Predict Herbicide Resistance Population Shifts:
The Evolutionary Ecology of s-Triazine Resistant Brassica

Explain the resistant problem:
Prediction, Step 4:  Determine the Role the Resistant Biotype Plays in Crop Fields

Explaining and understanding the nature of the resistant biotypes (R) is more difficult.  Will the problem get worse or better in the future? What is the biological basis of predicting these risks? How do we predict the ecological role a resistant weed species will play, in the absence of herbicide use? Resistant weeds are more than just resistant. All individual weed species fill a specific niche, or role, in weed-crop communities. Herbicide resistance is but one trait important to the weed species in fulfilling its ecological role, its fitness, its threat to crop production. What ecological role does R play in the absence of the new herbicide?

The s-Triazine Resistant Phenotype
TR is a chronomutant. There exists a consistent, differential, pattern of photosynthetic function between TR and TS Brassica napus over the course of a diurnal light period and over their life history. Photosynthetic superiority of the S or R biotype is a function of the time of day, the age of the plant and the temperature of the environment.

Diurnal Effects on Photosynthetic Function in R and S
Diurnal patterns of photosynthetic function of chlorophyll fluorescence and carbon assimilation differ between R and S phenotypes:
Chlorophyll fluorescence. Differential diurnal patterns of Chl a flourescence (Ft) exist in R and S plants:

Carbon assimilation. Differential diurnal patterns of carbon assimilation exist in R and S Brassica plants during development. The diurnal patterns between R and S changes with the ontogeny of plant. R carbon assimilation greater early and late in diurnal; and, R carbon assimilation greater overall late in ontogeny:

       8 1/2-9 1/2 leaf plants at 25° C

Temperature Effects on Photosynthetic Function in R and S
Controlled leaf temperature. R and S biotypes of Brassica napus differed in terms of carbon assimilation and stomatal function in response to changing leaf temperatures (15-35° C). R carbon assimilation rates responded to decreasing leaf temperatures more rapidly than S, a form of cold tolerance conferred by the pleiotropic increase in lipid fluidity and polarity.

R and S carbon assimilation rates at different leaf temperatures:
        -15° C:           R = S, where electron transport rate is not limited
        -25-35° C:      S > R, where electron transport rate is limited

      
Tom Sharkey Laboratory, University of Wisconsin-Madison, ca. 1989

Stomatal function in R versus S:
        -R leaf temperature always cooler
        -R equal or greater total conductances to water vapor (g)
        -R equal or greater leaf intercellular CO2 partial pressure (Ci)

Diurnal patterns and plant temperature. The diurnal pattern of carbon assimilation differs between R and S at different air temperatures and stages of development: R is greater than S at higher air temperature, and late in ontogeny.

  3-4 leaf plants at 35° C

When leaf temperature is not controlled, greater stomatal conductance and leaf cooling in R can overcome other disadvantages.

Selective Advantage of s-Triazine Resistant Weeds
R biotypes may have an adaptive advantage over S in certain unfavorable ecological niches independent of the presence of s-triazine herbicides:

        -cool, low-light environments early and late in the day
        -high temperatures
        -late in the plant's development

Light adaptation. Pleiotropic changes in R result in chloroplast and whole leaf morphology similar to that of low light, dark-adapted, plants. The acquisition of shade-adapted morphology in TR is not a phenotypic plasticity response to environment.
Heat-cold tolerance. R possesses greater heat tolerance than S due to leaf cooling from greater stomatal conductance and leaf intercellular CO2 partial pressure. R also possesses tolerance to cool temperatures conferred by pleiotropic membrane lipid changes.  These advantages may maintain R biotypes in Iowa cropping fields in the absence of s-triazine herbicide use:

    -as a preadaptation before herbicide introduction
    -as a component of populations after discontinuing herbicide use

Where do we go from here?
Weed and crop management is the management of selection pressures leading to the weed adaptations that plague our fields and interfere with our crops. We need to explain and understand the importance of these weedy adaptations to Iowa cropping systems if improved management systems of the future are ever to be developed. We can begin by understanding the ecological roles played by individual herbicide resistant species and understand the roles these weeds played before the introduction of the herbicide to which they are resistant (preadaptation, exaptation). The evolutionary past is the key to unlock the future.

Should improved management system information DESCRIBE or EXPLAIN?

“Two overall strategies for management of changes in weed communities are apparent. One is to continue the present essentially responsive approach in which shifts in weed composition and development of herbicide resistance are attacked with newly developed herbicides and complex mixtures of existing materials. This approach guarantees a continuing market for new chemical technologies, but leaves the grower with a generally increasing bill for weed control.
    The alternative is to take a more methodical approach in which principles are elucidated that predict the response of weeds to control measures, and strategies are developed to intercept problems before they become severe. The growing interest among weed scientists in modeling the dynamics and genetic composition of weed populations is a first step in implementing this alternative approach to the management of incipient weed problems. However, new categories of higher-level models are needed to understand and predict phenomena like species shifts, the spread of weeds within and between regions, and the evolution of herbicide resistance in taxa that are currently susceptible. Such phenomena occur at spatial and temporal scales that exceed the boundaries of farms and the attention span of individual growers. Consequently, the extension of human understanding of weeds into larger scales will make management decisions at the community, regional, and national levels both practical and desirable. Developing higher-level theory of weeds probably represents the greatest challenge for weed science in the coming century. Implementation of that understanding by farmers, communities, and government agencies may prove equally challenging.”
(Mohler. 2001)

References  

Summary

©jdekker-2011