My friends and I were reading an article and something drew out attention was about ammonium toxicity on plant. We came up with the idea that it is due to the ammonium inflow rate to the root of barley is higher than to rice. We were not quite sure if there are any more reasons to explain that kind of adaptation?
If, as you say, barley roots can take up more ammonium relative to rice, then the barley must also process the ammonium too. Barley has a higher protein content than rice, 9.9 vs. 7.1 g per 100 g. So, perhaps barley roots take in more ammonium so that more amino acids can be made (detoxifying the ammonium and storing nitrogen), which can allow more protein to be made. Just a thought.
- You are right, at high external NH4 concentration, barley root cells experience a breakdown in the regulation of NH4 influx, leading to the accumulation of excessive amounts of NH4 in the cytosol. see http://www.pnas.org/content/98/7/4255.abstract
- and http://webs.hogent.be/~fdbe129/amtox.pdf for review.
I think that rice has tolerance to ammonium due to better adaptation to anaerobic conditions in soil under water.
I think it relates to the magic physiology of rice, may be its distinctive atmosphere and environment, rice can live in marshy lands... you can search about a suspected enzyme in the cholorophyll of rice, which is called PEPC...
I don't know much about barley, but I guess ammonium inflow rate to the root may be higher in rice as rice roots have high activity of ammonium transporter. Ammonium is somewhat harmful even to rice root tissues, but glutamine synthetase (GS) in the rice root can immidiately use the absorbed ammonium. The glutamine is then converted to various amino acids (e.g., asparagine).
The activity of glutamine synthetase may be weaker in upland crops such as barley, wheat and maize. Besides, some upland rice varieties are rather sensitive to ammonium.
The reason might be the rice is adapted to anaerobic environment where the concentration of the ammonia is lesser compared to aerobic conditions where barley is gron. Moreover, the rice is tropical to sub-tropical crop where nitrification rates are faster and ammonia conc is more but barley is cool season crop where nitrification rates are slower, hence more NH4 conc. in soil and thus it become sensitive. I agree that rice physiology as well play a role into it.
To: Shankar Lal Jat
Most nitrogen is ammonia nitrogen in flooded paddy soil, while most nitrogen in barley field is nitrate nitrogen．
To: Moazzam Jamil
I agree that aerenchyma plays an important role for the growth and function of rice roots under flooded conditions. For example, rice roots may oxidize bivalent iron to trivalent iron. But as for ammonium ions, rice roots can absorb and use them without oxidization to nitrate.
Well, as far as I know, rice is tolerant to NH4 + because tolerates acidic media, which are precisely flooded soils where nitrification is inhibited. The question then is, why does rice grow without problems in acid soils and not does barley? The answer may lie in the mechanisms of pH homeostasis, where PM H+-ATPase is the key. Indeed, its activity is very high in rice and others acidophilic plants, and this allows to maintain the pH gradient in the plasma membrane. A lower activity of that enzyme depolarize the membrane and can strongly alter metabolisms and damage cells. The futile cycle that Alex Ignatov refers to is due in part to the efforts of the root cell to supply ATP to the pump (mainly thorough more root respiration, as it is based on the finding that NH4+ per se is not
translocated to the shoot in most plants), in those plants that cannot maintain higher PM H+ATPase activity (as barley).
Hope I have helped you something. My work is currently focused on the toxicity of ammonium.
Zhu Y et al. 2009. Adaptation of plasma membrane H+-ATPase of rice roots to low pH as related to ammonium nutrition. Plant Cell Environ, 99: 415–421.
Zhu Y et al. 2010. Involvement of Plasma Membrane H+-ATPase in Adaption of Rice to Ammonium Nutrient. Rice Science, 2011, 18(4): 335−342
Be careful about the source of your data. Many papers are treating observations from experiments on nutritive solution experiments (with fixed nitrogen species concentrations) when in the fields plants have not nitrogen in a non limiting fashion. Cases of toxicity in nature are not frequent because nitrogen is often a scarce ressource and plants have evolved to use this resssource as rapidly as they can (by diverting photosynthates fom the respiratory pathway to the assimilation pathway). For ammoniacal nutrition (rice in paddy soils), this process occurs predominantly within the root. So there is a strong interest to measure the activity of enzymes regulating the orientation of C (oxo-glutarate) towards the N assimilation (by GS-GOGAT).
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