Elevated CO2 and plant structure: a review

Consequences of increasing atmospheric CO2 concentration on plant structure , an important determinant of physiological and competitive success, have n ot received sufficient attention in the literature. Understanding how incre asing carbon input will influence plant developmental processes, and result ant form, will help bridge the gap between physiological response and ecosy stem level phenomena. Growth in elevated CO2 alters plant structure through its effects on both primary and secondary meristems of shoots and roots. A lthough not well established, a review of the literature suggests that cell division, cell expansion, and cell patterning may be affected, driven main ly by increased substrate (sucrose) availability and perhaps also by differ ential expression of genes involved in cell cycling (e.g. cyclins) or cell expansion (e.g. xyloglucan endotransglycosylase). Few studies, however, hav e attempted to elucidate the mechanistic basis for increased growth at the cellular level. Regardless of specific mechanisms involved, plant leaf size and anatomy are often altered by growth in elevated CO2, but the magnitude of these change s, which often decreases as leaves mature, hinges upon plant genetic plasti city, nutrient availability, temperature, and phenology. Increased leaf gro wth results more often from increased cell expansion rather than increased division. Leaves of crop species exhibit greater increases in leaf thicknes s than do leaves of wild species. Increased mesophyll and vascular tissue c ross-sectional areas, important determinates of photosynthetic rates and as similate transport capacity, are often reported. Few studies, however, have quantified characteristics more reflective of leaf function such as spatia l relationships among chlorenchyma cells (size, orientation, and surface ar ea), intercellular spaces, and conductive tissue. Greater leaf size and/or more leaves per plant are often noted; plants grown in elevated CO2 exhibit ed increased leaf area per plant in 66% of studies, compared to 28% of obse rvations reporting no change, and 6% reported a decrease in whole plant lea f area. This resulted in an average net increase in leaf area per plant of 24%. Crop species showed the greatest average increase in whole plant leaf area (+37%) compared to tree species (+14%) and wild, nonwoody species (+15 %). Conversely, tree species and wild, nontrees showed the greatest reducti on in specific leaf area (-14% and -20%) compared to crop plants (-6%). Alterations in developmental processes at the shoot apex and within the vas cular cambium contributed to increased plant height, altered branching char acteristics, and increased stem diameters. The ratio of internode length to node number often increased, but the length and sometimes the number of br anches per node was greater, suggesting reduced apical dominance. Data conc erning effects of elevated CO2 on stem/branch anatomy, vital for understand ing potential shifts in functional relationships of leaves with stems, root s with stems, and leaves with roots, are too few to make generalizations. G rowth in elevated CO2 typically leads to increased root length, diameter, a nd altered branching patterns. Altered branching characteristics in both sh oots and roots may impart competitive relationships above and below the gro und. Understanding how increased carbon assimilation affects growth processes (c ell division, cell expansion, and cell patterning) will facilitate a better understanding of how plant form will change as atmospheric CO2 increases. Knowing how basic growth processes respond to increased carbon inputs may a lso provide a mechanistic basis for the differential phenotypic plasticity exhibited by different plant species/functional types to elevated CO2.