing. Kinetin, at a concentration which inhibits the decrease
in pigment, completely prevents the respiratory rise;
instead, the oxygen consumption drifts downwards. Lower kinetin
concentrations have a proportional effect, 50% reduction
of respiration being given by about 0.1 mg/l. About one-fifth
of the respiratory rise may be attributed to the free amino
acids which are liberated during senescence; several amino
acids are shown to cause increases of almost 50% in the oxygen
consumption when supplied at the concentrations of total
amino acid present during senescence. A smaller part of the
rise may also be due to soluble sugars liberated during senescence,
largely coming from the hydrolysis of a presumptive
fructosan. The remainder, and the largest part, of the increase
is ascribed to a natural uncoupling of respiration from phosphorylation.
This is deduced from the fact that dinitrophenol
causes a similar large rise in the oxygen consumption of the
fresh leaves or of leaf segments kept green with kinetin, but
causes only a very small rise when the oxygen consumption is
near its peak in senescent controls. The respiration of these
leaves is resistant to cyanide, and 10 mM KCN even increases
it by some 30%; in contrast, etiolated leaves of the same age,
which undergo a similar rise in oxygen consumption over the
same time period, show normal sensitivity to cyanide. The respiratory
quotient during senescence goes down as low as 0.7,
both with and without kinetin, though it is somewhat increased
by supplying sugars or amino acids; glucose or alanine at 0.3
M bring it up to 1.0 and 0.87, respectively.