FREE NITROGEN COMPOUNDS IN PLANTS 685 
hese experiments, using much lower cell densities and much higher concentrations 
of the [14C]|substrate in the ambient medium. Under these latter circumstances a 
constant turnover of protein does take place, even in the exponential phase of growth, 
and these authors estimate that the bulk protein had a half-life of 3-4 days. 
Therefore, protein turnover does appear to take place even in actively growing 
cells, though the degree of importance may well be different at different stages along 
the growth curve and in the development of the cell population. 
The ability of cells in plant storage organs to re-synthesize protein may have 
some significance here. In the resting cells of a potato tuber the balance between the 
soluble nitrogen and the protein nitrogen remains quite stable during the life of the 
storage organ. It has, of course, been shown that this is drastically disturbed when 
I, PROTEOLYTIC ACTIVITY (/N V/VO & V/TRO) 
2.02 CONSUMPTION 
3. OXIDATIVE ENZYME ACTIVITY 
4. CARBOHYDRATE SYNTHESIS (GLUCOSE; CELLULOSE ) 
5.EXCHANGE ABILITY OF POOL AMINO ACIDS 
a. SPECIFIC ACTIVITY OF POOL reRnAL{ SI METHenR) 
b. SPECIFIC ACTIVITY OF PROTEIN ( 
6 RATIO OF SPECIFIC ACTIVITY ETHANOL INSOLUBLE 
avian (ike TY “ETHANOL SOLUBLE PROTEIN 
\ENDOGENOUS 
"\ PROTEIN 
‘\e 
~~) 
“. 
SS 
SN. POOL SIZE 
=e 

HOURS 
Fig. 2. A schematic representation of various biochemical changes occurring during differentiation 
in the slime mold (From WRIGHT AND ANDERSON”*), 
the cells commence to grow again, as in the behavior of cells at the surface of a cut 
slice. However, this condition of balance in the cells of the tuber, which may well be 
maintained by a constant but slow process of breakdown and re-synthesis, is dras- 
tically disturbed if the tubers are stored at a low temperature, particularly at a tem- 
perature of the order of +1°. It has been shown (cf. ref. 42, p. 244) that under these 
circumstances the ability of the cell to re-synthesize protein entirely disappears, so 
that the suggested normal cycle of breakdown and re-synthesis would be disturbed. 
Under these circumstances the tissue is unable to maintain many of its vital activities, 
notably the ability to retain solutes against distilled water and to absorb ions against 
an adverse diffusion gradient. 
Another critical point in the life of resting or storage cells is the climacteric, as this 
is shown in various fruits; notably the apple and the banana. Again it appears that 
there is a relatively stable content of protein in the mature organ, but the climacteric, 
which is associated with a temporary increase of respiration and also of protein, is 
References p. 692/693 
