Non-destructive limits to seed growth and leaf protease activities in nodulating and non-nodulating soybean isolines

Leaf senescence leads to a progressive decline in the photosynthetic compet ence of the leaf. This paper describes some effects of source:sink imbalanc e on leaf protein catabolism and senescence in soybean. We manipulated pod growth by restricting 100 or 50 % (PR-100 or PR-50, respectively) of young pods at the R4 stage in plastic drinking straws. This effectively reduces f inal seed mass without interrupting the vascular connections of pods. Nodul ating (NOD+) and non-nodulating (NOD-) isolines of the 'Clay soybean were g rown in drainage lysimeters and three pod-restriction (PR) treatments were compared. Pod restriction decreased seed biomass per plant as a result of l ower individual seed mass, which was only partially balanced by the increas e in seed number. The nitrogen concentration in seeds remained unchanged in NOD+ plants, while it increased with the degree of sink restriction in see ds of NOD- plants. Leaf soluble protein, CO2 exchange rate and seed nitroge n content were consistently lower in NOD- plants; the leaf protein level re mained stable with time in PR-100 plants, decreased for PR-50 and dropped f or controls. Endoprotease (HBase) and carboxypeptidase (CPase) activities w ere significantly lower in leaves from PR-100 plants, while aminopeptidase activity was enhanced, indicating a de novo synthesis of leaf protein. This is consistent with the reported accumulation of vegetative storage protein s (VSPs) in soybean and other legumes after moderate or severe sink reducti on. Thus, small modifications of the source:sink ratio such as those obtain ed by the nondestructive PR technique have an impact on leaf protein catabo lism. Nodulating and non-nodulating soybean isolines showed similar respons es to PR in terms of leaf senescence initiation and progression, but the ra te of the processes appear to be largely influenced by plant N status.