ELEVATED CO2 AND PLANT-GROWTH - CELLULAR MECHANISMS AND RESPONSES OF WHOLE PLANTS

Much research has focused on the photosynthetic responses of plants to elevated CO2, with less attention given to the post-photosynthetic ev ents which may lead to changes in the growth of tissues, organs and wh ole plants. The aim of this review is to identify how plant growth is altered in elevated CO2 and to determine which growth processes or cel lular mechanisms are sensitive to carbon supply. For leaves, both the expansion of individual leaves and the initiation of leaf primordia ar e stimulated in elevated CO2. When lamina growth is promoted, this is usually associated with increased leaf cell expansion rather than incr eased leaf cell production. Using several clones of hybrid poplar (Pop ulus euramericana, P. interamericana) two native herbs (Plantago media , Sanguisorba minor) and bean (Phaseolus vulgaris) we have identified the mechanism through which leaf cell expansion is promoted in elevate d CO2. Changes in the water relations, turgor pressure (P) and yield t urgor (Y) of growing leaves cannot explain increased cell expansion; t his appears to occur because cell wall loosening is promoted, as sugge sted by three pieces of evidence. (i) The rate of decline of water pot ential (psi) with time is accelerated when growing leaves are placed i n psychrometers and allowed to relax, (ii) Instron-measured cell wall extensibility (WEX), is greater for leaves exposed to elevated CO2 and (iii) the activity of the putative wall loosening enzyme, XET is incr eased for leaves of P. vulgaris exposed to elevated CO2. Species diffe rences do, however, exist; in the herb Lotus corniculatus small stimul ations of leaf growth in elevated CO2 are due to increased leaf cell p roduction and decreased cell size in elevated CO2. These results are d iscussed in relation to the concept of functional types. There is evid ence to suggest that both cell production and cell expansion are promo ted in roots of plants exposed to elevated CO2. For native herbs (Anth yllis vulneraria, Lotus corniculatus, P. media and S. minor), increase d root growth in elevated CO2 is due to increased cell elongation. In contrast to leaves, this appears to occur because both root cell turgo r pressure (P) and root cell wall extensibility (WEX) are promoted by exposure of shoots to elevated CO2. In longer-term studies on root gro wth, the effects of additional carbon on the production of root primor dia and root branching are of overriding importance, suggesting that c arbon supply may influence some aspect of the cell cycle, when effects on the extension of individual roots may not be apparent.