The results presented in this dissertation suggest the highly chemical-, species-, and research technique- specific nature of the environmental fate of CECs. For example, cell cultures often form amino acid conjugates while whole plants form sugar conjugates during xenobiotic metabolism. Additionally, more toxicological data are needed on the effects of these and other compounds in terrestrial invertebrates, especially for those of agricultural importance. From the research conducted in this dissertation, future research should focus on the impacts of exposure and the potential for transformation of CECs under different conditions and in multiple species. Future studies should place emphasis on experimentation using bio-solids and TWW with inherent compounds and field conditions to improve environmental relevance. Future risk assessments should be conducted by taking into account the formation of biologically active and conjugated metabolites, and with regard to the potential toxicity of CECs in non-target terrestrial organisms. During the past 3–4 decades, parallel expansions of populations of non-indigenous rhizomatous grasses have occurred in aquatic and estuarine habitats around the nation. Among these expansions are Spartina alterniflora invasions in the salt marshes of the Pacific Northwest , take-over of large sections of the riparian ecosystems in Southern California by Arundo donax and Phragmites australis invasions in the upper regions of salt marshes in the mid-Atlantic states . The east coast salt marsh grass Spartina alterniflora has been invading west coast salt marshes from San Francisco Bay through Washington since the early 1970’s . Although S. alterniflora can produce seeds that are spread by water movement, it spreads mainly through the expansion of its underground rhizomes. In Washington salt marshes, S. alterniflora is large compared to other salt marsh species,blueberry pot size and it is altering ecosystem structure by affecting benthic structure and species diversity .
Likewise, in San Francisco Bay, S. alterniflora displaces the native wetland plants, as well as eelgrasses and algae. In the mid-Atlantic region, Phragmites australis has invaded many wetlands , and replaced much of the upper marsh vegetation, that was characterized by Spartina patens and Distichlis spicata. The 2–3 m tall stems die back at the end of the growing season, but the densely grown shoots remain in place, affecting sedimentation patterns, surrounding less tall vegetation, and use of the marsh by mammals and birds. The rhizomatous grass Arundo donax that in the riparian ecosystems of Southern California has expanded into large, self-sustaining populations, has become an ecological and economical pest. The populations expand through the distribution of vegetative propagules, in the form of stem and rhizome fragments by the rivers. Currently the expansions of rhizomatous grasses like these are combated with mechanical and/or chemical methods. Both these approaches have pros and cons, and their application is affected by the local environmental conditions, funding, and available manpower .The overall goal of this project was to increase the knowledge of the ecophysiology of the internal processes targeted by, or the ecological processes resulting from control efforts, and to facilitate conveyance of this type of information and knowledge to agencies and individuals engaged in control effort of rhizomatous grasses in aquatic and estuarine habitats. The physiological process that we will focus on is allocation of photosynthates to different parts of the plant and the role of internal nitrogen in this process, in order to determine the most effective time for herbicide application.In the hydroponic culture described earlier, the S. alterniflora seedlings that were precultured in deionized water showed an initially slow, but increasing growth, until the growth is reduced near the end of the experiment at week 12 . Specifically, the growth of leaves and stems stopped , as the plants exhaust the N supply from their nutrient solution , and the leaves’ internal N decreases down to their CNC . Before the leaves reached their CNC at the end of week 11, rhizome biomass showed a slow increase to 1.93 + 0.257 g. After the leaves reached their CNC in the last five weeks of the experiment, approximately 6.11 g was added to the rhizome biomass.
While at CNC, internal leaf N content was seen to increase above CNC for a short period, after nutrient additions to the culture solution. In this period, the leaves showed slight biomass increase as well . As the rhizomes grow, new roots and tillers develop at many of their internodes, and the biomass of these tissues show a significant increase , that is related to the increase of S. alterniflora rhizome length.The leaves that were collected from tall and short S. alterniflora in the DE salt marsh, showed differences in the seasonal pattern of the internal N content . At the start of the growing season in March, the leaf N/C ratio in the tall S. alterniflora is high at almost 0.06 g N/gC. In the short S, alterniflora the N/C was lower than both the tall S. alterniflora from the same marsh, and the S. alterniflora in the hydroponic greenhouse experiment early in their growing season . The internal N/C ratio in the leaves of the short S. alterniflora does not change much during the entire growing. In the second half of the growing season, the internal leaf N/C ratios in the tall S, alterniflora leaves decreases and becomes indistinguishable from the N/C ratio in the short S. alterniflora’s leaves.The difference between the growth patterns of the two groups was most pronounced in the growth of the green leaves and the stems. These variables show a reduction in the nitrogen-deprived plants when compared to the plants that continue to receive nitrogen, especially the leaves . In addition to the seemingly smaller amount of above ground plant material, the leaves of the nitrogen-deprived plants were a much lighter color green, and leaf senescence was more common for these plants . As a result of this combination of responses, the nitrogen-deprived plants appeared much smaller than those that continued to receive nitrogen. At the time of reduced leaf growth by the no-nitrogen plants, the internal N content in the nitrogen-deprived plants dropped significantly below the N content in the leaves of the N-supplied plants . This was observed through both the carbon-based and the dry-weight based determination of the tissue N content. The lowest mean N/C ratio was 0.038 + 0.001, and % N dropped below 2%, to 1.69 + 0.052%, at the time that the N content in the leaves of the control plants was lowest as well, with N/C = 0.077 + 0.004 and N = 3.446 + 0.167%.During nitrogen deprivation the growth of mature green leaves was limited or nonexistent, due to lack of mobile nitrogen. Both the function and growth of leaf tissues have a high requirement for nitrogen.
Each functional leaf cell will need to contain a minimum amount of nitrogen for both photosynthetic pigments and enzymes such as chlorophyll and Rubisco, and a full complement of DNA and nucleic acids for RNA production. Once the tissue nitrogen has been diluted to the lowest functional level, also referred to as the Critical Nitrogen Content , each cell will only contain this minimum amount of cellular nitrogen allowing it to function at maintenance level. Therefore these cells will not contain enough nitrogen to produce another complement of DNA and enzymes,blueberry plant size preventing them from dividing and the tissue to grow. During internal nitrogen limitation when leaf biomass did not increase anymore, photosynthesis was not reduced in Ipomoea batatas . Photosynthesis continued after the leaves have reached their CNC and the external nitrogen was depleted. The carbohydrates produced cannot be stored in the leaf tissue since further input of carbon compounds into leaf cells would lower the nitrogen to carbon ratio below their CNC, thus interfering with cell function. Instead of having been incorporated into the leaves, these carbohydrates were translocated to the sinks that are the rhizomes, roots, and for S. alterniflora, the rhizome tip tillers. This S. alterniflora study has shown that growth of that species’ vegetative reproductive structures, the rhizomes, occurs after the internal nitrogen content in the leaves is too low to allow for growth of the leaves. The manufacturers of systemic herbicides advise application of their product when there is substantial translocation to the below ground tissues of the plant. For the control of S. alterniflora to be effective, a systemic herbicide has to be carried in the phloem stream to the below ground permanent structures for winter survival and spring regrowth, the rhizomes and the associated rhizome tip tiller. Most rhizome growth and nearly all growth of rhizome tip tillers occur when the leaf N content has reached its CNC. It is obvious therefore that most photosynthate transport and incorporation in the rhizomes and rhizome tip tillers occurs at the time of low leaf N/C ratios, and not before.
We can expect substantial ‘delivery’ of the active ingredient in systemic herbicides which are carried in the phloem, such as glyphosate, to the target tissues would occur at the time of low leaf N/C. Therefore, it may be beneficial to take leaf N/C ratios into consideration when determining the timing of systemic herbicide applications. The field sampling in this study showed that the leaf N/C ratio of short S. alterniflora that grow away from the creek banks is almost always at its CNC level, while the leaf N/C ratio of the taller plants that grow on the creek banks showed a high level at the beginning of the growing season, and a decrease with time not unlike that of the plants in our greenhouse study. This indicates that our experimental conditions were a reasonable mimic of creek bank salt marsh conditions. Additionally, it indicates that N availability in the fine grained, and anoxic marsh sediment away from the creek banks is significantly different from that in more coarse grained creek bank sediments, through which, rather than over which, the flooding water moves during tidal movements. The low oxygen availability in this marsh sediment may interfere with the uptake of the nutrients, or the nitrogen content of this soil may be lower since exchange between the interstitial soil water and the periodically overlaying nutrient rich sea/marsh water will be limited due to the dense and fine-grained nature of the soil. In a series of studies by our laboratory that observed the role of leaf N content in the allocation of growth on multiple species, the P. australis study was the first in which we controlled the N concentration in the hydroponic nutrient solution. Continuous high concentrations were maintained for all plants until the 11th week of the experiment, at which time the N was removed from the nutrient solution for half of the remaining plants. The growth of the plants in both the no-nitrogen and the nitrogen supplied groups showed the importance of external and internal N on the allocation of growth in P. australis plants. It was interesting that even after the removal of the external N, the plants increased their living biomass by 39 g dry weight, which was almost as much as the 49 g biomass increase of the plants supplied with N. The biomass increase of the plants without external N was most likely supported by the pool of internal N in the plant. At the time the external N supply was removed, the N content in the plants was above their CNC. The critical nitrogen content of a tissue is defined as the lowest amount of nitrogen in tissues that will allow for growth the growth of that tissue. Total plant biomass increased after the removal of the external N, which was evidence that photosynthesis must have continued. As more carbohydrates are produced and incorporated into the plant without an external supply of new nitrogen, the internal N/C ratio in the tissues will decrease. As described earlier, among the different tissues, leaves have the highest CNC, and will therefore reach this CNC earlier than tissues with lower CNC values. When the leaves reached their CNC, they could no longer incorporate more C into their own tissues, since this would reduce their N/C below the CNC and interfere with their function . At this point two scenarios are possible; one is that photosynthesis could shut down, and the other is that all the carbohydrates produced in the leaves could be transported to other tissues of the plant.