The antisenescent activity of naturally occurring cytokinins (bases and ribosides) has been evaluated by measuring chlorophyll retention in detached wheat (Triticum vulgare) leaf segments. 6‐(3‐Methyl‐2‐butenylamino)‐2‐methylthiopurine (ms2ip) was the most active cytokinin followed by 6‐(4‐hydroxy‐3‐methyl‐trans‐2‐butenylamino)purine (tZ). 6‐(4‐Hydroxy‐3‐methyl‐cis‐2‐butenylamino)‐9‐β‐D‐ribofuranosylpurine (cZR), 6‐(4‐hydroxy‐3‐methyl‐trans‐2‐butenylamino)‐2‐methylthio‐9β‐D‐ribofuranosylpurine (MstZR), and 6‐(4‐hydroxy‐3‐methyl‐cis‐2‐butenylamino)‐2‐methylthio‐9‐β‐D‐ribofuroanosylpurine (mscZR) were essentially inactive. 9‐Ribosyl substitution did not affect the activity of tZ, (±)‐6‐(4‐hydroxy‐3‐methylbutylamino)purine (DHZ), or 6‐(3‐methyl‐2‐butenylamino)purine (2ip), but lowered the activity of 6‐(o‐hydroxybenzylamino)purine (OHBA) and 6‐(4‐hydroxy‐3‐methyl‐cis‐2‐butenylamino)purine (cZ). 2‐Methylthio substitution increased the activity of 2ip and DHZ, decreased the activity of tZ, and had no effect on the activity of cZ. The activities of the simultaneously substituted 2‐methylthio‐9‐ribosyl compounds are lower than those of their corresponding unsubstituted or 2‐methylthio substituted bases with the exception of DHZ. Structure‐activity relationships for chlorophyll retention did not parallel many of the relationships found for callus tissue growth stimulation.

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