This question also relates, to some degree, with questions 2 and 3 above. We have seen that Nitrogen must be converted to ammonium to be assimilated; but that it also requires sugars. We also saw that, for the most part, ammonium is not trasnported via the stem.
The reasoning behind this being true becomes more obvious. Since we require sugars to assimilate all nitrogen sources; then the "location" of those sugars and quantities become a significant factor for ammonium to nitrate ratio.
In addition to this, ammonium to nitrate ratio is also related to the typical pH that a particular plant is used to. Plants that prefer alkaline soils will generally accept higher nitrate; whereas acid loving plants will accept higher ammonium.
reference 1
"Ammonium is metabolized in the roots, where it reacts with sugars. These sugars have to be delivered from their production site in the leaves, down to the roots. On the other hand Nitrate is transported up to the leaves, where it is reduced to Ammonium and then reacts with sugars."
.....
"As we already established, sugars need to be transported down from the leaves to the roots to meet the Ammonium. In growing fruits and plants in which the majority of the growth is in the leaves (e.g. Chinese cabbage, lettuce, spinach), sugars are consumed quickly near their production site and are much less available for transport to the roots. Thus, Ammonium will not be efficiently metabolized and use of a lower Ammonium/Nitrate ratio is preferred."
reference 2
"Free ammonium cannot stay as ammonium in the root not it moves upward as ammonium. In the cytoplasm the pH is above 7, which instantaneously will change the NH4+ partially into ammonia (NH3), which is very toxic to the respiration system in the cell. The ammonium, therefore, immediately reacts with the sugar in the cell to produce amino acid. The nitrate is transported in the ionic form to the leaf where it is reduced to ammonium. In the leaf, the site of sugar production there is always excess of sugar as compared to the amonia produced during nitrate reduction. In the roots the situation is different. The sugar has to move from the leaves to the root."
"In the work of Moritsugu et al. 1983, ( slide4Fig 5.04), it is clearly seen that there is a difference in the sensitivity of various plants to the ammonium to nitrate ratio in the nutrient solution. Rice, barley, maize, sorghum, cucumber, tomato, lettuce, cabbage, Chinese cabbage, spinach, carrot and radish were given ammonium, nitrate or a mixture of ammonium and nitrate, with pH stabilization or without it, at a concentration of 5 mmol N (70 g N /m3). It was found that while rice and maize were relatively indifferent to the form of the nitrogen, Chinese cabbage and spinach plants died when they were given 5 mmol N as ammonium only ( slide4Fig. 5.04). The size of the root system and the sugar reserves in the roots explain the difference in the sensitivity of the various types of plants."
"In general it may be expected that crops, like Chinese cabbage, lettuce, spinach, with most of the growth is directed towards the leaves, will be more sensitive to high concentrations of ammonium than other plants that send large quantities of sugars to the roots."
reference 3
"the preferred form in which N is taken up depends on plant adaptation to soil conditions. Generally, plants adapted to low pH and reducing soils as found in mature forests or arctic tundra tend to take up ammonium or amino acids, whereas plants adapted to higher pH and more aerobic soils prefer nitrate"
