Polyamines and leaf senescence in pyrrolizidine alkaloid-bearing Heliotropium plants

Putrescine, spermidine and spermine were found in leaves and inflorescences of H. angiospermum and H. indicum plants; the levels of these amines declined with leaf age. In addition, homospermidine was identified in the inflorescence axes and youngest leaves of H. indicum. The youngest tissues exhibited the highest levels of both putrescine and pyrrolizidine alkaloids. The detection of homospermidine in the plants supports the theory that the pyrrolizidine moiety is derived from two molecules of putrescine with homospermidine as an intermediate. In the youngest organs, the pyrrolizidines represented over 5% of the total nitrogen content. Their level was 50-100 fold higher than that of the polyamines, including putrescine. When detached and kept in the dark for 100-120 hr, mature older Heliotropium leaves, with a very low polyamine content, exhibited only a weak senescence syndrome. By contrast, in detached, darkened leaves of Avena sativa and Nicotiana alata having high polyamine levels, the chlorophyll and protein degradation and increases in free amino acids were very pronounced.