Proline metabolism in senescing rose petals (Rosa hybrida L. 'First Red')

In the present study, an attempt has been made to unravel the pathways of proline metabolism associated with nitrogen-assimilating enzymes in senescing petals of the cut rose Rosa hybrida L. 'First Red'. Flower petals (n = 3) were harvested from the outermost whorl of each flower of 'First Red' rose at six different developmental stages. Our findings showed the existence of the glutamate as well as the ornithine pathways of proline biosynthesis in senescing rose petals. The glutamate pathway was more active and dominant over the ornithine pathway of proline biosynthesis. There was a progressive rise in proline accumulation up to Stage-5 (flowers 6 d after harvest, petals completely unfolded) which was positively correlated with higher activities of Delta(1)pyrrolline-5-carboxylate synthetase (P5CS) and ornithine-delta-aminotransferase (delta-OAT) in flower petals. It was apparent that a significant reduction in glutamine synthetase (GS) activity at Stage-5 (2.8-fold) and at Stage-6 (25.0-fold; flowers 9 d after harvest, completely senesced with petal blueing) might restrict the supply of glutamate as a substrate for P5CS. A higher activity of glutamate dehydrogenase (NADH-GDH) was recorded at Stage-5. This was necessary to maintain a continuous supply of glutamate for P5CS to function. In addition, high delta-OAT activity was also detected at Stage-5. These observations showed the co-operative role of NADH-GDH and delta-OAT in proline biosynthesis in the terminal stages (S5 - S6) of petal senescence. The relatively low activity of proline dehydrogenase (PDH) indicated a low turnover rate of proline throughout flower-bud senescence.