Fertilyser Nitrate / ammonium ratio

[Brabantia]

Usually it is say that fertilyser containing ammonium ions cannot be used at low temperatures (less 15°C ?). The usual reason given is that the microbial activity at low temperature is to weak to carry out nitrification.
This article demonstrates (clic:Here) that with a reasoning which appears to me correct can lead to a contrary conclusion with what is usually allowed.
What is your opinion about this subject?

 

[Stone]

Ammonium seems to be taken up more readily than nitrate at lower temps. However the production of sugar needs to be high enough for the plant to be able to convert the ammonium arriving at the roots into amino acids and to manufacture enough sugar the plant needs plenty of light. In other words you don't want the plant absorbing too much ammonium if it cannot use it (in winter). Because orchids are tropical, they also require higher temperatures along with the light to function properly.
Personally I give them nothing in winter unless they are warm and under lights. Then you can do what you want.

 

[troy]

60° farenheit nights & 75° days under lights is ok to fertilize ?

 

[Stone]

60° farenheit nights & 75° days under lights is ok to fertilize ?

Well those are growing season temps for many species.

 

[troy]

Those are my winter temps

 

[myxodex]

One of the interesting aspects of the NH4/NO3 ratio is the synergism between the two N forms when both are applied. For example in species that cannot tolerate high NH4/NO3 ratios and will grow on NO3 as sole N-source, actually assimilate NO3 more rapidly if a small amount of NH4 is present (i.e. the total increase in the rate of N assimilation cannot be accounted for by the amount of NH4 added). There doesn't seem to be a neat and simple explanation for exactly why this happens apart from the fact that both NH4 and NO3 affect gene expression differently, each turning up and down different groups of genes in response to signals from the roots, and the levels of these genes directly affect metabolic balance. The increase of respiration in response to NH4 as mentioned in the link you provided is one of these effects. This regulation of these metabolic enzymes is also affected by light and so some of it is diurnal. The consensus is that most plants (with the exception of a few specialists) grow optimally if both NH4 and NO3 are present in the feed.

Some plants are very sensitive to NH4 and under particular circumstances it can become toxic. Sensitivity to NH4 is very species specific and the strongest correlation seems to be ecological adaptation more than species relatedness. Barley is sensitive, while rice is quite happy to grow on NH4 alone. Late successional plants tend to be NH4 tolerant while early successional plants tend to be sensitive. I haven't read anything about orchids and I suspect that most are not especially ammonium sensitive under typical culture conditions. I have read some speculation that certain Vanda and Phalaenopsis species which do not do well in standard invitro seed propagation media, might be NH4 sensitive and require low NH4/NO3 ratios but the evidence for this is not conclusive.

As Stone says, the supply of carbs from photosynthesis is necessary for NH4 assimilation and so inadequate light is an issue here. Of course NO3 is converted to NH4 in the plant's leaves and so you might think that the N-source is irrevelant as both need carbs for assimilation. However, the enzymes that convert NO3 to NH4 are regulated by feedback and so the production and accumulation of NH4 will not occur in the case of carb limitation when only NO3 is supplied and means that the less toxic NO3 accumulates instead of NH4. Incidentally, at very high light intensity NH4 toxicity can actually become more severe (see review link below).

In barley, when NH4 is supplied in the feed at concentrations in excess of 1mM (approx 14 ppm N as NH4) the switch from high affinity uptake to low affinity uptake happens and NH4 flows into the roots via an unregulated process and toxicity results. Potassium can inhibit this uptake of NH4 and does so at concentrations above 1.5mM (or approx 28 ppm as K2O). The protective effect of K against NH4 toxicity is well known, and has been reported for a number of species. The practical implication of this is; do not to be tempted by reports of the synergistic benefits of having both NH4 and NO3 in feed, to supplement Klite (which is almost all NO3) with NH4, without also adding extra K. Even moderate NH4 levels and low K are a potentially toxic combination and not necessarily only at low light levels.

There is a good review on ammonium toxicity from Herbert Kronzucker's lab in which they discuss some of these issues.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.383.9319&rep=rep1&type=pdf
The same lab have put forward an alternative model for NH4 toxicity that somewhat controversially suggests that it is not the high cellular levels of NH4 that are toxic per se (as some plants have high cytoplasmic NH4 concs and are not affected), but rather that it is the response of sensitive species to actively pump NH4 out of the cells in a futile process which consumes a large percentage of the plants total energy expenditure and so exacerbating any carb limitation issue. Explained in this paper; http://jxb.oxfordjournals.org/content/59/2/303.full
It is also known that exogenous application of krebs cycle organic acids can help alleviate NH4 toxicity which fits in quite neatly with this idea.

 

[consettbay2003]

What are the symptoms of NH4 toxicity?

 

[myxodex]

It seems as though there are no unique symtoms of ammonium toxicity (i.e. no symptoms that define the issue as ammonium toxicity).
Researchers have recreated ammonium toxicity in defined conditions in order to study it. The reported symptoms are somewhat dependent on other factors. Slow growth, chlorosis of leaves and poor root development are the most commonly reported. Technical measurements of plants suffering from NH4 toxicity reveal very high respiration rates and excessive ethylene production, both of which are signs of stress. Germination and survival of seedlings in high NH4 feeding is low or absent in NH4 sensitive species.

The reason I have looked into the whole NH4/NO3 ratio issue comes from failed experiments with trying to grow Neofinetia seeds. I got early stage protocorms quite quickly but these became glassy and died. This glassiness problem is called either vitrification or hyperhydricity in the plant tissue culture field. This problem has been reported for a few Vanda and Phalaenopsis species at protocorm stages and it was speculated that they need a lower NH4 / NO3 ratio to grow and develop in vitro. NH4 toxicity is more pronounced in vitro possibly because there is no nitrification by bacteria, but more likely due to limited ventilation. A symptom of NH4 toxicity in vitro is hyperhydricity, although this might be caused by, or at least exacerbated by other factors such as poor ventilation and/or small head space in the growing container. Excessive ethylene production is generally found in cases of hyperhydricity as it is with NH4 toxicity ... so these two issues are inter-related and difficult to separate out. It has been suggested that hyperhydricity is always caused by NH4 toxicity, while other researchers only list it as one of a number of causes.

The only information that I could get on in vitro cultivation of Neofinetia was the english abstract of a Korean publication. They used a Japanese fertiliser called Hyponex No.1 (NPK = 6.5 - 6 -19) as the basal medium. This has been used quite a lot in orchid seed media for production in Taiwan and Korea. I have read on another forum that this medium has a NH4:NO3 ratio of 15:85; which is lower than that of most orchid seed media (>= 33 % NH4). It has also been used successfully to grow some Vanda and Phalaenopsis species from seed, but unfortunately Hyponex do not give much technical information about their media. I have read that in tissue culture the growth and differentiation of callus into roots and shoots can be very sensitive to the NH4/NO3 ratio for some species. It has also been shown that the NH4:NO3 ratio affects the production of plant hormones (cytokinins particularly) and the optimum for development does vary a lot between species and can be affected by relatively small changes in this ratio.

I suspect that ammonium toxicity is unlikely to be common in general orchid culture for a number of reasons. Firstly, NH4 toxicity is less likely to occur in the presence of abundant nitrate, especially when the NH4/NO3 ratio is less than 0.5 (i.e. less than a third of total N), which it is in most fertilisers. Secondly, most fertilisers are high in K (at least K > NH4) which helps protect against ammonium toxicity. In addition bacterial nitrification will lower the NH4 concentration applied so elevated NH4 amounts will only be transient. If some orchid species are really NH4 sensitive then they will most likely also be difficult to propagate from seed on the usual media which are generally high in NH4.

 

[Bjorn]

Interesting......I have been using my own fertiliser mix that contributes with approx 10ppm NH4 and 6 ppm NO3 tot he water I use, without seeing any negative effects of any kind? That means that I have a ratio of 10/6 = 1.66. I do not believe so much in the story with the nitrosomas either , since all my water contains this nutrient mix. Another thing that talks against nitrification is the fact that it works excellently on mounted plants like Cattleyas that dry up in a matter of hours-if not minutes after watering. Not an ideal environment for bacteria I presume? These are all observations from deflasked plants, seedlings in an (unventilated) environment might be a totally different story. Problems due to anarobic conditions have been mentioned before, amongst other by Roth regarding propagation of tigrinum, perhaps related to the current topic?

 

[myxodex]

Interesting......I have been using my own fertiliser mix that contributes with approx 10ppm NH4 and 6 ppm NO3 tot he water I use, without seeing any negative effects of any kind? That means that I have a ratio of 10/6 = 1.66. I do not believe so much in the story with the nitrosomas either , since all my water contains this nutrient mix. Another thing that talks against nitrification is the fact that it works excellently on mounted plants like Cattleyas that dry up in a matter of hours-if not minutes after watering. Not an ideal environment for bacteria I presume? These are all observations from deflasked plants, seedlings in an (unventilated) environment might be a totally different story. Problems due to anarobic conditions have been mentioned before, amongst other by Roth regarding propagation of tigrinum, perhaps related to the current topic?

I don't expect that you would see any issues. Many NH4 sensitive species would probably grow in your feeding regime without issue and we do not have any evidence that orchids are NH4 sensitive anyway (although it would not surprise me if some orchid species were found to be NH4 sensitive).

The concentration of NH4 you are feeding is low and in the high affinity range (i.e. less than 1 mM). The issue with higher concentrations, those at which low affinity transport kicks in, is that we enter a range in which the plant cannot resist uptake, so it becomes a bit like force feeding. With NH4 even this is not a problem so long as the rate of assimilation can keep up with the rate of uptake so that cytoplasmic NH4 accumulation is not excessive. The rate of assimilation is dependent on other factors including the rate of photosynthesis and the level of NO3 (although it is unclear exactly why?). Incidentally it has been shown that photosynthesis is disrupted by high NH4 levels, a key process for reducing them.
The rate of uptake is dependent on the external concentration and increases with increasing concentration in a linear relationship that doesn't show saturation in contrast to high affinity uptake which does show saturation. In barley, and reported for other plants, this NH4 uptake is competitively inhibited (at least partly) by K when it is also in the low affinity uptake range (kicks in between 47 and 70 ppm K2O).

The authors of the review I linked in a previous post above, suggest that NH4 toxicity is probably universal in plants, what varies are the levels which can be tolerated and this in turn is dependent on other factors such as light, NO3, K and so on. My thinking is more that optimisation of growing conditions implies reducing stress, as stress incurs a respiratory burden and so wasting a sizeable proportion of photosynthetic carbon fixation, even before it becomes symptomatic. For example in their paper ( http://jxb.oxfordjournals.org/content/59/2/303.full ) in which they show how K can alleviate the respiratory burden of NH4 toxicity (figure 7), they also show that at the highest level of K (40 mM or 1880 ppm K2O) the respiratory burden of NH4 export is now joined by a significant respiratory burden of exporting K. So at very high levels of K, it too can become a stressor and impair the growth of the plant, ... and even at optimal K in terms of growth (5mM - 235 ppm K2O) the combined burden of over-feeding both NH4 and K comes to over 40% of total energy costs. This shows that even agricultural plants can struggle in high feeding scenarios when the balance of nutrients is poor. Bjorn, your feeding regime is at the lower end of the range used by most orchid growers and you are below the range of force feeding. I'm not suggesting that feeding at higher levels is necessarily bad, but just that the potential impact of any nutrient imbalances, in terms of stress incurred and carbohydrate wasted, increases quite a lot when we move into concentrations which involve force feeding.

In tissue culture and orchid seed propagation media the concentrations are generally high (low affinity uptake levels), and force feeding in this scenario could possibly be a requirement for success. Scan the literature in this field and you will soon discover how critical the NH4/NO3 ratio can be in it's effects on both the growth of callus and its differentiation into roots and shoots, and indeed the ratio of shoot to root growth. Even in pot culture the shoot to root ratio is affected by NH4/NO3 ratios which varies across species, but with a general trend of high NH4/NO3 ratios favouring shoot over root growth. At sub-millimolar concentrations these effects might not be apparent ... who knows ?

As for nitrification, you have a point, and I used the wrong word. I was thinking more of combined microbial processes that consume NH4. There are a lot of bacteria and fungi that will assimilate NH4 as a source of N. It has been pointed out on this forum previously that bark decays more rapidly with combined NH4 + NO3 feeding than it does with NO3 alone. This is most likely a consequence of NH4 boosting the growth of microbial polysaccharide degraders for which N is probably the limiting nutrient, and most of these will not be able to use NO3.

 

[Bjorn]

Thanks myxo for this elaborate explanation. If I understand you right, the explanation to the many reported good effects of growing at very low nutrient levels is, at least partly, due to the fact that amongst others the ammonium hovers in a range where it is not getting toxic?