Thylakoid polypeptide components, Mg~(2+) -induced Chl a fluorescence at 77K and surface chargechanges were measured to investigate the functional sites of Mg~(2+) in the regulation of energy distri-bution between two photosystems in completely and incompletely developed barley chloroplasts. It wasfound that in contrast to the completely developed chloroplasts, the incompletely developed chloroplastslacked Chl b and did not contain the 23KDa and 25KDa polypeptide components of LHC-PSII. In themeantime, these membranes did nor present Mg~(2+) -induced Cbl a fluorescence and surface charge changesof thylakoids. These results provided strong evidence that the 23KDa and 25KDa polypeptides ofLHC-PSII are the specific acting sites of the cation that induced these two phenomena. It is suggested in this paper that Mg~(2+) -induced change of excitation energy distribution betweentwo photosystems is produced by the mechanism due to electrostatic neutralization of LHC-PSII bythe cation to cause a structural or con Thylakoid polypeptide components, Mg ~ (2+) -induced Chl a fluorescence at 77K and surface chargechanges were measured to investigate the functional sites of Mg ~ (2+) in the regulation of energy distri-bution between two photosystems in completely and incompletely developed It wasfound that in contrast to the completely developed chloroplasts, the incompletely developed chloroplastslacked Chlb and did not contain the 23KDa and 25KDa polypeptide components of LHC-PSII. In themeantime, these membranes did nor present Mg ~ (2+) - These results provided strong evidence that the 23KDa and 25KDa polypeptides of LHHC-PSII are the specific acting sites of the cation that induced these two phenomena. It is suggested in this paper that Mg ~ (2+ ) -induced change of excitation energy distribution betweentwo photosystems is produced by the mechanism due to electrostatic neutralization of LHC-PSII by the cation to cause a structural or con