The primary function of mature leaves in photosynthesis is supported by the venation network that (a) delivers water via the xylem to the mesophyll tissue as well as the phloem and (b) exports the sugar products of photosynthesis out of the leaf. To ascertain the plasticity of the foliar venation network in association with plant life habit, multiple features of foliar minor-vein capacity for water import and sugar export (vein density, number and size of xylem and phloem cells, phloem cell wall ingrowths), photosynthesis, and transpiration were characterized in leaves of winter annual, summer annual, biennial, and perennial herbaceous species that had been grown under a variety of different environmental conditions.
Results/Conclusions
Winter annuals had the fewest minor veins per leaf area with small but numerous vascular cells, summer annuals exhibited an intermediate to high vein density with large but few vascular cells per minor vein, and biennials had a high vein density, an intermediate number of vascular cells per minor vein, and small water conduits (tracheary elements). Smaller tracheary elements in the winter annuals and biennials compared to the summer annuals are likely adaptive with regard to freeze-thaw induced cavitation resistance. Transpiration was consistently associated with number of tracheary elements and vein density, while photosynthesis rate was consistently associated with minor-vein phloem features. The greatest level of leaf vascular phenotypic plasticity in response to growth environment was observed in the winter annual Arabidopsis thaliana. While not apparent in leaves grown under moderate conditions, significant differences among ecotypes (from Sweden, Poland, and Italy) were evident in leaves grown under extremes of temperature or light depending on temperature and precipitation characteristics of the habitats from which each ecotype originated. Similar differences among species (annuals versus perennials) and ecotypes (from central Canada to Texas) of sunflower (Helianthus) are under investigation. Such features have implications for plant response to climate change and crop resilience.