[1] We present the first direct, multisite observations in support of the hypothesis that atmospheric aerosols affect the regional terrestrial carbon cycle. The daytime growing season (summer) CO 2 flux observations from six sites (forest, grasslands, and croplands) with collocated aerosol and surface radiation measurements were analyzed for high and low diffuse radiation; effect of cloud cover; and effect of high and low aerosol optical depths (AOD). Results indicate that, aerosols exert a significant impact on net CO 2 exchange, and their effect may be even more significant than that due to clouds. The response appears to be a general feature irrespective of the landscape and photosynthetic pathway. The CO 2 sink increased with aerosol loading for forest and crop lands, and decreased for grassland. The cause for the difference in response between vegetation types is hypothesized to be canopy architecture.

Measurement of shikimic acid accumulation in response to glyphosate inhibition of 5-enolpyruvylshikimate-3-phosphate synthase is a rapid and accurate assay to quantify glyphosate-induced damage in sensitive plants. Two methods of assaying shikimic acid, a spectrophotometric and a high-performance liquid chromatography (HPLC) method, were compared for their accuracy of recovering known amounts of shikimic acid spiked into plant samples. The HPLC method recovered essentially 100% of shikimic acid as compared with only 73% using the spectrophotometric method. Relative sensitivity to glyphosate was measured in glyphosate-resistant (GR) and non-GR cotton leaves, fruiting branches, and squares (floral buds) by assaying shikimic acid. Accumulation of shikimic acid was not observed in any tissue, either GR or non-GR, at rates of 5 mM glyphosate or less applied to leaves. All tissues of non-GR plants accumulated shikimic acid in response to glyphosate treatment; however, only fruiting branches and squares of GR plants accumulated a slight amount of shikimic acid. In non-GR cotton, fruiting branches and squares accumulated 18 and 11 times, respectively, more shikimic acid per micromolar of translocated glyphosate than leaf tissue, suggesting increased sensitivity to glyphosate of reproductive tissue over vegetative tissue. GR cotton leaves treated with 80 mM of glyphosate accumulated 57 times less shikimic acid per micromolar of translocated glyphosate than non-GR cotton but only 12.4- and 4-fold less in fruiting branches and squares, respectively. The increased sensitivity of reproductive structures to glyphosate inhibition may be due to a higher demand for shikimate pathway products and may provide an explanation for reports of fruit abortion from glyphosate-treated GR cotton.

Plant spacing greatly affects leaf area, light interception, and canopy apparent photosynthesis (CAP) in soybean [GLycine max (L.) Merr.]. This study examined relationships among these variables in four population‐density and row‐width treatment combinations for two seasons. Wide‐row (0.96 m) treatments were 3,11, and 18 plants m−2 in 1988 and 3, 15, and 24 in 1989; narrow‐row (0.43 m) treatments were 12 plants m−2 in 1988 and 13 in 1989. Narrow‐row treatments tended to have greater CAP rates during early growth in 1988, but not 1989, when rates lagged behind the wide‐row, high population. The reduction in CAP by the narrow‐row treatment in 1989 corresponded to its reduced light interception as measured parallel to the row base. Both total radiation (300‐2500 nm wavelengths) interception measured in 1988, and average photosynthetically active radiation (400‐700 nm) interception measured in 1989, were linearly related to CAP prior to, but not after, canopy closure. Similarly, leaf area indices (LAI) were curvilinearly related to percent light interception until canopy closure. After canopy closure, light interception did not decline at a rate commensurate with the loss of leaf area, indicating abscission of leaves not involved in light interception. Canopy photosynthesis was not different for any treatment after Reproductive Stage R5 in either year; however, significant differences were evident between Rl and R5 in 1989. These differences corresponded to the significantly lower yield of the low population in that year. Yield differences between the medium populations of the wide and narrow rows were not explained by CAP during reproductive growth. The data indicate that when LAI is above critical levels in post‐anthesis soybean canopies, factors other than photosynthesis may be involved in the response of seed yield to variation in plant density.