Elevated CO2 and plant growth: cellular mechanisms and responses of whole plants

Abstract Much research has focused on the photosynthetic responses of plants to elevated CO2 , with less attention given to the post-photosynthetic events which may lead to changes in the growth of tissues, organs and whole plants. The aim of this review is to identify how plant growth is altered in elevated CO2 and to determine which growth processes or cellular mechanisms are sensitive to carbon supply. For leaves, both the expansion of individual leaves and the initiation of leaf primordia are stimulated in elevated CO2. When lamina growth is promoted, this is usually associated with increased leaf cell expansion rather than increased leaf cell production. Using several clones of hybrid poplar (Populus 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 elevated CO2. Changes in the water relations, turgor pressure (P) and yield turgor (Y) of growing leaves cannot explain increased cell expansion; this appears to occur because cell wall loosening is promoted, as suggested by three pieces of evidence. (i) The rate of decline of water potential (ψ) with time is accelerated when growing leaves are placed in 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 increased for leaves of P. vulgaris exposed to elevated CO2. Species differences do, however, exist; in the herb Lotus corniculatus small stimulations of leaf growth in elevated CO2 are due to increased leaf cell production and decreased cell size in elevated CO2. These results are discussed in relation to the concept of functional types. There is evidence to suggest that both cell production and cell expansion are promoted in roots of plants exposed to elevated CO2. For native herbs (Anthyllis vulneraria, Lotus corniculatus, P. media and S. minor), increased root growth in elevated CO2 is due to increased cell elongation. In contrast to leaves, this appears to occur because both root cell turgor pressure (P) and root cell wall extensibility (WEX) are promoted by exposure of shoots to elevated CO2. In longer-term studies on root growth, the effects of additional carbon on the production of root primordia and root branching are of overriding importance, suggesting that carbon supply may influence some aspect of the cell cycle, when effects on the extension of individual roots may not be apparent.