Re: What effect does lemon juice have on plants?

According to the USDA food database (first reference), bottled or canned lemon
juice contains 2.4% sugar by weight (24 grams per liter). A high concentration
of sugar reduces the availability of water to the seed. If the seed cannot
absorb water, it cannot germinate. As you saw, sugars can also promote microbe
growth. Microbes produce waste products that can be toxic to plants. 

Plant cells absorb water because they contain a higher concentration of
dissolved substances than the soil solution. If the soil solution contains high
concentrations of dissolved substances, such as sugars or salts, the seed or
root cannot absorb water.

Irrigating plants with sugar or human beverages that contain sugar, such as
sodas or fruit juices, seems to be a very popular project judging from the large
number of questions submitted to MadSci. I don't recommend projects that involve
irrigating plants with human beverages for the following  reasons:

1. Human beverages are too expensive to be practical alternatives to tap water
or fertilizer solutions for plant irrigation.

2. Photosynthetic plants are autotrophic so beverages designed for heterotrophic
humans do not make sense biologically for use on plants. Autotrophic organisms
make all the sugars and other organic compounds they require.

3. Students are often unable to explain results of such projects. 

4. There are many, many better plant projects relating to plant nutrition and
plant water relations such as the following:

a. Plant growth with different types or amounts of fertilizers.
b. Plant growth in soil and without soil (hydroponics).
c. Plant growth with and without carbon dioxide.
d. Plant nutrient deficiency symptoms.
e. Effects of soil salinity on plant growth.
f. Plant growth with different amounts of water.

A long list of potential plant projects is found in the following answer:


Re: could u give me some ideas about a science project with beans?

The second reference suggests some good experiments involving sugar and plants.
Sugar can be used to extend the vase life of cut flowers or to keep albino corn
seedlings alive.


References


USDA National Nutrient Database for Standard Reference


Re: What is a good plant to test the effects of sugar or sweetener?


Re: Why did the liquids kill the plants? Why did the tea do well?


Re: Why will plants grow in Diet Coke and water and not other sodas?


Re: Can bean seeds grow with milk?


Re: will plants consume milk and orange juice?