Photosynthetic pathway and hydraulic architecture in higher plants

In terrestrial plants, large quantities of water evaporate from leaves during photosynthesis, an inevitable dilemma for plants to open stomata to allow CO2 entry into mesophyll cells. Evolution of C4 photosynthetic pathway resulted in enhanced water use efficiency (WUE) compared to C3 plants. C4 plants have 2- to 4-fold lower stomatal conductance and potentially lower transpiration rates per unit carbon gain than C3 species of similar ecological function. Consequently, for equivalent rates of photosynthesis in identical climates, C4 plants do not need to acquire and transport as much water as C3 species. Because the structure and function of xylem reflect a balance between water demand by the leaf canopy and resistance to xylem embolism under high tension, variation in water transport requirements due to changes in WUE could affect the evolution of xylem characteristics. In a comparison of xylem hydraulic conductivity and vascular anatomy between numerous C3 and C 4 species, it was found that C4 plants consistently have lower hydraulic conductivity per unit leaf area than similar C3 species. The majority of C4 species also exhibited lower specific conductivity, water flow per unit wood area, compared with C3 counterparts. C4 species generally produced shorter and narrower vessels, indicating the xylem of C4 plants is less prone to cavitation. The resistance of xylem to cavitation in C4 compared to C3 species was confirmed in five pairs of C3 and C4 species. In every case, C4 species showed 50% loss of hydraulic conductivity at relatively lower pressure potential of the xylem. Because the C4 xylem is less prone to cavitation induced by water stress, and some C 4 species can produce high biomass and exhibit an arborescent habit, this research indicates C4 species could be developed into productive wood fibre crops in semi-arid and arid regions of the world where production by C3 species is poor and uneconomical. In addition, because WUE differences between C3 and C4 plants are proposed as the cause of the changes in xylem properties, our results suggest that other environmental factors affecting WUE, namely atmospheric CO 2 content, could be important modifiers of wood anatomy in higher plants.