Alyson Tobin asked for suggestions for an experiment on nitrogen nutrition.
Alyson,
We do an experiment in our Introduction to Plant Biology I-II course... a
two-quarter course for non-majors... which might be interesting to your
students:
Working in pairs, the students plant soybeans in four 4" pots filled with
Pro-Mix potting medium (a seed-starter mix nearly free of fertilizer). One
pot (H+) has high nitrate fertilizer (supplied by inserting in the soil TWO
of the 13-4-5 fertilizer "sticks" for houseplants) and the seeds are
inoculated with commercially available Rhizobium inoculant. Another (H-)
has high nitrate but the seeds are NOT inoculated (the potting medium is
presumed sterile). The third pot (L+) has no fertilizer but is inoculated,
and the fourth (L-) has no fertilizer and no inoculant. We plant two seeds
in each pot. Inoculation is by wetting the seeds and rolling them in the
black, powdered inoculant. (Students must be warned to plant their
uninoculated seeds first so that they do not contaminate them with
inoculant which might be on their hands.)
The pots are put in a glasshouse and watered regularly. After
approximately 6 weeks, the pots are brought into the lab and the students
make various qualitative visual comparisons of the shoots. Then, they cut
off the shoots at soil level and weigh them (wet weight). They carefully
remove the roots from the soil and count and weigh the nodules. All data
is placed into the spreadsheet for the entire class and the spreadsheet
does the calculations and draws the histograms. The students use the data
to write a lab report.
The experiment works nearly always provided our inoculant is fresh (we can
obtain it only in the spring and keep it in the refrig) and the plants are
consistently watered. It demonstrates that the soybeans will tend NOT to
form the mutualistic association when nitrate is readily available in the
soil. In other words, the most and largest nodules are in the L+ pot. We
also have the students cut open a nodule and look at it under the
dissecting scope to see the pink color of the leghemoglobin.
The experiment provides an excellent launching point for discussions on 1)
symbiotic relationships, 2) the reasons for not forming the relationship
when nitrate is plentiful (the soybean would needlessly lose energy
reserves by feeding a bacterium which it doesn't need), 3) the role of
leghemoglobin, 4) the value of agricultural research, and 5) agroeconomics.
The experiment would be even better if we could follow it until the plants
reach maturity and produce seed because most of the protein production (and
hence the need for nitrogen) is in the seed-forming phase of development.
If we could go that long (we can't because we're on a 10-week quarter
system) we would measure and compare number and weight of pods, and number
and weight of seeds in addition to the other measures.
Dr. David W. Kramer
Department of Plant Biology
Ohio State University at Mansfield
1680 University Drive
Mansfield, OH 44906
(419) 755-4344 FAX: (419) 755-4367
e-mail: kramer.8 at osu.edu
