Towards better understanding of assimilate transfer and phloem loading in conifer trees

Leaf assimilate transport and phloem loading in conifers are less understood than in angiosperms. Among other peculiarities, the semi-permeable endodermis bundle sheath acts as a bottleneck for assimilate transport, when mass flow of water from the xylem opposes diffusion of assimilates towards the phloem. We propose that transitory starch storage in the endodermis and transfusion parenchyma tissues from the needle veinlet may facilitate steady carbohydrate supply for phloem loading, especially during periods of high transpiration. 13C labelling and temporally resolved localization of labelled starch with high-resolution SIMS imaging can shed light on the storage and transport mechanisms.