Cold acclimation of photosynthesis in the context of phloem loading and possible anatomical constraints

Plants load sugars into the phloem using either an apoplastic (via H +/sucrose symporters) or symplastic (via plasmodesmata-connected cytoplasm) pathway, and symplastic loading is purported to be sensitive to cold temperatures. Furthermore, recent evidence suggests the symplastic phloem-loading mode may limit photosynthetic acclimation in mature leaves to conditions that increase sugar production (e.g. higher light intensity). For the symplastic loaders, complete photosynthetic acclimation is achieved only when new leaves develop with greater foliar vein density and, presumably, greater sugar export capacity. The present study was undertaken to test the effect of cold temperature on photosynthetic acclimation in two symplastic loaders (Cucurbita pepo and Verbascum phoeniceum) and one apoplastic loader (Spinacia oleracea). Plants were grown under warm temperature and then subjected to either short-term (1 week) or long-term (several weeks) exposure to moderately cold temperatures (constant leaf temperature of approximately 12.5°C). Short-term exposure tested the effect of cold temperature on fully expanded, warm-grown leaves, while long-term exposure allowed new leaves to develop under the cold temperature regime (cold-grown leaves). Short-term exposure to cold temperature had little impact on any of the physiological or anatomical parameters measured. Long-term exposure, however, resulted in many differences across the three species and within phloem loading mode. Cold-grown C. pepo (symplastic loader) exhibited no measurable acclimation to cold temperatures. Cold-grown V. phoeniceum (symplastic loader) and cold-grown S. oleracea (apoplastic loader) exhibited increased photosynthetic capacity and thicker leaves, while cold-grown V. phoeniceum also exhibited greater foliar loading vein density. These results provide further evidence that in species that vary foliar vein density in response to the environment, high vein density is apparently necessary for full photosynthetic acclimation. Furthermore, the generalization that symplastic loaders are inherently sensitive to cold temperatures is too simplistic. In addition to foliar vein density, the role of other anatomical constraints on photosynthetic acclimation to cold temperatures is discussed.