Date of Award

1-1-2010

Document Type

Campus Access Thesis

Department

Environmental Health Sciences

First Advisor

Dwayne E Porter

Abstract

Atrazine is the second most widely used herbicide in the U.S. with over 64 million pounds applied annually for a variety of weed control applications. Atrazine targets photosynthetic pathways in plants, disrupting electron transport along the photosystem II pathway (Huber, 1993), often resulting in inhibition of photosynthesis in many plant species. Aquatic microalgae and macrophytes are the most sensitive plant species tested to date (Solomon et al., 1996) and sublethal plant stress within these species may be important in developing new techniques to assess atrazine stress. This is of particular importance within the coastal zone of the U.S. as there is wide-spread usage of atrazine in both residential and commercial applications (e.g. golf courses and power line right of ways), which may adversely affect estuarine plant species, such as smooth cord grass Spartina alterniflora. Spartina alterniflora is the most dominant macrophyte in many estuarine regions of the U.S., particularly in the southeastern U.S. and may be often subject to chronic atrazine exposure. This study assessed the use of vegetation indices (PRI, NDVI and Carter Indices) and conventional ecotoxicology end points for survival, growth and biomass in Spartina alterniflora chronically exposed to atrazine for 21 days at ambient light, photoperiod, and temperature during both the summer and the fall.

Results of this study have indicated that chronic atrazine exposure adversely affected growth and survival in Spartina alterniflora and that the sublethal plant growth stress can be detected using hyperspectral imagery techniques. Specific results indicated that chronic atrazine exposure adversely affected including water quality including pH, dissolved oxygen and % oxygen saturation in both chronic 21 day atrazine tests (summer and fall),were significantly (p < 0.05 – 0.001) reduced in a general dose dependant manner (e.g. increasing reductions with increasing atrazine concentrations). While there were also significant differences in water temperature and salinity, these differences were only 1.1 oC for temperature and 1.5 ppt in comparisons between controls and atrazine exposure. Thus while there were significant differences between temperature and salinity comparisons, these levels were well within the normal range of temperatures and salinities encountered by Spartina alterniflora throughout the estuary and were not as significant as the reduced oxygen (21-23% reduction) and pH (3-10% reduction) levels observed, which were more indicative of lowered water quality (e.g. hypoxia).

Spartina alterniflora survival was also adversely affected by chronic atrazine exposure with 21 day LC50 values of 1,528 ug/L with 95% CL ranging from 466.1 to 5,011 ug/L in Test1 during summer conditions and a 21 day LC50 values of 1,107 ug/L with 95% CL ranging from 239.9 to 5,108 ug/L in Test2, during fall conditions. The overlapping confidence limits observed between the two tests indicated that these differences observed between the two tests were not significant.

Sublethal effects in Spartina alterniflora included observations of shoot stress (e.g. chlorosis) which indicated 7-21 day EC50 values ranging from < 1.00 to 1,528 ug/L ( 95% CL ranging from 83.6 – 3.120 ug/L) in Test1, during summer conditions and from < 1 to 412.5 ug/L (95% CL ranging from 0.75 – 5,368 ug/L) in Test 2, during fall conditions. The overlapping confidence limits observed between the two tests indicated that these differences observed between the two tests were not significant. Plant shoot growth was also assessed and results in Tests 1 and 2 were slightly different, with greater effects on growth observed during the Test 2. Results for Test 1 indicated that there were no significant differences in the number of shoots produced/plant and shoot growth (mm/day) in all sized shoots (small = < 20mm; moderate = >20 -100mm; and large = >100 mm) throughout the 21 days of exposure. Only the number of stressed and dead shoots were significantly (P < 0.05 – 0.007) increased in all atrazine treatments (1 - 10, 000 ug/L) in comparisons with controls.

Plant shoot growth in Test 2 also indicated that were significant (p < 0.01 – 0.004) increased numbers of stressed and dead shoots but only at 1 – 100 u/L atrazine doses. In addition, plant growth (mm/day) was significantly (p < 0.001 - 0.004) reduced in moderate sized shoots (> 20 -100 mm) at days 7, 14, and 21 days in all atrazine doses (1 – 10,000 ug/L) and in large shoots (> 100 mm) at days 14 and 21 in 1 and 10 ug/L atrazine doses, when compared to controls. Plant growth in small shoots (< 20 mm) was not significantly affected by atrazine exposure throughout the test.

Total (root + shoot) biomass (g/0.05 m2) was not significantly affected in Test 1 but was significantly (p < 0.01 – 0.001) reduced in all atrazine treatments (1 -10,000 ug/L) in Test 2. These differences between Tests 1 and 2, may be suggestive that atrazine exposure during the fall during declining temperatures, photoperiods and light intensity may be more stressful to Spartina alterniflora than during the summer during periods of peak temperature, photoperiod and light intensity. Since atrazine targets photosynthetic plant pigments, this stress on affecting growth may be more obvious as light intensities and photoperiods are declining in the fall than during periods of optimum light intensity and photoperiods as would be observed during the summer exposure in Test1. Root biomass (% root and g/0.05 m2 ) was also significantly (p <0.02 -0.05) reduced in Test 1 for the 100 – 10,000 ug/L atrazine treatments. In Test 2, root biomass (% root and g/0.05 m2 ) was also significantly (0.01 -0.001) reduced in all atrazine treatments (p < 1 – 10,000 ug/L). This suggests that atrazine significantly affects below ground production in the roots of Spartina alterniflora in both tests and that this effect was more pervasive during Test 2 during periods of reduced temperatures, photoperiod and light intensity.

Shoot biomass (% shoot and g/0.05 m2 ) was also significantly 0.05) reduced in Test 1 but was less pervasive as effects were only noted in the 100 ug/L atrazine treatment. In Test 2, shoot biomass (% shoot and g/0.05 m2 ) was not significantly (p > 0.05) in all atrazine treatments ( 1 – 10,000 ug/L). This suggests that atrazine did not significantly affect above ground production of biomass as much as below ground production in Spartina alterniflora. This may be in part related to the increased production shoots observed in Test 2 in some atrazine doses (1; 10 ug/) as well as the fact that dead shoots and stressed roots also contribute to above ground PRI Indices measurements revealed differing responses between Tests 1 and 2. PRI Indices measurements in Test 1 were not significantly (p > 0.05) different in comparison among atrazine treatments and control at days 7 and 14 of exposure; however, at day 21, reflectance was significantly (p < 0.004 – 0.0001) increased in most atrazine treatments (10 – 10,000 ug/L) when compared to controls. This was indicative of increased chlorosis observed in Spartina alterniflora, following repeated, chronic atrazine exposure. In Test 2, there were no significant ( p > 0.05) differences in PRI Indices measurements in comparisons among atrazine treatments and control at days 7, 14 and 21 of exposure. This suggests that PRI measurements may be more sensitive during the peak summer growing season (during peak temperatures, photoperiod and light intensity) than in the fall during cooler temperatures and declining photoperiod and light intensity.

NDVI measurements also revealed differing responses between Tests 1 and 2. NDVI measurements in Test 1 were not significantly (p > 0.05) different in comparisons among atrazine treatments and control at days 7 and 14 of exposure; however, at day 21, reflectance was significantly (p < 0.004 – 0.0001) increased in the two highest atrazine treatments (1,000 and 10,000 ug/L) when compared to controls. This was indicative of increased chlorosis observed in Spartina alterniflora, following repeated, chronic atrazine exposure but only at very high atrazine exposure levels. In Test 2, there were no significant ( p > 0.05) differences in NDVI measurements in comparisons among atrazine treatments and control at days 7 and 21 of exposure.; however at day 14 reflectance was significantly (p < 0.01 – 0.07) increased in the two atrazine treatments (1 and 10,000 ug/L) when compared to controls. This suggests that NDVI measurements were not as sensitive as PRI measurements, although both indices did discern evidence of plant stress following chronic atrazine exposure (days 14-21). In addition the PRI appeared to discern stress at a wider range of atrazine exposure (10 – 10,000 ug/L PRI versus 1,000 -10,000 ug/L) during Test 1. In Test 2, the NDVI Indices was somewhat more sensitive than the PRI, suggesting that the NDVI was better able to discern stress during lower temperature, light intensity and photoperiods. Carter Index revealed differing responses between Tests 1 and 2. Carter Indices measurements in Test 1 were not significantly (p > 0.05) different in comparisons among atrazine treatments and control at days 7 and 14 of exposure; however, at day 21, reflectance was significantly (p

< 0.004 – 0.0001) increased only in the highest atrazine treatments (10,000 ug/L) when compared to controls. In Test 2, there were no significant ( p > 0.05) differences in Carter Indices measurements in comparisons among atrazine treatments and control at days 7, 14, and 21 of exposure.

Comparison of atrazine exposure levels in agricultural row crops in TX, South FL, and SC with the LOEC for biomass (1 ug/L) and Hyperspectral PRI (10 ug/L) in Spartina alterniflora, indicated that only atrazine exposure levels from agriculture in row crops and vegetable crops exceeded the LOEC for biomass (1 ug/L), primarily in widespread areas in TX (locations adjoining agricultural production, as well as second order streams and bay sites). Atrazine exposure in SC urban areas only approached or occasionally (<33% of the time) exceeded the LOEC for biomass (1 ug/l). Comparisons with the LOEC for Hyperspectral PRI (10 ug/L) indicated that only atrazine exposure from row crops, exceeded levels that would discern sublethal stress (chlorosis) in Spartina alterniflora. Maximum atrazine exposure levels from urbanization and vegetable farming/golf courses (1.00-1.80 ug/L) were well below the LOEC for Hyperspectral PRI (10 ug/L), suggesting that hyperspectral approaches would be of limited value for assessments within these regions.

These findings clearly indicate that Vegetation Indices, such as PRI, NDVI, and the Carter Index, may be able to discern atrazine induced plant stress at concentrations at or below traditional sublethal shoot growth and biomass determinations effects. The costs, ease of measurement and wide spread assessment capabilities for hyperspectral measurements using remote sensing technologies, make hyperspectral measurements a new field assessment tools for assessing the effects of herbicides and other stressors that cause chlorosis in Spartina alterniflora. Future studies using vegetation indices measurements of Spartina alterniflora should be focused on other herbicides and other emerging chemicals of concern (e.g. flame retardants, nanomaterials, and pharmaceuticals) , as well as osmoregulatory/ion imbalance effects associated with climate change.

Share

COinS