Endogenous Factors Controlling Photoperiodism of Crop Plants.

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Various developmental changes in crop plants (higher plants), such as flowering, tuberization and termination of stem growth, are under the control of photoperiods. Both tuberization in potato plants and termination of stem growth of soybean plants are induced by short days. We have previously found that potato tuberization is induced by tuberonic acid (TA) and its glucoside which seem to be formed in the leaves under the short days. Jasmonic acid (JA) which is structually related to tuberonic acid has various inhibitory effects on plant growth, for example, it inhibits the growth of seedlings. The possible involvement of JA in the termination of stem growth of soybean plants was examined in indeterminate and determinate isolines of soybean [Glycine max (L.) Merril cv. Harosoy].JA-like activities in leaves of both isolines were very low 20 days before the termination of stem growth of determinate isoline. The activity increased rapidly thereafter and reached a maximum near the time of the termination. Although the activities in leaves of both isolines fluctuated in a similar manner, the activity in the determinate isoline was much higher than that in the indeterminate isoline. The presence of JA in the leaves of the determinate isoline was confirmed by purification by high-performance liquid chromatography and by mass spectrometry. The level of JA in the leaves was 0.18 mg/kg fresh weight (nearly 10^<-6> M), on an average of four samples harvested different times. Exogenous application of JA to cultured shoot apices of the indeterminate isoline at concentrations above 10^<-7> M strongly inhibited growth. Above a concentration of 10^<-6> M, some shoots without any growth at all were observed. These results suggest that JA is a major endogenous factor that controls the growth habit of soybean plants. It is possible that the levels of JA and TA are under the control of photoperiod.