Different phloem-loading machineries correlated with the climate

A few years ago, phloem loading in source leaves was assumed to be a universal process (reviews Giaquinta 1983; Delrot 1987). In the transfer of photo-assimilates from mesophyll to sieve element an apoplastic step was invoked to explain the accumulation of sugars in the sieve tubes. This step involves leakage from the mesophyll and subsequent accumulation by the SE/CC-complexes of the minor vein. Sugar transport, driven by a large proton-motive force (ApH+) across the membrane of the SE/CC-complex, results in a high sugar concentration in the sieve tubes. The thermodynamic logic of this model impeded for some time the development of other views on the phloem-loading mechanism. Recent evidence, however, is beginning to turn this rigid conception into a more flexible one. There is a tendency to see phloem loading as a process which is different in various plant families. The phloem-loading mechanism may even shift, within certain limitations, in individual plants in response to changing environmental conditions. This paper presents the ultrastructural and physiological arguments leading to a more flexible model of phloem loading; the highly variable plasmodesmatal connectivity between the mesophyll and the phloem elements (minor-vein configuration), the physiological evidence for different modes of loading, the coincidence between the minor-vein configuration and the mode of phloem loading, the correlation between the minor-vein configuration and plant taxonomy and the global distribution of plant families. The emerging picture of a multiprogrammed phloem-loading within an ecophysiological context is discussed.