In 27 C 4 grasses grown under adequate or deficient nitrogen (N) supplies, N-use efficiency at the photosynthetic (assimilation rate per unit leaf N) and whole-plant (dry mass per total leaf N) level was greater in NADP-malic enzyme (ME) than NAD-ME species. This was due to lower N content in NADP-ME than NAD-ME leaves because neither assimilation rates nor plant dry mass differed significantly between the two C 4 subtypes. Relative to NAD-ME, NADP-ME leaves had greater in vivo (assimilation rate per Rubisco catalytic sites) and in vitro Rubisco turnover rates (k cat ; 3.8 versus 5.7 s 21 at 25°C). The two parameters were linearly related. In 2 NAD-ME (Panicum miliaceum and Panicum coloratum) and 2 NADP-ME (Sorghum bicolor and Cenchrus ciliaris) grasses, 30% of leaf N was allocated to thylakoids and 5% to 9% to amino acids and nitrate. Soluble protein represented a smaller fraction of leaf N in NADP-ME (41%) than in NAD-ME (53%) leaves, of which Rubisco accounted for one-seventh. Soluble protein averaged 7 and 10 g (mmol chlorophyll) 21 in NADP-ME and NAD-ME leaves, respectively. The majority (65%) of leaf N and chlorophyll was found in the mesophyll of NADP-ME and bundle sheath of NAD-ME leaves. The mesophyll-bundle sheath distribution of functional thylakoid complexes (photosystems I and II and cytochrome f ) varied among species, with a tendency to be mostly located in the mesophyll. In conclusion, superior N-use efficiency of NADP-ME relative to NAD-ME grasses was achieved with less leaf N, soluble protein, and Rubisco having a faster k cat .C 4 photosynthesis involves the close collaboration of two photosynthetic cell types, the mesophyll (M) and bundle sheath (BS). A key characteristic of the C 4 syndrome is the operation of a CO 2 concentrating mechanism, which serves to raise the CO 2 concentration in the BS around Rubisco to levels high enough to suppress photorespiration and almost saturate photosynthesis in air (Hatch, 1987). This explains the commonly observed high photosynthetic rates of C 4 relative to C 3 leaves, when comparisons are made under high light and temperature. C 4 plants also have greater photosynthetic rates and accumulate more biomass than C 3 plants for less leaf nitrogen (N) and Rubisco (Bolton and Brown, 1978;Brown, 1978;Schmitt and Edwards, 1981;Ghannoum et al., 1997;Ghannoum and Conroy, 1998;Makino et al., 2003). The C 4 photosynthetic pathway is divided into three biochemical subtypes following the major C 4 acid decarboxylation enzyme (NAD-malic enzyme [ME], NADP-ME, and phosphoenolpyruvate carboxykinase; Hatch, 1987). C 4 grasses with different biochemical subtypes have characteristic leaf anatomy (Hattersley, 1992) and different geographic distribution according to rainfall, such as seen in Australia (Hattersley, 1992) and South Africa (Ellis et al., 1980). With increasing rainfall, NADP-ME grasses increase in abundance, whereas NAD-ME grasses become less abundant. The aforementioned observations triggered our interest in the comparative physiology of the C 4 subtypes, espec...

Despite mounting evidence showing that C 4 plants can accumulate more biomass at elevated CO 2 partial pressure (p(CO 2 )), the underlying mechanisms of this response are still largely unclear. In this paper, we review the current state of knowledge regarding the response of C 4 plants to elevated p(CO 2 ) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO 2 ) can stimulate the growth of both well-watered and waterstressed C 4 plants. First, through enhanced leaf CO 2 assimilation rates due to increased intercellular p(CO 2 ). Second, through reduced stomatal conductance and subsequently leaf transpiration rates. Reduced transpiration rates can stimulate leaf CO 2 assimilation and growth rates by conserving soil water, improving shoot water relations and increasing leaf temperature. We argue that bundle sheath leakiness, direct CO 2 fixation in the bundle sheath or the presence of C 3 -like photosynthesis in young C 4 leaves are unlikely explanations for the high CO 2 -responsiveness of C 4 photosynthesis. The interactions between elevated p(CO 2 ), leaf temperature and shoot water relations on the growth and photosynthesis of C 4 plants are identified as key areas needing urgent research.

(277 words)1 2 Resolving ecophysiological processes in elevated atmospheric CO 2 (C a ) at scales larger than 3 single leaves poses significant challenges. Here, we describe a field-based experimental system 4 designed to grow trees up to 9 m tall in elevated C a with the capacity to control air temperature 5 and simultaneously measure whole-tree gas exchange. In western Sydney, Australia, we 6 established the Hawkesbury Forest Experiment (HFE) where we built whole-tree chambers 7 (WTC) to measure whole-tree CO 2 and water fluxes of an evergreen broadleaf tree, Eucalyptus

The rising levels of atmospheric CO2 are likely to increase biomass production of C3 species in both natural and managed ecosystems because photosynthetic rates will be higher. The greatest absolute increase in productivity will occur when nitrogen and phosphorus availability in the soil is high. Low nitrogen does not preclude a growth response to high CO2, whereas some C3 species fail to respond to high CO2 when phosphorus is low, possibly because insufficient phosphorus is available to maintain maximum photosynthetic activity at high CO2. C3 plants response to high CO2 because the flux of carbon through the photoreductive cycle is increased and photorespiration is suppressed. This change in metabolism appears to alter the foliar nutrient concentration required to promote maximum productivity (critical concentration). Higher phosphorus concentrations are needed at elevated CO2, whereas the nitrogen requirement is reduced by CO2 enrichment. Since critical concentrations are used to evaluate nutrient status of crop and forest species and to manage fertiliser programs, they will need reassessing as the atmospheric CO2 concentration rises. Another consequence of the altered nutrient requirement at high CO2 is that the nitrogen concentrations of foliage, roots and grain are consistently lower in plants grown at elevated CO2, irrespective of availability of nitrogen in the soil. In natural ecosystems, the lower nitrogen to carbon ratio of the litter may alter rates of nutrient cycling. For farmers, the rising CO2 concentrations could cause reductions in grain nitrogen, and therefore protein content. This could have important implications for baking quality of hard wheats as well as affecting the nutrient value of grain such as rice.

To investigate if Eucalyptus species have responded to industrial-age climate change, and how they may respond to a future climate, we measured growth and physiology of fast-(E. saligna) and slow-growing (E. sideroxylon) seedlings exposed to preindustrial (290), current (400) or projected (650 lL L À1 ) CO 2 concentration ([CO 2 ]) and to current or projected (current 1 4 1C) temperature. To evaluate maximum potential treatment responses, plants were grown with nonlimiting soil moisture. We found that: (1) E. sideroxylon responded more strongly to elevated [CO 2 ] than to elevated temperature, while E. saligna responded similarly to elevated [CO 2 ] and elevated temperature; (2) the transition from preindustrial to current [CO 2 ] did not enhance eucalypt plant growth under ambient temperature, despite enhancing photosynthesis; (3) the transition from current to future [CO 2 ] stimulated both photosynthesis and growth of eucalypts, independent of temperature; and (4) warming enhanced eucalypt growth, independent of future [CO 2 ], despite not affecting photosynthesis. These results suggest large potential carbon sequestration by eucalypts in a future world, and highlight the need to evaluate how future water availability may affect such responses.