Restrictions to Carbon Dioxide Conductance and Photosynthesis in Spinach Leaves Recovering from Salt Stress

Salt accumulation in spinach (Spinacia oleracea L.) leaves first inhibits photosynthesis by decreasing stomatal and mesophyll conductances to CO2 diffusion and then impairs ribulose-1,5-bisphosphate carboxylase/oxygenase (S. Delfine, A. Alvino, M. Zacchini, F. Loreto [1998] Aust J Plant Physiol 25: 395–402). We measured gas exchange and fluorescence in spinach recovering from salt accumulation. When a 21-d salt accumulation was reversed by 2 weeks of salt-free irrigation (rewatering), stomatal and mesophyll conductances and photosynthesis partially recovered. For the first time, to our knowledge, it is shown that a reduction of mesophyll conductance can be reversed and that this may influence photosynthesis. Photosynthesis and conductances did not recover when salt drainage was restricted and Na content in the leaves was greater than 3% of the dry matter. Incomplete recovery of photosynthesis in rewatered and control leaves may be attributed to an age-related reduction of conductances. Biochemical properties were not affected by the 21-d salt accumulation. However, ribulose-1,5-bisphosphate carboxylase/oxygenase activity and content were reduced by a 36- to 50-d salt accumulation. Photochemical efficiency was reduced only in 50-d salt-stressed leaves because of a decrease in the fraction of open photosystem II centers. A reduction in chlorophyll content and an increase in the chlorophyll a/b ratio were observed in 43- and 50-d salt-stressed leaves. Low chlorophyll affects light absorptance but is unlikely to change light partitioning between photosystems.

Abundance of Globigerinoides sacculifer and Globigerinoides ruber and oxygen and carbon stable isotopes of different planktonic foraminifers at DSDP Holes 78-541 and 78-543

The stable isotope study of monospecific planktonic foraminifer samples recovered at Sites 541 and 543 during Deep Sea Drilling Project Leg 78A indicates a warming during the early Pliocene about 4.7 to 4.3 Ma. The changes in the late Pliocene oxygen isotope record around 2.9 to 2.7 Ma coincide with changes in the circulation pattern resulting from the closure of the Panama seaway and the beginning of the Northern Hemisphere glaciation. The Pleistocene record is characterized by 0.5 to 1.0 per mil fluctuations in the d18O record. These fluctuations reflect salinity changes, rather than temperature changes, as indicated by Globigerinoides ruber and G. sacculifer abundances. The salinity changes may be explained by a drifting of (1) the highly saline Central Water Mass of the southern Sargasso Sea, and (2) lower-salinity ocean water displaced by the northward shift of the Intertropical Convergence Zone into the Caribbean region during cooler intervals.

(Table 1) Carbon- and oxygen-isotopes for bulk sediment carbonate at DSDP Holes 86-577 and 86-577A

We have obtained a detailed carbon-isotope stratigraphy of the Paleocene sections recovered from Deep Sea Drilling Project (DSDP) Holes 577 and 577A. This 13C record is useful in stratigraphically correlating the two holes and in interpreting the magnetostratigraphic data. Comparison with the published data for Site 527 (Southeast Atlantic Ocean) shows that 13C stratigraphy is also valuable for long-distance correlation.