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  • Author or Editor: James A. Bunce x
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It is known that, in a number of plant tissues, the diffusivity of water vapors is larger than that of CO2. We have measured the water potential of both intact potato (Solomon tuberosum, cv. Russet Burbank) tubers, using a Wescor due point probe, and tissue slices, using the liquid exchange method. The water potential measured by both methods was similar. The results show that the diffusivity of water vapors is larger than that of CO2. The difference in diffusivity between CO2 and water vapors is attributed to a simultaneous loss of water through diffusion and surface evaporation. The results indicate that the contribution of the latter mechanism to water loss is more significant than simple diffusion.

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Application of a complete nutrient solution (CNS) on apple seedling leaves reduced stomatal conductance (gs). Tween 20 and CaCl2 were components of the CNS which induced gs reduction. Tween 20 alone, however, did not cause stomatal closure, but CaCl2 (24.8 mm) had a consistent, negative effect on gs when applied alone. Application of CaCl2 in combination with one of the other macrocomponents of the CNS (MgSO4, urea, or K2SO4 + KH2PO4) produced less consistent gs reductions indicating that the CaCl2 effect on gs can be modified by the presence of these compounds. Urea, MgSO4, or K2SO4 + KH2PO4 had little effect on gs when applied separately. Application of MgCl2 or KCl, which were not the CNS components, decreased and had no effect on gs, respectively. In addition to gs reduction, CaCl2 sprays reduced net photosynthesis (Pn). The equivalence of intercellular CO2 concentration in sprayed and unsprayed seedlings implied that the Pn drop following CaCl2 sprays resulted from decreased capacity of mesophyll for CO2 fixation and not from reduction in the stomatal aperture. Two possible explanations for stomata closure are discussed: a direct effect of CaCl2 on stomata and an indirect effect of CaCl2 spray through changes in mesophyll CO2 fixation capacity. Reductions in gs and Pn following treatments with different salts were not associated with visible leaf injury.

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