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Title:   Author  item
                                            Gleason, Seanitem
                                            Wiggans, Dustinitem
                                            Bliss, Claytonitem
                                            Comas, Louiseitem
                                            Cooper, Mitchellitem
                                            DeJonge, Kendallitem
                                            Young, Jasonitem
                                            Zhang, Huihui
  Submitted to: American Society of Plant Biologists
                                    Publication Type: Abstract Only
                                    Publication Acceptance Date: 1/15/2016
                                    Publication  Date: 1/15/2016
                                    Citation: Gleason, S.M., Wiggans, D.R., Bliss, C.A., Comas, L.H., Cooper, M.S., DeJonge, K.C., Young, J.S., Zhang, H. 2016. Electron transport, pep carboxylase activity, and maximal net co2 assimilation exhibit coordinated and proportional decline with loss of hydraulic conductance during water stress in Zea mays. American Society of Plant Biologists. 25th Western Photosynthesis Conference.Interpretive Summary: Efforts to improve plant performance in dry habitats have largely been focused on the process that converts sunlight, water, and CO2 into simple organic molecules (photosynthesis).  We argue here that the ability of a plant to transport water (hydraulics) and CO2 to the points where photosynthesis takes place are also important processes, and that during drought, all three of these processes (photosynthesis, hydraulics, and CO2 transport) decline together.  This suggests that drought affects these processes similarly and that efforts to improve species in dry habitats will necessarily require careful consideration of all physiological systems, not just photosynthesis.
                                Technical Abstract: Efforts to improve the photosynthetic performance of species are presently focused on leaf-level traits (e.g., quantum efficiency, mesophyll osmoregulation, stress protein regulation).  Here, we emphasize that efforts to improve plant performance in arid environments would benefit from also considering the efficiency and safety of the hydraulic network (i.e., xylem) to supply water to the sites of photosynthesis.  Maximal rates of electron transport (ETR), PEP carboxylase activity (Vpmax), net CO2 assimilation (Amax), stomatal conductance (gs), whole-plant conductance (Kplant), and xylem conductance (leaves and stems) were measured during dry-down experiments on field- and greenhouse-grown maize plants (inbred line B73).  Whole-plant conductance, midrib conductance, and stem conductance all exhibited similar levels of embolism resistance during stress, with P50 values ranging from -1.29MPa to -1.82MPa, suggesting coordinated xylem response to declining leaf water potential ('leaf).  This loss in whole-plant conductance was aligned with a proportional decline in photosynthetic functioning (ETR, Vpmax, Amax) and gs_max, also suggesting an integrated whole-plant response to declining water potential.  Furthermore, the six hydraulic and photosynthetic traits examined in this study ('leaf, Kplant, ETR, Vpmax, Amax, gs_max) formed a single axis of variation, with 83% of the total variation among these traits being explained by a single principal component.  Considering that much of the residual variation in this analysis (17%) must include measurement error, this represents a remarkable degree of alignment among hydraulic, stomatal, and photosynthetic functioning.  We suggest that the covariation among these physiological systems is the logical outcome of natural selection and emphasizes that improvement to one trait (e.g., Amax) may result in the failure of some other “linked” system component (e.g., xylem conductance), rather than leading to a marked improvement in growth.  (Supported by USDA-ARS.)


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