Ua. Hartwig et al., THE NITROGEN-SINK IS INVOLVED IN THE REGULATION OF NITROGENASE ACTIVITY IN WHITE CLOVER AFTER DEFOLIATION, Physiologia Plantarum, 92(3), 1994, pp. 375-382
In an attempt to manipulate plant nitrogen-sink strength, various defo
liation treatments were applied to white clover (Trifolium repens L. c
v. Ladino) grown in a controlled environment. Nitrogenase activity and
its oxygen limitation were measured as H-2 evolution in Ar:O-2 using
a flow-through gas exchange system. An experiment to monitor the respo
nse of nitrogenase activity to various degrees of defoliation showed t
hat the removal of up to 50% of the leaf area had no effect on nitroge
nase activity within 6 h. If more than 50% of the leaf area was remove
d, the nitrogenase activity decreased in relation to the loss of leaf
area. This was accompanied by a corresponding increase in the O-2 limi
tation of nitrogenase activity. In the experiment to determine the N-s
ink strength after defoliation, the dry weight increase was initially
unaffected by the removal of 40% of the leaf area, whereas removal of
85% or 100% of the leaf area resulted in a dry weight loss for several
days. The time course of nitrogen assimilation was similar to that of
dry weight increase. This study provides substantial evidence that af
ter a severe defoliation the nitrogen demand is temporarily restricted
due to the lack of dry weight increase. Since the plant's ability to
store organic nitrogen is very limited, it seems plausible that nitrog
en assimilation in the still fully intact symbiotic system had to be d
own-regulated. Consequently, to avoid ammonia toxicity, nitrogenase ac
tivity had to be reduced. Such an interpretation is supported by the f
act that complete defoliation of nitrogen-starved plants caused a much
milder decrease in nitrogenase activity compared to the decrease in p
lants well supplied with nitrogen. The present data are consistent wit
h the hypothesis that after defoliation nitrogenase activity is adjust
ed in response to the reduced demand for symbiotically fixed nitrogen
(nitrogen-sink strength). It is proposed that such an adjustment could
be made by a nitrogen feedback mechanism that regulates a Variable ox
ygen-diffusion barrier in the nodules.