Differences in submersed/emergent leaf areas and petiole lengths

[Thomas Barr <tcbiii at earthlink_net>]

I recently finished up a phenology project on leaf area changes in 4 species
of aquatic plants. I choose 4 test subjects (victims). Echinodorus
cordifolius, Ludwigia acurata, Glossostigma and Lobelia cardinalis.

I hypothesized that all plants would increase their leaf areas in response
to rising water levels resulting in submergence.

My question was:

Do the quantitative leaf area and petiole length changes differ when a plant
is submerged during the wet season? If so, how much do they differ?

I found large significant differences in all plant species during this
treatment. The time frame was 3 weeks. Leaf areas where measured by using a
graph paper square grid(each area unit is .25cm^2) and tracing the leaf on
the paper then counting up the squares for total leaf area. This is very
simple method to quantify leaf area. Petiole length is a straight forward
measurement.

A number of different plants were selected to investigate a more comparative
approach in general trends and possible other mechanisms of response to
submersion.

Gloss:
Yielded about a 4X increase in leaf area after 3 weeks or submersion.
Petiole length increased by 234%.

Echinodorus: 
Yielded about a 4X DECREASE in leaf area after 3 weeks or submersion.
Petiole length increased by 310%.

Lobelia:
Yielded about a 4X increase in leaf area after 3 weeks or submersion.
Petiole length not measured.

Ludwigia:
Yielded about a 4X increase in leaf area after 3 weeks or submersion.
Petiole length not measured.

SE, SD where very low in all cases.

What is interesting is that the Sword plant, Echinodorus which is a weed
here in CA, actually decreased the leaf area in response to water level
rise. This was NOT predicted in the original hypothesis.
 
The three plants, Lobelia, Glossostigma and Ludwigia all showed marked
increases in leaf area. The cost of producing larger leaves seems to be the
mechanism for dealing with submersion with these three plants although a
large increase in petiole occurred in Glossostigma, it is not large compared
to water level changes found in nature (it increased 1.62 cm) but may be
enough to contribute to its survivorship in it's natural vernal pool and
seeps habitat where water levels changes are slight relative to lake and
deeper/permanent water aquatic plant habitats.

Time period and depth of submersion also can vary. These leaf increases may
be adapted for shallow submersion depths and beyond a certain depth, the
cost of producing a long aerial leaf and the amount of light may be too
great for the plant and it simply dies. If the time of high water is only a
week or or so, them aerial advantage might not be worth the cost of
producing these types of leaves. If it is longer than for example a month or
some time frame, then the cost might be worth producing aerial leaves.
"Plant memory" also seems to play a role in that once a plant knows where
the water surface is, it will continue to produce aerial leaves even if the
water levels is raised.

 Both Ludwigia(Red submersed=> Green emmersed) and Lobelia(Green submersed=>
Red emmersed)  also underwent a color change.

Echinodorus showed a decrease in leaf area. This is interesting since it
appears that this plant is adapted to allocating its resources more for the
aerial advantage rather than increasing leaf area. The plant is structurally
more adapted to handle more wave and current stress and also produces much
longer petioles. In high water currents, a larger leaf might act as a big
sail. In Ech berteroi which is found in less current (lake, pools) submersed
leaves are larger and more undulated. Ech cord is often found in high flow
and low flow areas. It's petiole diameter increaed by 210% although this was
not investigated, simply cutting a stem showed a massive well developed
aerenchyma system had formed quickly(less than 3 weeks) in the aerial leaves
after submersion.
 
In conclusion:
Three of tested aquatic plants appear to be well adapted to rapidly change
their leaf morphology in order to survive submersion by increased leaf
areas. Echinordorus appears to use a different mechanism to overcome the
light, CO2 and O2 changes of submersion by investing vegetative growth into
extending it's petiole length and reducing it's leaf size rather than leaf
area increases but it should be noted that not all species in that genus
exhibit this pattern.

Folks can easily do this type of plant project as it takes no special
equipment.

Regards, 
Tom Barr