Study of cytokinin content related to the loss of viability of long term stored maize seeds

Loss of maize seed viability following long term storage was studied in relation to cytokinin content using two populations ( recent crops (NS) and long term stored seeds(S)) of each of three different maize varieties: 'Tuxpeno Crema' from subtropical regions,'Criollo del Mezquital' from dry, high lands, and 'Compuesto Universal' from temperate high lands. Embryonic axes were used, since 82-85% of the zeatin type ribosides were found in this tissue. A comparative analysis of the endogenous zeatin type ribosides in quiescent seeds showed that the cytokinin content was higher in S than in NS seeds. This difference was greater in the highland seeds than in the subtropical variety which had the lower germinability. Total RNA yield was lower for S seeds than for NS seeds in the three varieties. Comparison of changes in cytokinins during germination revealed an increase in zeatin type ribosides at the beginning of the germination process, both in S and NS seeds. Zeatin type nucleotide content in axes from S quiescent seeds was higher than in NS axes. The accumulation of nucleotides of NS seeds axes increased from the beginning of imbibition until 12 hours, and showed a sustained content of nucleotides. S seeds had a higher content of nucleotides at the beginning of imbibition, which increased up to 5 h, after which the level of zeatin nucleotides declined. Apparently, the loss of seed viability is not primarily dependent on the zeatin riboside contents of the quiescent seeds nor on the production of these compounds during germination but, among other biochemical degradative processes, on the inability of the NS axes to maintain the exchange of cytokinins and on the extensive degradation of RNA.