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A comparison of the effect of thiosulfate and sulfate on sunflower growth Judith Hooper, Ph.D., Pima Research Co.; for Best Sulfur Products, Fresno, CA July 1, 2000 |
A comparison of the effect of thiosulfate and sulfate on sunflower growth
Judith Hooper, Ph.D., Pima Research Co.; for Best Sulfur Products, Fresno, CA July 1, 2000
Introduction
One of the questions that has been asked about plant sulfur nutrition is if the particular source of sulfur impacts how well it is taken up in the plant. It is easy to imagine that physical properties such as water solubility and particle size can play a role in how readily sulfur is taken up, and there are many articles in the literature that report such effects, 1, 2, 3. Whether or not subtler chemical properties have an effect is less clear.
A recent study done at Rutgers suggests that the chemical differences between thiosulfate and sulfate are not distinguished by the plant. They found that axenically grown Arabidopsis – grown in the absence of any other living organism -- grew equally well with thiosulfate as the sole sulfur source as they did with sulfate 4. This result is not very surprising for a number of reasons. First, precedent already exists for direct thiosulfate uptake by living organisms; thiosulfate not only appears in the pathway for microbes such as fungi and blue green algae, but a number of soil bacteria use thiosulfate as a preferred sulfur source 5 , 6. Second, there is a long, though spare, history in the literature of plant metabolism documenting the use of alternatives to sulfate as the sole sulfur source. These alternatives include bulky, organic candidates such as cysteine (an amino acid) and glutathione (an "anti-stress" molecule) that have no resemblance to sulfate other than they contain an atom of sulfur somewhere in their structure 7. Thus, that plants should be capable of utilizing thiosulfate, a molecule that is structurally very similar to sulfate, seems reasonable.
Experimental Design and Methods
In an effort to extend these results to conditions more similar to what might be encountered in the field, a series was set up using sunflower seedlings that compared the benefit of sulfate versus thiosulfate against a control that had no sulfur supplementation. Plants were grown in 1 gallon pots using a soiless nursery media (Scott's) under greenhouse conditions. All plants were watered daily and fed with an NPK fertilizer that contained no sulfur (8.3-8.5-7.5). Three sets of 15 plants each were randomly arranged in a single plot. In addition to biweekly NPK fertilization at a rate of ½ cup per plant of a 1:64 dilution, one series received supplemental sulfur in the form of potassium sulfate (5.2S), while a second series receive potassium thiosulfate (5.1S). The third series (the control) received just potassium (as KCl/KOH). The NPK of each series was formulated individually and adjusted to minimize any differences between the series other than the sulfur source.
Plants were harvested individually when they reached full bloom. The height of the plant was measured, the total above-ground weight taken, and the flower head weight determined. The results were subjected to statistical analysis. Despite fairly large variations within each series, it was possible to identify certain trends and specific difference that were significant.
Results
Figure 1 shows the results of all three series (total weight and flower weight on the left axis, and height on the right axis.) As mentioned above, there was significant variation between plants within a given treatment. However, trends were apparent, indicating that any sulfur supplementation was better than nothing, and thiosulfate performed better than sulfate.
Figure 2 shows that the largest differences were seen in flower weight, with nearly a 50% increase over the control for thiosulfate treated plants. With respect to height, on the other hand, there was only an apparent 2% increase which, after statistical analysis, proved not significant. Similarly, the differences in total weight appeared not significant, especially when the effect of the flower head weight was removed.
Ignoring trends and looking strictly at the statistically significant values an interesting relationship emerged: the difference between the flower weight of the control and the sulfate treatments was not significant, and the difference between the sulfate and thiosulfate treatments was not significant, but the difference between thiosulfate and the control was significant. This can be represented graphically as a Venn diagram:
The control and sulfate have some part of the population in common, and the sulfate and thiosulfate treatments have some part of the population in common, but the thiosulfate is definitely unique from the control.
In addition to significant changes in flower head weight, there was a trend toward earlier harvest with the thiosulfate treated plants by about a week. There also was no mortality within either of the sulfur treated series while 3 out of the 15 control plants died.
Conclusions and continued work
The results of the sunflower experiments support the results obtained earlier on Arabidopsis and indicate that thiosulfate may exert it's own unique effect on plant growth in general and, perhaps, on sulfur metabolism in particular. There are a number of hypotheses that could account for these differences, from improved soil microbiology to improved energetic "economics" with thiosulfate. But sorting out the underlying mechanism of action requires a better understanding of the real benefits. This could include, for example, evidence for a different distribution of proteins in the flower, improved resistance to stress, more seeds and/or larger seed, or simply a bigger flower in general. In addition, the effect of the soil microbiology may well play a critical role in generating the different results between treatments.
Current experiments are aimed at unraveling some of these issues. Most importantly, the sample size has been doubled in order to eliminate some of the "noise" due to the variation within a given treatment, and allow a clearer demonstration of what statistical difference exist between sulfate and thiosulfate. At harvest the flowers will be evaluated more closely for differences in protein composition and total nutrient analysis (N,P,K,S). It is also hoped that by the time harvest arrives it will be possible to determine the general microbiology of the growth media and whether or not it might play a role in improved growth characteristics.
Addendum: An accounting of energetic requirements of sulfur metabolism
There is an energetic cost associated with the metabolism of sulfate to it's final end product: cysteine (see Fig. 4). Each step in the pathway results in a transformation, and that transformation has a cost. Costs are involved in transport across the membrane, in getting electrons from the plant (reduction) and in enzyme activation requirements.
When thiosulfate is used as the starting material rather than sulfate, the energetic costs may be substantially reduced because some of these steps are omitted. While still theoretical at this point, an accounting of relative energetic costs between sulfate and thiosulfate uptake indicate thiosulfate may have a strong advantage.
Acknowledgements:
We are grateful to T. Leustek, Rutgers University, who was kind enough to fund the Arabidonpsis pilot experiments and share the results; and G. DeGrandi-Hoffman at the USDA/ARS Carl Hayden Bee Research Center, Tucson, AZ, provided essential insights in the unfamiliar area of population statistical analysis.
