PLANT-RESPONSES TO ATMOSPHERIC CO2 ENRICHMENT WITH EMPHASIS ON ROOTS AND THE RHIZOSPHERE

Empirical records provide incontestable evidence of global changes; fo remost among these changes is the rising concentration of CO2 in the e arth's atmosphere. Plant growth is nearly always stimulated by elevati on of CO2. Photosynthesis increases, more plant biomass accumulates pe r unit of water consumed, and economic yield is enhanced The profitabl e use of supplemental CO2 over years of greenhouse practice points to the value of CO2 for plant production. Plant responses to CO2 are know n to interact with other environmental factors, e.g. light, temperatur e, soil water, and humidity. Important stresses including drought, tem perature, salinity, and air pollution have been shown to be ameliorate d when CO2 levels are elevated In the agricultural context, the growin g season has been shortened for some crops with the application of mor e CO2; less water use has generally, but not always, been observed and is under further study; experimental studies have shown that economic yield for most crops increases by about 33% for a doubling of ambient CO2 concentration. However, there are some reports of negligible or n egative effects. Plant species respond differently to CO2 enrichment, therefore, clearly competitive shifts within natural communities could occur. Though of less importance in managed agro-ecosystems, competit ion between crops and weeds could also be altered. Tissue composition can vary as CO2 increases (e.g. higher C:N ratios) leading to changes in herbivory, but tests of crop products (consumed by man) from elevat ed CO2 experiments have generally not revealed significant differences in their quality. However, any CO2-induced change in plant chemical o r structural make-up could lead to alterations in the plant's interact ion with any number of environmental factors-physicochemical or biolog ical. Host-pathogen relationships, defense against physical stressors, and the capacity to overcome resource shortages could be impacted by rises in CO2. Root biomass is known to increase but, with few exceptio ns, detailed studies of root growth and function are lacking. Potentia l enhancement of root growth could translate into greater rhizodeposit ion, which, in turn, could lead to shifts in the rhizosphere itself. S ome of the direct effects of CO2 on vegetation have been reasonably we ll-studied, but for others work has been inadequate. Among these negle cted areas are plant roots and the rhizosphere. Therefore, experiments on root and rhizosphere response in plants grown in CO2-enriched atmo spheres will be reviewed and, where possible, collectively integrated To this will be added data which have recently been collected by us. H aving looked at the available data base, we will offer a series of hyp otheses which we consider as priority targets for future research.