Re: Would watering a plant with sugar solutions positivly effect the grow of th

Complete question:

I introduced one of each carbohydrate (sucrose,fructose,glucose,lactose, 
and starch) solutions at 0.05m concentrations to different hydroponic units 
containing Brassica rapa after the Brassica were a week old. I expected the 
plants to incorporate the carbos into themselves by diffusion through the 
roots and then transport into leaves where they would be stored as starch 
molecules. In turn, I expected the starch concentration to increase along 
with growth of the plant. But I had some strange results; although the 
trend seemed to be a higher concentration of starch, the plants looked 
stunted and turned red/purple/yellow(they should be green!) maybe due to 
osmotic stress? We felt results were hard to interpret and wondered if any 
one had ever documented an experiment like this before. I have looked for 
days through several data bases and haven't found it yet.  I also wondered 
what kinds of carbohydrates are synthesized by Wisconsin Fast Plants 
(Brassica rapa). Could you please help or point me in the direction of a 
similar experiment? Thank you!

Reply:

Generally, intact plant roots cannot absorb sugars in appreciable amounts and 
use them in metabolism. Bits of plant tissue in sterile culture do require an 
external sugar supply.

Your hypothesis that it is an osmotic effect is reasonable.  Another 
possibility is the sugar promoted microbial growth, and the microbes competed 
with the Fast Plants for mineral nutrients. Plants deficient in phosphorus 
often have purplish leaves. Several mineral nutrient deficiencies result in 
yellow leaves. Could you detect any bad odors that might indicate microbial 
growth? Rapid microbe growth might also deplete rootzone oxygen or result in 
toxic waste products. Either of those could cause root injury and result in 
stunting and leaf symptoms.

Were your hydroponic systems just solution culture or was there a solid medium, 
such as sand or gravel? Was the nutrient solution aerated? What was the formula 
for your nutrient solution? Did you have a control (no sugar) that grew well?

I assume you tested Fast Plant leaves for starch using the iodine test so you 
know they contain starch. You might contact the Wisconsin Fast Plants website 
and see if they have info on other sugars they contain. Sucrose is the most 
common sugar produced by plants.

The kind of plant experiment you describe may have been done in the early 1900s 
or even in the 1800s so you would have to search the older literature at a big 
university library. A good place to start would be the book by Hewitt, which 
summarizes early literature on plant nutrition research.



References


Re: would plants grow more quickly if watered with a sugar-water solution


Wisconsin Fast Plants

Hewitt, E.J. 1966. Sand and Water Culture Methods Used in the Study of Plant 
Nutrition. Commonwealth Bureau of Horticulture and Plantation Crops, East 
Malling, England. Technical Communication 22.