Vol. 16—1962 - NorthEastern Weed Science Society

a copious precipitate deposits after ten minutes of boiling. Subsequent
experiments have revealed that this effect is proportional to the concentration
of 2,4...D employed, both in stElllla, where 2,4-D promotes growth, and in
roots, where 2,4-D inhibits growth. The minimal effective concentration
appears to be about 10-6 molar in both tissues and the effect rises linearly
with the logarithm of the conoen~rat1on emplqyed. There is no evidence of
an optimum. Certain of these facts have, of course,. compelled us to doubt
the physiological significance of this phenomenon, for, as you are aware,
a graph relating growth effect to concentration of auxins applied to plant
tissues usually shows a sharp optimum concentration, for growth promotion.
I believe, however, that this is not a serious stumbling block, for in our
laboratory we have obtained evidence that the growth inhibitional phase of
auxin action on stems, at least, is quite separable from the grolvth promotional
phase •. For example, in experiments with excised pieces of etiolated
pea stem, it can readily be shown that when sugar is added to the medium
there is an auxin optimum, but when sugar is omitted from the mediumthere
is no auxin optimum. Regarding the lack of difforential effect on root
and stem in our protein test system as contrasted with differential effects
of auxin on root and short in vivo, we need only remark that there is good
evidence to believe that the basic action of auxin in all tissues is fundamentally
the same. Whether one aohieves growth promotion or growth inhibition
as a result of auxin application is probably the result of secondary
reactions of various types in the different tissues. At any rate, we do
not believe, at the moment, that we need to relegate the results we have
found to the limbo of the physiologically meaningless.
What evidenoe do we have that the phenomenawe have discovered are
in fact biologically meaningfUl? In the first place, the effect appears to
be elicited only by those t,ypes of molecules which show auxin activity.
For example 2,4-D and indoleacetio aoid are most active, phenylacetic
acid and.2-3-5-triiodobenzoic acid are moderately active, and the antiauxin
p-chlorophenaxyisobut,yTic acid is cqinpletely inactive. Secondly, only
those cells of both etiolated and green pea plants which are capable of
responding to auxin from a growth point of view show the altered coagulability
pattern of proteins. Thirdly, kinetic studies have convinced us
that altered heat coagulability maymnnifest itself as soon as two hours
after application of auxin, although the usual time lag is four hours.
Since we can first detect growth differences between .control and treated
tissues in about one hour or so we are at least close to the range in which
our effect must operate to be of significance in the control of growth.
Fourthly, gibberellin, which produces no marked effect on the growth of
excised pea stem tissue, is also without effect on alteration of the heat
coagulability of proteins, though it does appear to synergize slightly with
auxin, as it does in growth itself.
an'the·chemicalside we have also done certain experiments to try to
understand the nature of changes wrought in the proteins. The first question
would appear to be this1 Is it protein itself which is changed by auxin
trea"bnent or can we possibly be altering something else in the tissue which
results in an altered stability of the proteins toward heat? The first
thing to remark here is that the altered coagulability persists after prolonged
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