Temperature effects on symplasmic and apoplasmic phloem loading and loading-associated carbohydrate processing.

Given the fact that plant species with a type 1 (symplasmic) minor vein ultrastructure seem to dominate in the tropics and subtropics, and species with a type 2 (apoplasmic) minor vein ultrastructure in the temperate and boreal climate zones, a cold sensitivity of symplasmic phloem loading was postulated. Electron microscopic observations were taken as support for this proposal. The objective of the present work was to test this postulate by measuring physiological parameters correlated to phloem loading. Carbohydrate levels in the leaf, minor vein loading of 14 CO2-derived assimilates in leaf segments and exudation of sugars and 14 C-labelled compounds in several species from families with known phloem-loading pathways were compared in 10 and 20°C-adapted plants at both 10 and 20°C. No essential differences in response to temperature between symplasmically and apoplasmically phloem-loading species were observed. Carbohydrate availability for export was essentially similar, phloem loading was fully operative at 10°C, and exudation of sugars equally reacted to low temperature in symplasmic and apoplasmic species. Apparently, the geographical distribution of type 1 and 2 species is not explained by a difference in temperature sensitivity of the phloem-loading mode.