Why are wetland plants limited to waterlogged soils?
The growth of the wetland plants, Phragmites communis, Mentha aquatica,
Phalaris arund. nacea, Juncus effusus, and Oenanthe crocata were all
significantly reduced when grown in dry soil and/or with competition from
L. perenne in a glasshouse. The interaction of water stress with competition
caused the greatest reduction in growth. Nitrate reductase activity was
highest in those wetland plants grown in soil with minimum watering.
Results from field trials, indicated that the spread of M. aquatica and
P. arundinacea was restricted because of competition from other naturally
occuring species, whereas P. communis was not. Levels of nutrients in
glasshouse grown plants did not vary with watering regime or competition
from L. perenne. In the field, levels of N and P were lower in the wetland
species from drier soils.
Particle size of the soil did not restrict the growth of the above wetland
species in comparison to a mesophyte (Zea mays). Increased bulk density
restricted root growth of Z. mays to a greater extent than the wetland
species. When the above wetland plants and Z. mays were grown in tubes
that were slowly droughted, Z. mays was less affected by water stress
compared to the wetland species. Z. mays had lower stomatal conductance,
greater photosynthetic rates, and less negative leaf water potentials than
the wetland species. Z. mays also had much faster and deeper root growth.
In contrast root growth of P. communis and M. aquatica were restricted by a
relatively small drop in soil water potential. P. arundinacea had greater root
growth rates than the other wetland species, but its roots were restricted
to the surface soil layers. Few adventitious roots formed on the wetland
species even in fully watered controls.
Leaf water potentials were more negative than expected in the wetland
species and in particular P. communis, in comparison to usual values for
mesophytes. Investigations showed that this was not an artifact of the
psychrometer measurements. Calculated hydraulic resistance was high in
P. communis. Metaxylem vessels had smaller diameters and were less
abundant in many parts of the wetland species compared to Z. mays.
P. communis had more negative leaf water potentials in intact plants
compared to excised leaves or stems suggesting existance of high hydraulic
resistance in the root or rhizome system. In addition this species showed
a loss of water from the leaves at night. The above factors were thought
to contribute to the negative leaf water potentials of the wetland species.