FREE NITROGEN COMPOUNDS IN PLANTS 669 
or asparagine seems also to be a function of the time of year in which the fruit develops. 
In this later phase of growth of the fruit, an unusual feature is the accumulation of 
relatively large amounts of free histidine and, after passing the crisis known as the 
climacteric, so that ripening may ensue, there is a relatively massive conversion of 
what was previously amide nitrogen into free histidine. This example is but one of 
what may be many kinds of conversion which have hitherto passed unnoticed for 
lack of the ready means of their detection which chromatography has provided. 
When such massive accumulations of soluble compounds occur, this seems to be 
associated either with the slowing down of growth and of the consequential demand 
for nitrogenous compounds to build protein, or it may be due to a block in metab- 
olism for genetic or environmental reasons. 
It is now apparent that the course of nitrogen metabolism and the storage of the 
soluble nitrogen compounds in plants is dramatically influenced by the inorganic 
nutrients. A typical example is the mint plant, where effects due to the so-called 
macro nutrient elements have been described*®. Potassium tends to promote one 
type of change in leaves and calcium another, whereas deficiencies of an element 
such as sulphur may result in massive accumulations of amide, mainly glutamine, 
and of the amino acid arginine. 
However, not only nutrition but also the environment in which the plant grows 
has a determining effect on the course of its nitrogen metabolism and on the soluble 
compounds which are formed and stored. A diurnal cycle in the mint plant tends to 
promote glutamine in leaves in the light and asparagine in the dark, and this in 
turn is associated with a general trend toward protein synthesis in the light and 
toward protein breakdown in the dark**. A similar conversion of {!4C}proline, when 
added to leaf discs of tobacco, to glutamine in the light and to asparagine in the 
dark (Fig. 1) has also been observed in this laboratory by POLLARD AND RocHow?s, 
working with one of us (F.C.S.). MoTHEs also reported to the International Botanical 
Congress at Montreal in 1959 the formation from |!4C|urea of asparagine in the dark 
and glutamine in the light. A very important feature of nitrogen metabolism is the 
way in which it may be modified by such features of the environment as the length 
of day and night and the fluctuating diurnal cycle of temperature. For the mint plant, 
where these relationships have been worked out in some detail, it is of interest to 
note that the environmental variables also interact with the nutritional ones, so 
that effects which are due to length of the day and to potassium supply tend to work 
together; whereas effects due to calcium nutrition and to short days and long nights 
also tend to work together. But, surprisingly, the overriding variable is that of night 
temperature. Low night temperature causes asparagine to appear in otherwise long- 
day-high-potassium mint plants, in the leaves of which it would not normally oceur 
but where it replaces the amide glutamine. Such effects as these clearly present a 
complex picture of the soluble nitrogen pool which, for its interpretations, requires 
both physiological and biochemical investigation. 
Effects upon the course of nitrogen metabolism are not confined to the so-called 
macro-nutrients, for the trace elements, which are required only in very minute 
amounts, may also have a very prominent effect as shown by ZACHARIUS (see ref. 46), 
and they also interact both with each other and with other physiological variables. A 
prominent example of this concerns molybdenum and manganese, the effects of 
which on the nutrition and nitrogenous composition of the tomato plant require the 
References p. 692/693 
