Citation: Pratt, R. B., V. Castro, J. C. Fickle, A. Madsen, and A. L. Jacobsen. 2020. Factors controlling drought resistance in grapevine (Vitis vinifera, chardonnay): application of a new microCT method to assess functional embolism resistance. PREMISE:Quantifying resistance to embolism in woody plants is important for understanding their drought response. Methods to accurately quantify resistance to embolism continue to be debated. METHODS:We used a new microCT-based approach that quantifies embolized conduits and also analyzes conductive conduits by using an x-ray-dense, iodine-rich tracer that moves though the vascular system and can easily be observed in microCT images. Many previous microCT studies assumed that all conduits were initially conductive, which may not be the case if there are developing or occluded conduits. We compared microCT results to a standard benchtop dehydration method and a centrifuge method. During dehydration, we measured gas exchange and quantified water potential at mortality. RESULTS:Our microCT curves agreed with previously published microCT curves from the same greenhouse-grown cultivar. We found a significant difference in embolism estimates if we assumed that all water-filled conduits were functional rather than only those containing tracer. Embolism estimates from microCT differed from both the benchtop and centrifuge methods. The benchtop and centrifuge methods did not differ from one another. CONCLUSIONS:The new microCT method presented here is valuable in sampling species that may contain nonconductive conduits. Disagreement between microCT and two other methods was likely due to differences in the ways they quantify embolism. MicroCT assess the theoretical effect of embolism, whereas benchtop and centrifuge methods directly measure hydraulic conductivity. The theoretical approach does not fully account for the resistances of flow through a complex 3D vascular network.
Along the distal stems of woody plants, nodes occur along the stem length separated by internode regions. Nodes typically include a leaf or leaf scar and an axillary bud that are connected to the xylem tissue within the stem through vascular leaf and bud traces. The diversion of xylem tissue into these lateral appendages creates a node gap that is typically occupied by parenchyma. We hypothesized that node-associated changes in structure within the stem tissues would result in alterations to stem biomechanics and hydraulic transport. We examined four deciduous species, Juglans californica, Populus trichocarpa, Quercus robur, and Rhus aromatica and measured node frequency, stem density, biomechanics, and hydraulic conductivity in 36 stems from each species. Vessel diameters within nodes and internodes were measured on a subset of these stems, as well as measures of xylem, pith, and node gap areas. Increased node frequency was correlated with decreased stem strength (modulus of rupture; MOR), decreased stem stiffness (modulus of elasticity; MOE), increased stem density, and decreased hydraulic conductivity. There were no differences in vessel diameter or xylem area between node and internode regions. Reduced hydraulic conductivity with increasing node frequency could have been due to increased vessel termini associated with nodes as has been found in prior research. Increased length of hydraulic pathways due to divergence of vessels around node gaps could also decrease hydraulic conductivity. Variation within the tree crown in node frequency may be an important morphological feature that has implications for crown tolerance of periodic mechanical stresses, such as wind events and fruit load.
SUMMARYThe leaf diffusive resistance to the transfer of water vapour and COj exchange in rice plants {Oryza sativa L., cv. IR-20) measured from the late vegetative stage to maturity, varied in response to differing CO^ enrichments. The leaf diffusive resistance of plants treated with 900 /tl r* CO2 ranged from one-half to twice that of the control plants (330//ll"'CO2).This difference was more pronounced during the heading and ripening stages. In plants treated with 600 /tl ri CO2, the diffusive resistances appeared to be smaller than in the control plants for most of the day. In the diurnal cycles, resistances were highest during the early morning and before sunset, the maximum again occurring in the plants treated with 900 fi\ T' CO^.The rate of transpiration was inversely related to the diffusive resistance, and was 20 to 100 % lower in the plants treated with 900 //I T' COj than in the controls, both in the seasonal and diurnal cycles. In plants treated with 600 //11"' CO^, the rate of transpiration was lower than in the controls in the morning but distinctly higher during the afternoon. However, the integrated total daily transpiration in 600 /il T' CO2 did not exceed that of the controls. In all cases, COa-treated plants, through their increased biomass production and economic yield, appeared to have a higher water use efficiency than the control plants.
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