Changes in the Ability of Photophosphorylation and Activities of Surface-Bound Adenosine Triphosphatase and Ribulose Diphosphate Carboxylase of Chloroplasts Isolated from the Barley Leaves Senescing in Darkness

Dark-induced aging of detached primary leaves of 11-day-old barley seedlings brings about a significant decline in the rates of ferricyanide Fe(CN)₆]^3? reduction and photophosphorylations of isolated chloroplasts. Ferricyanide-supported noncyclic photophosphorylation is somewhat more susceptible to leaf aging than phenazine methosulfate (PMS)-supported cyclic phosphorylation. Non-latent membrane-bound adenosine triphosphatase (ATPase) and ribulosediphosphate carboxylase (RuDPCase) lose about half of their initial activities after 24 h, while during this period the electron transport and photophosphorylation activities are much less affected. Also, the loss of RuDPCase is almost complete, while chloroplasts still exhibit a significant level of Fe(CN)₆]^3? reduction and photophosphorylations after 7 days of dark incubation. This would suggest that the enzymatic dark reactions are more sensitive to aging stress than the primary photochemical reactions of chloroplasts. Studies on the effect of divalent cations such as Mg²⁺ and Ca²⁺ on non-latent ATPase activity revealed that the dark stressed aging of detached leaves brings about a time dependent alteration in the response of this enzyme to Mg²⁺, but not to Ca²⁺. The former showed inhibitory as well as stimulatory response, whereas the latter always caused the usual stimulation. Addition of kinetin (50 μM) ensured retention of Fe(CN)₆]^3? reduction, photophosphorylations and RuDPCase activity in chloroplasts during leaf aging, but it failed to preserve the initial loss in the activity of the non-activated membrane-bound ATPase.