Since evolution and crop breeding programs have not favored HUP+ symbioses over HUP" symbioses, it may be possible that H2 evolution by nodules is beneficial to the growth and yield of the symbiosis.
To test this hypothesis, plant growth was compared in soils that were previously exposed to air or to H2 in air at an H2 exposure rate that increased from 63 to 250 pinoles L"1 soil h"1 over a 9 week period. This H2 exposure rate was similar to that experienced by soils adjacent to legume nodules during plant growth (Dong, Layzell 2001). After 43 d, soybean plants grown in H2 treated soil had 14% more dry weight than plants grown in untreated soil (Figure 1) (Wu, Dong, Layzell, unpublished).
A plant growth response was also observed in non-legumes. For example, 44 d old barley plants grown on H2 treated soils were 18% larger than plants grown on air treated soil (Figure 1). In similar studies with 35 d old canola, the H2 treated soil stimulated growth by 18% compared to untreated soil (Figure 1) (Wu, Dong, Layzell, unpublished). In spring wheat, soil pretreated with H2 for 30 d was found to enhance plant biomass of 29 d old plants by 32% compared with plants grown in air treated soil (Figure 1) (Dong, Layzell, unpublished). These results show that H2 treatment of soil enhances fertility and promotes plant growth, not only for legumes, but also for various non-leguminous crops that may grow concurrent with or subsequent to the time of H2 fertilization.
In spring wheat, growth promotion can also be achieved in soils in which only 5% of the soil volume was previously treated with H2; the balance of the soil being untreated (Figure 1) (Dong Layzell, unpublished). Similarly, the plant growth promoting activity of H2 treated soils has been shown to be extractable since aqueous extracts of the soil (3 mL per seed) enhanced the growth of 25 d old barley plants by 32% when compared with seeds watered with extracts of air treated soil (Willms, Layzell, unpublished).
Growth in H2 Treated
Soybean Barley Canola Wheat Wheat
Figure 1. The effect of soil pretreatment with H2 on the growth of soybean, barley and canola relative to growth in untreated soils. In spring wheat, a growth response was seen in soils in which 100% or 5% of the total soil volume was treated with H2.
To assess whether the plant growth promoting activity of H2 treated soil was due to bacteria or fungi, spring wheat was grown in a soihpromix (1:19) mixture in the presence and absence of antibiotics (penicillin and streptomycin) and a fungicide (benomyl). The antibiotic treatment eliminated the growth response observed in the H2 treated soils when compared with the untreated control soils (Figure 2) (Dong, McLearn, unpublished).
These studies illustrate that exposure to H2 at levels similar to that which occurs next to legume nodules greatly enhances the ability of that soil to support the growth of both legumes and non-legumes. The growth promotion activity seems to be associated with the growth and hydrogen oxidation capacity of soil bacteria. Moreover, the activity is extractable and is present even when the soil is diluted to only 5% of the total soil volume.
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