Geographical variation in populations of Chenopodium album resistant and susceptible to atrazine. II. Photoperiod and reciprocal transplant studies

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Abstract
Population differences in phenology and growth characteristics of atrazine-resistant (R) and atrazine-susceptible (S) populations of C. album collected from contrasting climatic regions of southern Ontario [Canada] were investigated in 2 experiments. The 1st was a reciprocal transplant of S populations into contrasting gardens, and the 2nd was a photoperiod study comparing both R and S populations under short- and long-day conditions. In the reciprocal transplant trial, individuals of 7 S populations comprising a northern group from an area characterized by 1670-1890 mean annual growing degree-days > 6.degree. C and 7 S populations comprising a southern group from an area of 2220-2440 degree-days were grown in garden plots at Harrow (southern garden) and Ottawa (northern garden). Individuals in the populations from the northern group were earlier to flower, produced less total and reproductive biomass, but had a greater reproductive effort and greater seed weight than individuals in the populations from the southern group. Growth patterns for each group were very similar in both gardens, although some between-garden population differences occurred. There were, in addition, differences in growth patterns between populations within both groups in both gardens. Electrophoretic studies conducted on leaf tissue indicated both between- and within-population differences in enzyme patterns, with the more southerly populations showing greater variability than the northern group. In the photoperiod study, individuals of 22 populations, consisting of northern and southern groups for each of the R and S plant types, were grown under 8- and 16-h photoperiods. For all populations the differential effect of short vs. long days was evident in both seedlings at 45 days and mature plants. All populations were photoperiodically indeterminate with plants under the 8-h photoperiod favoring earlier flowering than those under the 16-h photoperiod. Under the 16-h regime flowering was earlier in the more northerly R and S populations than in their respective southern populations. Under the 8-h photoperiod, these differences in phenology (and corresponding biomass production) were no longer apparent between the more northerly and southerly groups. Under the 8-h photoperiod, R populations were slower to flower than S populations and exhibited smaller reproductive efforts. Greater within-group variation in time to 1st flowering was evident under exposure to a 16-h photoperiod and within both light regimes R populations were less variable than S populations and southern S and R groups were more variable than the northern groups.