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The objective of this study was to determine how adjustment in stomatal conductance (g ) and turgor loss point (Ψ) between riparian (wet) and neighboring slope (dry) populations of Acer grandidentum Nutt. was associated with the susceptibility of root versus stem xylem to embolism. Over two summers of study (1993-1994), the slope site had substantially lower xylem pressures (Ψ) and g than the riparian site, particularly during the drought year of 1994. The Ψ was also lower at the slope (-2.9±0.1 MPa; all errors 95% confidence limits) than at riparian sites (-1.9±0.2 MPa); but it did not drop in response to the 1994 drought. Stem xylem did not differ in vulnerability to embolism between sites. Although slope-site stems lost a greater percentage of hydraulic conductance to embolism than riparian stems during the 1994 drought (46±11% versus 27±3%), they still maintained a safety margin of at least 1.7 MPa between midday Ψ and the critical pressure triggering catastrophic xylem embolism (Ψ). Root xylem was more susceptible to embolism than stem xylem, and there were significant differences between sites: riparian roots were completely cavitated at -1.75 MPa, compared with -2.75 MPa for slope roots. Vulnerability to embolism was related to pore sizes in intervessel pit membranes and bore no simple relationship to vessel diameter. Safety margins from Ψ averaged less than 0.6 MPa in roots at both the riparian and slope sites. Minimal safety margins at the slope site during the drought of 1994 may have led to the almost complete closure of stomata (g =9±2 versus 79±15 mmol m s at riparian site) and made any further osmotic adjustment of Ψ non-adaptive. Embolism in roots was at least partially reversed after fall rains. Although catastrophic embolism in roots may limit the minimum Ψ for gas exchange, partial (and reversible) root embolism may be adaptive in limiting water use as soil water is exhausted.

Background:The phospholipid cardiolipin undergoes acyl chain remodeling after biosynthesis, which has been hypothesized to optimize mitochondrial function. Results: ⌬cld1 yeast, containing unremodeled cardiolipin, have no mitochondrial morphology or oxidative phosphorylation defects. Conclusion: Cardiolipin remodeling is not required for optimal mitochondrial bioenergetic function in yeast. Significance: Cardiolipin remodeling may be important for presently unknown mitochondrial processes and/or have unappreciated physiological functions.

The Sec61 translocon of the endoplasmic reticulum membrane forms an aqueous pore that is gated by the lumenal Hsp70 chaperone BiP. We have explored the molecular mechanisms governing BiP-mediated gating activity, including the coupling between gating and the BiP ATPase cycle, and the involvement of the substrate-binding and J domain–binding regions of BiP. Translocon gating was assayed by measuring the collisional quenching of fluorescent probes incorporated into nascent chains of translocation intermediates engaged with microsomes containing various BiP mutants and BiP substrate. Our results indicate that BiP must assume the ADP-bound conformation to seal the translocon, and that the reopening of the pore requires an ATP binding–induced conformational change. Further, pore closure requires functional interactions between both the substrate-binding region and the J domain–binding region of BiP and membrane proteins. The mechanism by which BiP mediates translocon pore closure and opening is therefore similar to that in which Hsp70 chaperones associate with and dissociate from substrates.

The vulnerability of xylem conduits to cavitation theoretically determines the maximum flow rate of water through plants, and hence maximum transpiration (E), stomatal conductance (g,), and leaf area (A^-Field-grown Betula occidentals with a favourable water supply exhibit midday xylem pressures (