CA-2+-BINDING SITES AND PHOSPHATASE-ACTIVITIES IN SIEVE ELEMENT RETICULUM AND P-PROTEIN OF CHICKPEA PHLOEM - A CYTOCHEMICAL AND X-RAY-MICROANALYSIS SURVEY

In stem of Cicer arietinum, the loss of ribosomes attached to the rough ER cisternae during sieve element ontogeny results in the formation of "sieve element reticulum" (SER). By enhancing contrast of the SER, the OsFeCN postfixation/staining of material prefixed in glutaraldehyde in presence of calcium enables a good visualization of this membrane system. The pattern of staining in the SER is slightly lower when Mg2+ is substituted for Ca2+. These results support the view that the OsFeCN staining requires divalent cations and that the SER can accumulate Ca2+. The detection of Ca2+ by means of the pyroatimonate method in conjunction with X-ray microanalysis and the cytochemical localization of Ca2+-ATPase in the SER cisternae provides evidence for Ca2+ sequestration by the SER. On the other hand, Ca2+-binding sites and ATPase activity are localized in P-protein. The ability to bind Ca2+ seems to enable the SER to function as an effective Ca2+ sink which may participate-with the sieve tube plasma membrane and mitochondria-in the maintenance of low Ca2+ concentration in phloem sap. In addition, the close association between P-protein and SER membranes exhibiting Ca2+-binding capabilities suggests that a Ca2+-mediated functional relationship may exist between the two structures. It is postulated that the SER may play a role in putative Ca2+ control of P-protein organization.