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C photosynthesis at high light is often modeled by assuming limitation by the maximum capacity of Rubisco carboxylation (V) at low CO concentrations, by electron transport capacity (J) at higher CO concentrations, and sometimes by triose-phosphate utilization rate at the highest CO concentrations. Net photosynthetic rate ( P ) at lower light is often modeled simply by assuming that it becomes limited by electron transport (J). However, it is known that Rubisco can become deactivated at less than saturating light, and it is possible that P at low light could be limited by the rate of Rubisco carboxylation ( V ) rather than J. This could have important consequences for responses of P to CO and temperature at low light. In this work, P responses to CO concentration of common bean, quinoa, and soybean leaves measured over a wide range of temperatures and PPFDs were compared with rates modeled assuming either VC or J limitation at limiting light. In all cases, observed rates of P were better predicted by assuming limitation by VC rather than J at limiting light both below and above the current ambient CO. One manifestation of this plant response was that the relative stimulation of P with increasing the ambient CO concentration from 380 to 570 µmol mol did not decrease at less than saturating PPFDs. The ratio of VC to V at each lower PPFD varied linearly with the ratio of P at low PPFD to P at high PPFD measured at 380 µmol(CO) mol in all cases. This modification of the standard C biochemical model was much better at reproducing observed responses of light-limited P to CO concentrations from pre-industrial to projected future atmospheric concentrations. [ABSTRACT FROM AUTHOR]

To examine the impact of climate change on vegetative productivity, we exposed fallow agricultural soil to an in situ temperature and CO2 gradient between urban, suburban and rural areas in 2002. Along the gradient, average daytime CO2 concentration increased by 21% and maximum (daytime) and minimum (nighttime) daily temperatures increased by 1.6 and 3.3°C, respectively in an urban relative to a rural location. Consistent location differences in soil temperature were also ascertained. No other consistent differences in meteorological variables (e.g. wind speed, humidity, PAR, tropospheric ozone) as a function of urbanization were documented. The urban-induced environmental changes that were observed were consistent with most short-term (~50 year) global change scenarios regarding CO2 concentration and air temperature. Productivity, determined as final above-ground biomass, and maximum plant height were positively affected by daytime and soil temperatures as well as enhanced [CO2], increasing 60 and 115% for the suburban and urban sites, respectively, relative to the rural site. While long-term data are needed, these initial results suggest that urban environments may act as a reasonable surrogate for investigating future climatic change in vegetative communities. [ABSTRACT FROM AUTHOR]