Background
The authors monitored ground-level atmospheric CO2 concentrations for several months at three locations - one each in Australia, Japan and the United States - to determine if atmospheric CO2 concentrations measured at Mauna Loa, Hawaii are representative of what plants actually experience in field conditions.  Data from the three sites exhibited a similar diurnal pattern, with maximum CO2 concentrations of 440-540 ppm occurring during a three-hour pre-dawn period that was followed by a concentration decrease to 350-400 ppm by mid-morning, after which there was a slow but steady increase in the late afternoon and early evening that brought the cycle back to its pre-dawn maximum.  Thus, the ambient CO2 concentrations at the three locations were not static over a 24-hour period but rose to 20-45% more than the global background level evident in the Mauna Loa data during the hours just before dawn.

What was done
After identifying these pre-dawn spikes in the ambient atmospheric CO2 concentration, the authors decided to see if they impacted plant growth in any way.  Thus, they grew tomato, soybean and velvetleaf plants for one month in controlled-environment chambers under three different sets of conditions: 1) constant 24-hour exposure to an atmospheric CO2 concentration of 370 ppm, 2) constant 370 ppm CO2 exposure during the day followed by a constant 500 ppm exposure at night, and 3) CO2 exposure of 500 ppm from 2200 to 0900 followed by a decrease to 370 ppm by 1000, which was maintained until 2200, somewhat mimicking the CO2 cycle they observed in nature.

What was learned
The 24-hour exposure to 370 ppm CO2 and the 370-ppm-day/500-ppm-night treatments gave essentially identical results in terms of biomass production after 29 days.  However, the CO2 treatment that mimicked the authors' real-world atmospheric CO2 observations, i.e., the one that had an initially-elevated but subsequently-declining CO2 concentration during a portion of the early morning, resulted in total plant biomass increases of 10, 20 and 21% in tomato, soybean and velvetleaf, respectively.

What it means
As cities and their appetites for fossil fuels grow ever larger and their nighttime CO2 concentrations consequently grow in magnitude (see Urban CO2 Dome in our Subject Index), plants located within the spheres of their influence will likely grow better than plants not exposed to the extra CO2 they produce, all else being equal.