Can sucrose content in the phloem sap reaching field pea seeds (Pisum sativum L.) be an accurate indicator of seed growth potential?

The composition of the translocates reaching the seeds of pea plants having various nitrogen (N) nutrition regimes was investigated under field situations. Sucrose flow in the phloem sap increased with the node number, but was not significantly different between N nutrition levels. Because N deficiency reduced the number of flowering nodes and the number of seeds per pod, the sucrose flow bleeding from cut peduncles was divided by the number of seeds to give the amount of assimilates available per seed. The sucrose concentration in phloem sap supplied to seeds at the upper nodes was higher than that at the lower nodes. The flow of sucrose delivered to the seeds during the cell division period was correlated with seed growth potential. Seeds from the more N-stressed plants had both the highest seed growth rate and received a higher sucrose flux per seed during the cell division period. As seed growth rate is highly correlated with the number of cotyledonary cells produced during the cell division period, sucrose flow in phloem sap is proposed to be an important determinant of mitotic activity in seed embryos. The carbon (C)/N ratio of the flow of translocates towards seeds was higher under conditions of N-deficiency than with optimal N nutrition, indicating that N flux towards seeds, in itself, is not the main determinant of seed growth potential.