Potassium Research and Agricultural Production - The International ...

ciency in plants: (a) soluble carbohydrates and reducing sugars accumulate; (b) starch
and glycogen syntheses are impaired; (c) amino acids accumulate and protein synthesis
is blocked; (d) the utilisation of respiratory substrates is retarded; (e) oxidative phosphorylation
and photophosphorylation rates are decreased, etc. Potassium ions have a
fundamental influence on a whole series of different metabolic processes in plants. In
the main lecture H.J.Emns and R.A. Wildes had described the fundamental role of
potassium, to be a co-factor for a large number of enzymes that participate in several
major metabolic processes. Of all univalent cations present in normal living tissues.
only potassium has the appropriate properties, and is present in cells in sufficient concentration,
to fulfil! the requirements of the great majority of enzymes which depend
for their activity on univalent cations. The influence of potassium on photosynthesis
was not discussed in the session; but shortage of potassium does lessen photosynthesis
activity, partly as a result of alterations in protein metabolism and the impaired functioning
of a specific enzyme. Potassium supply also affects the production of chlorophyll,
on which photosynthesis depends.
1.2. Second Session
This session was closer to agriculture and dealt with uptake and distribution of K in
plants and its interactions with other ions, effects on transport processes and on the
water economy in plants. The use of plant analyses in assessing nutrient status was discussed.
These are some of the points Professor Mengel made in summarising the session:
Nitrogen nutrition and nitrogen metabolism have large effects on uptake and distribution
of inorganic ions. A balance between cations and anions is essential for normal
plant life. This balance is important both in considering gross uptake and release
by plant roots and also balance in other plant parts. Uptake of ions from the nutrient
solution involves a balance between inorganic ions, whereas in the plant itself the organic
anions have a large part in balancing the bulk of cations. Ion content and ion distribution
do not depend only on N metabolism, but also on the ability to be actively
transported through biological membranes. SCOll Russell had said potassium can be
transported against an electrochemical potential gradient, for calcium and sodium
such active uptake by cells of higher plants has not yet been proved. This unique feature
of potassium absorption enables the root to take up large amounts of potassium,
even from dilute solutions. This property is inherent in the whole length of plant roots,
and a short segment can supply the entire plant with potassium, provided the environment
supplies enough K to this segment. Therefore the potassium transported to the
roots, rather than the root surface, controls total potassium uptake; however we do
not know which part of a plant's root system takes up nutrients in the field. This
unique property of potassium, to be accumulated in plant cells by active uptake mechanisms,
affects the water economy too. An accumulation of potassium in the xylem
vessels gives an osmotic gradient, which is responsible for water uptake from the root
environment and for root pressure. Also the potassium accumulation in the guard cells
of the stomata, affecting stomatal opening and closing, plays a major role in water
economy.
The high potassium concentration found in sieve tube vessels justifies the assumption
that potassium is actively secreted into the sieve tubes where it is the dominant cation
balancing the various organic anions. Possibly the favourable influence of potassium
on the transport of photosynthates from the leaves to the storage organs is caused by
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