PLANT PHYSIOLOGY WHOLE PLANT AND ECOPHYSIOLOGY

Plant {delta}15N Correlates with the Transpiration Efficiency of Nitrogen Acquisition in Tropical Trees

Cernusak, L. A., Winter, K., Turner, B. L. — Tue, 03 Nov 2009 12:22:28 PST

Based upon considerations of a theoretical model of ¹⁵N/¹⁴N fractionation during steady-state nitrate uptake from soil, we hypothesized that, for plants grown in a common soil environment, whole-plant ¹⁵N (_P) should vary as a function of the transpiration efficiency of nitrogen acquisition (F_N/v) and the difference between _P and root ¹⁵N (_P – _R). We tested these hypotheses with measurements of several tropical tree and liana species. Consistent with theoretical expectations, both F_N/v and _P – _R were significant sources of variation in _P, and the relationship between _P and F_N/v differed between non-N₂-fixing and N₂-fixing species. We interpret the correlation between _P and F_N/v as resulting from variation in mineral nitrogen efflux-to-influx ratios across plasma membranes of root cells. These results provide a simple explanation of variation in ¹⁵N of terrestrial plants and have implications for understanding nitrogen cycling in ecosystems.

Vascular Function in Grape Berries across Development and Its Relevance to Apparent Hydraulic Isolation

Choat, B., Gambetta, G. A., Shackel, K. A., Matthews, M. A. — Tue, 03 Nov 2009 12:22:28 PST

During the latter stages of development in fleshy fruit, water flow through the xylem declines markedly and the requirements of transpiration and further expansion are fulfilled primarily by the phloem. We evaluated the hypothesis that cessation of water transport through the xylem results from disruption or occlusion of pedicel and berry xylem conduits (hydraulic isolation). Xylem hydraulic resistance (R_h) was measured in developing fruit of grape (Vitis vinifera ‘Chardonnay’) 20 to 100 d after anthesis (DAA) and compared with observations of xylem anatomy by light and cryo-scanning electron microscopy and expression of six plasma membrane intrinsic protein (PIP) aquaporin genes (VvPIP1;1, VvPIP1;2, VvPIP1;3, VvPIP2;1, VvPIP2;2, VvPIP2;3). There was a significant increase in whole berry R_h and receptacle R_h in the latter stages of ripening (80–100 DAA), which was associated with deposition of gels or solutes in many receptacle xylem conduits. Peaks in the expression of some aquaporin isoforms corresponded to lower whole berry R_h 60 to 80 DAA, and the increase in R_h beginning at 80 DAA correlated with decreases in the expression of the two most predominantly expressed PIP genes. Although significant, the increase in berry R_h was not great enough, and occurred too late in development, to explain the decline in xylem flow that occurs at 60 to 75 DAA. The evidence suggests that the fruit is not hydraulically isolated from the parent plant by xylem occlusion but, rather, is "hydraulically buffered" by water delivered via the phloem.

Most Water in the Tomato Truss Is Imported through the Xylem, Not the Phloem: A Nuclear Magnetic Resonance Flow Imaging Study

Windt, C. W., Gerkema, E., Van As, H. — Thu, 01 Oct 2009 06:20:41 PDT

In this study, we demonstrate nuclear magnetic resonance flow imaging of xylem and phloem transport toward a developing tomato (Solanum lycopersicum) truss. During an 8-week period of growth, we measured phloem and xylem fluxes in the truss stalk, aiming to distinguish the contributions of the two transport tissues and draw up a balance between influx and efflux. It is commonly estimated that about 90% of the water reaches the fruit by the phloem and the remaining 10% by the xylem. The xylem is thought to become dysfunctional at an early stage of fruit development. However, our results do not corroborate these findings. On the contrary, we found that xylem transport into the truss remained functional throughout the 8 weeks of growth. During that time, at least 75% of the net influx into the fruit occurred through the external xylem and about 25% via the perimedullary region, which contains both phloem and xylem. About one-half of the net influx was lost due to evaporation. Halfway through truss development, a xylem backflow appeared. As the truss matured, the percentage of xylem water that circulated into the truss and out again increased in comparison with the net uptake, but no net loss of water from the truss was observed. The circulation of xylem water continued even after the fruits and pedicels were removed. This indicates that neither of them was involved in generating or conducting the circulation of sap. Only when the main axis of the peduncle was cut back did the circulation stop.

New Insights into the Mechanisms of Water-Stress-Induced Cavitation in Conifers

Cochard, H., Holtta, T., Herbette, S., Delzon, S., Mencuccini, M. — Thu, 01 Oct 2009 06:20:41 PDT

Cavitation resistance is a key parameter to understand tree drought tolerance but little is known about the mechanisms of air entry into xylem conduits. For conifers three mechanisms have been proposed: (1) a rupture of pit margo microfibrils, (2) a displacement of the pit torus from its normal sealing position over the pit aperture, and (3) a rupture of an air-water menisci in a pore of the pit margo. In this article, we report experimental results on three coniferous species suggesting additional mechanisms. First, when xylem segments were injected with a fluid at a pressure sufficient to aspirate pit tori and well above the pressure for cavitation induction we failed to detect the increase in sample conductance that should have been caused by torus displacement from blocking the pit aperture or by membrane rupture. Second, by injecting xylem samples with different surfactant solutions, we found a linear relation between sample vulnerability to cavitation and fluid surface tension. This suggests that cavitation in conifers could also be provoked by the capillary failure of an air-water meniscus in coherence with the prediction of Young-Laplace's equation. Within the bordered pit membrane, the exact position of this capillary seeding is unknown. The possible Achilles' heel could be the seal between tori and pit walls or holes in the torus. The mechanism of water-stress-induced cavitation in conifers could then be relatively similar to the one currently proposed for angiosperms.

Evidence That Light, Carbon Dioxide, and Oxygen Dependencies of Leaf Isoprene Emission Are Driven by Energy Status in Hybrid Aspen

Rasulov, B., Huve, K., Valbe, M., Laisk, A., Niinemets, U. — Wed, 02 Sep 2009 10:00:33 PDT

Leaf isoprene emission scales positively with light intensity, is inhibited by high carbon dioxide (CO₂) concentrations, and may be enhanced or inhibited by low oxygen (O₂) concentrations, but the mechanisms of environmental regulation of isoprene emission are still not fully understood. Emission controls by isoprene synthase, availability of carbon intermediates, or energetic cofactors have been suggested previously. In this study, we asked whether the short-term (tens of minutes) environmental control of isoprene synthesis results from alterations in the immediate isoprene precursor dimethylallyldiphosphate (DMADP) pool size, and to what extent DMADP concentrations are affected by the supply of carbon and energetic metabolites. A novel in vivo method based on postillumination isoprene release was employed to measure the pool size of DMADP simultaneously with the rates of isoprene emission and net assimilation at different light intensities and CO₂ and O₂ concentrations. Both net assimilation and isoprene emission rates increased hyperbolically with light intensity. The photosynthetic response to CO₂ concentration was also hyperbolic, while the CO₂ response curve of isoprene emission exhibited a maximum at close to CO₂ compensation point. Low O₂ positively affected both net assimilation and isoprene emission. In all cases, the variation in isoprene emission was matched with changes in DMADP pool size. The results of these experiments suggest that DMADP pool size controls the response of isoprene emission to light intensity and to CO₂ and O₂ concentrations and that the pool size is determined by the level of energetic metabolites generated in photosynthesis.