Sorry it has taken me so long to get back to this thread. I have spent something like twenty hours reviewing studies on the net trying to make the link between exogenous application of NAA (a root hormone often used for adventitious rooting in cuttings) and stimulated root growth in transplants. This is what root stimulants are supposed to do. You would think that someone, somewhere, would have published a study to show if these root stimulants using NAA were really efficacious in a real world nursery situation. Finding evidence either for or against any effect is elusive, but I have found several references.
Bretts gave me several links to read, which I have done in addition to the thousand or more that I also found. I have found evidence from a few studies that treatment with NAA can be beneficial to seedling transplants in some situations for some species. Nearly all of the hits you get when you google the words exogenous, NAA, roots, will give you citations identical to, or similar to the ones Bretts cited. These studies for the most part were done in the '30's and were experiments on excised root tips to show that IAA (the natural equivalent of NAA) is the root growing hormone produced by the leaves, buds, and shoots. These studies are all done on non woody plants, often unbranched seedling roots in culture. One could read these and understandably make the leap that treating roots with a hormone solution might do the same thing in growing woody plants. But it is not that simple, as I pointed out with the car/gasoline metaphor.
In my opinion, although there are some effects of NAA treatments on woody plant roots in some situations, it is probably not significant in container culture in which all the environmental factors are tightly controlled. After reading all these studies, I have to stand by that assessment. The main reason that I still believe this is that any effect would be greatly overwhelmed by the other effects that are natural plant responses in an ideal environment. As I pointed out previously, the danger of buying into the root stimulant argument, is that people often will ignore the good, solid horticultural techniques that are proven and time tested in favor of the easy route of just adding a pill. Believing in a stimulant is easy, researching the applicable physiological solutions is more difficult. The strong tools for enhanced root growth are:
1) maintenance of as much foliage, bud, and shoot material as possible (given that transpiration is also a factor in transplants).
2) Root pruning, particularly the removal of primary root tips has a very strong effect on the growth of lateral roots, many times the effect, if any, of exogenously applied NAA, and is applicable to all species and all environmental situations.
3) Simultaneous moderate application of nitrogen fertilizer to the root zone (yes, avoiding nitrogen fertilization is myth).
4) Maintenance of an optimal stress free environment that allows retention of the maximum amount of buds, leaves, and shoots to drive the natural IAA hormone machine.
5)And this was a surprise, avoiding phosphorus in the fertilizer at transplant time. (avoid iron and sulfates as well)
As for Dr Linda Chalker-Scott's website and work, I applaud her myth busting work and agree with most everything she espouses, although I think she has a tendency to overdo it. She is also quite a controversial expert which you can get a taste of here:
http://forums2.gardenweb.com/forums/load/design/msg0213585812574.html
When you read what she has to say about myths, you must bear in mind that she is talking about landscaping for the most part, for which the only equivalent for us would be field growing. She does suggest that root stimulants MAY (her word) help in transplanting by redirecting resources to the roots instead of top growth, but I have not been able to find any study to confirm this for other than seedlings, and again, she is talking about field growing, not container growing, which a whole different ballgame. Container growing creates an ideal environment, field growing is about overcoming soil limitations.
I have written extensively about the effect of top pruning on root growth in transplant situations. Although I haven't done controlled studies, I have strong anecdotal evidence that indeed, retention of as much foliage and buds (especially terminal buds) has a strong effect on root growth. See this article on my website:
http://www.evergreengardenworks.com/rootprsd.htm
A study on the effect that top pruning can have on woody plant seedlings can be found here:
http://www.afs-journal.org/index.php?option=article&access=doi&doi=10.1051/forest:2006067
I have found several studies and references on the effect of NAA on root growth. I have considered them all in making my conclusion about the use of root stimulants. I have listed the pertinent ones below, and you can decide for yourself if you think an NAA transplant solution would be beneficial in your particular circumstance.
Exogenous application of NAA can stimulate lateral roots:
[endogenous] Auxin and cytokinins are key factors in determining lateral root initiation. Exogenous auxin induces a extra lateral roots in a dose dependent manner (ie. more auxin, more lateral roots). Conversely, many auxin mutants show a paucity of lateral roots. Environment also influences lateral root development. Lateral roots in nitrogen deficient environments will respond to applied NO3- by elongating. This requires the function of a myb transcription factor called ANR1. Lateral roots of antisense ANR1 plants do not respond to nitrate. This also demonstrates that this is a specific response to the nitrate stimulus and not simply a nutritional response because the antisense plants are otherwise healthy.
http://www.public.iastate.edu/~bot.512/lectures/Roots.htm
Also see this interesting study:
Transplanting often causes root damage, and rapid root growth following transplanting may help to minimize the effects of transplant shock. The objective of this study was to determine the effects of NAA and IAA on posttransplant growth of vinca (Catharanthus roseus L.). Bare-root seedlings were germinated in a peat-based growing mix and transplanted into diatomaceous earth 10 days after seeding. Immediately after transplanting, seedlings were drenched with several concentrations of IAA or NAA (62.5 mL/plant). Both auxins increased posttransplant root and shoot growth, but the response was dose-dependent. The maximum growth occurred at concentrations of 10 mg.L[-1](IAA) or 0.1 mg.L[-1] (NAA). The growth-stimulating effect of these auxins decreased at higher rates and NAA was highly toxic at 100 mg.L[-1], killing most of the plants. Unlike the growth of bare-root seedlings, plug seedling growth was not stimulated by drenching with NAA solutions. These results show that auxins have the ability to stimulate posttransplant growth of vinca, but their effects may depend on the application method, rate, and timing, and transplanting method.
http://cat.inist.fr/?aModele=afficheN&cpsidt=1611451
I find the last two sentences of this conclusion to be very interesting. This further points out my original premise that you must be very careful when trying to extrapolate the results of any experiment to a situation with different parameters.
Here is another study on the exogenous application of NAA on seedlings:
http://forestry.oxfordjournals.org/cgi/content/abstract/63/2/197
However, these are seedling experiments with NAA applications just before and just after cotyledon development. It is generally accepted that root hormones have a strong influence on seedling germination and root development in the first week or two after germination. It is a mistake to extrapolate this to woody plants with developed plants in a transplant situation.
NAA root applications may be of benefit in planting conifers in forestry situations where root development is difficult in native soils:
Primary lateral root production on Pinus contorta seedlings was increased 5–10 fold by immersion in auxin solutions. Watering auxin solutions onto P. contorta seedlings growing in soil or compost induced the development of large numbers of primary lateral roots in the root collar region. The number of primary lateral roots which developed depended mainly upon the auxin used and its application rate, seedling age and to a lesser extent on the soil characteristics. Instability of transplanted pines is partly due to their lack of primary lateral roots and their inability to regenerate new ones after planting out; this instability may be overcome by using auxin treated seedlings. Potential practical applications of this technique are discussed.
http://forestry.oxfordjournals.org/cgi/content/abstract/55/2/125
I found one study that concludes that NAA root treatments of a woody plant clearly had no benefit on overall plant growth. This study uses several measures for plant growth, that I think are worth noting.
http://72.14.253.104/search?q=cache...imulate+root+growth&hl=en&ct=clnk&cd=86&gl=us
(scroll down to page 110 to find the effect of NAA)
This study compared the effects of NAA and BA (a cytokinin) separately on the accumulation of CPT, a cancer fighting compound. However, it also measured the effect of NAA on the morphology of the plants including RSR (root to shoot ratio), plant weight, and plant height. The concentration of NAA used was from 0 to 4 mg/L, which is in the ballpark for the concentrations used in the other studies above that showed lateral root development in this range.
The conclusion is:
In conclusion, exogenous BA and NAA applications inhibited plant height and leaf
number of C. acuminata in a hydroponic culture system, and NAA applications also inhibited
plant weight and leaf length. Exogenous BA and NAA applications both increased RSR, and
NAA applications decreased SLW. BA applications significantly increased CPT accumulation,
whereas NAA applications had no effect on CPT accumulation. Conversely, BA applications had
no effect on CPT yield, and NAA application decreased CPT yield
Unlike potential positive effects in compromised environments, the effect of NAA solutions on roots after transplanting must be compared to the strong stimulation that can be effected by several known and accepted procedures as I pointed out above. Not so commonly known is the role of Phosphorus. Myth has it that phosphorus is good for roots and thus should be good for transplants (bonemeal addition to soil prior to planting for example). The actual effect is the opposite:
Plants typically respond to P deficiency by allocating more carbon to roots, thus increasing their root-to-shoot ratio. In addition, low P availability can dramatically alter the spatial configuration of the root system by increasing root hairs and promoting lateral root formation. Such plastic root alterations are believed to play a crucial role in exploring increased soil volumes in search of nutrient-rich patches (Neumann and Martinoia, 2002; López-Bucio et al., 2003). In Lupinus albus, P deficiency promotes the formation of cluster roots, also termed proteoid roots (Johnson et al., 1996). Increased root branching in response to P limitation has also been reported for Arabidopsis (Arabidopsis thaliana). Growth of Arabidopsis in a moderate-to-limiting supply of P results in a redistribution of root growth from the primary to lateral roots (Williamson et al., 2001). Reduced primary root elongation in low P conditions was accompanied by increased root branching, perhaps concentrating root biomass near the soil surface for more efficient nutrient foraging (López-Bucio et al., 2002, 2003
http://www.plantphysiol.org/cgi/content/full/137/2/681
But once again, I have to caution you that you have to consider the overall plant growth before developing a plan of attack. So, if you decide to withhold phosphorus because it promotes strong fast lateral root growth after a transplant, you would have to quickly reverse this to normal nutrient feeding as soon as the roots colonized or nearly colonized the pot to prevent the deleterious effects of P deficiency in other parts of the plant.
Nutrient levels can have a strong influence on root development:
Several soil nutrients can alter root hair development (Reviewed in López-Bucio et al, 2003). Fe or P deficiencies both induce more epidermal cells to differentiate as root hairs, and both induce root hairs to elongate more than normal. These two pathways appear independent. The Fe pathway appears to function through the ethylene and auxin pathways because ethylene and auxin mutants show altered responses to Fe deficiency. P appears to be ethylene and auxin independent. The increased root hairs increases the surface area of roots, increasing their capacity to absorb limited nutrients.
Several nutrients can also alter root architecture by altering lateral root formation or growth, or by altering primary root growth. High nitrate inhibits lateral root elongation if the root system is uniformly exposed. However if only a portion of the root system experiences high nitrate while the rest experiences deficiency, the section with high nitrate will show elongated lateral roots. A MADS box transcription factor, ANR1 is induced by local high nitrate and is required for the root architecture response.
Nitrate supply increase cytokinin content in the root. Takei et al (2004) have shown the adenosine phosphates-isopentenyltransferase AtIPT3 is a key determinant of nitrate-dependent cytokinin biosynthesis in Arabidopsis.
Phosphate deficiency induces the formation of lateral roots and inhibits root elongation. The result is a dense, highly branched root system. This is compounded by the effect on root hairs. In addition, expression of phosphate transporter genes and other physiological changes result in a root system highly adapted for efficient uptake of P. The effects on root growth are brought on by inhibition of the cell cycle and by low auxin concentrations in the root apical meristems.
Sulfate deficiency also increases lateral root density. The NIT3 (nitrilase3) gene is induced and thought to increase auxin synthesis.
Moreover the cytokinin receptor CRE1/WOL/AHK4 has been shown to be involved in the interaction between phosphate-starvation, sugar, and cytokinin signaling in arabidopsis (Franco-Zorilla et al., 2005)
Several lines of evidence suggest the nutrient ions may act directly as signaling molecules. Mutants in nitrate metabolism still show the normal response to Nitrate. Root systems on plants with adequate phosphate show the classic P starvation phenotype in localized areas of deficiency. Thus the changes in root architecture are not secondary effects of altered metabolism, but appear to be primary effects regulated by the ions themselves.
http://en.wikibooks.org/wiki/Plant_Sciences:Arabidopsis_root_development
The non uniform nitrate levels is especially interesting. First, this would indicate that high levels of nitrate should be avoided. But also, if irregular location of the nitrate can be achieved, this will increase root growth. This may be possible with pelletized time/temperature release fertilizers. I have been incorporating Osmocote into my soil mixes for years.
Lastly, in one of the studies (unfortunately I can no longer find the citation), it was shown that a single treatment of NAA stimulated the growth of lateral roots, and that subsequent treatments did not stimulate more rooting, and could in fact, inhibit further rooting. This makes sense since the role of NAA in stimulating rooting is to activate the tissues deep within the root to produce lateral roots, but the growth and elongation of the new lateral roots is due to the other factors. This is exactly equivalent to the role auxin plays in stimulating adventitious roots in stem tissue. Here also, a single application is beneficial, but subsequent applications result in no further rooting and may inhibit further lateral rooting.
In conclusion I think we can fairly and safely assume the following from the studies and references above:
NAA treatments can stimulate lateral roots in new seedlings.
Root stimulation is species dependent and in some species may be counterproductive.
Response level is greatest at very low doses with a possible maximum of 0.1 mg/L (equivalent to 0.1 ppm).
Higher dose rates are counterproductive and may even be toxic.
A single treatment is best, additional treatments with NAA do not further stimulate lateral roots or are inhibitory.
Treatment will work best with a moderate amount of nitrate and no phosphorus and no iron.
Continued in next post.......
Brent
