Introduction

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Evolutionary changes were necessary for plants to inhabit land. All resources reach aquatic plants from the surrounding water whereas terrestrial plants are nourished both from soil and directly from the atmosphere. Roots growing into soil absorb water and nutrients, while leaves, supported by a stem superstructure in the aerial environment, intercept sunlight and CO2 for photosynthesis. This division of labour results in assimilatory organs of land plants being nutritionally inter-dependent; roots depend on a supply of photoassimilates from leaves, while shoots (leaves, stems, flowers and fruits) depend on roots to supply water and mineral nutrients. Long-distance transport is therefore a special property of land plants. In extreme cases like the Australian mountain ash (Eucalyptus regnans), sap must move up to 100 m and overcome gravity to rise to tree tops.

Specialised vascular tissues have evolved to support the rates at which resources assimilated from the environment need to be transported within land plants. Mature xylem vessels conduct water and nutrients upwards by mass flow through a complex of non-living cells, energised during the day by evaporative water loss from leaves (Section 5.1). Fine vascular networks in leaves control distribution of this sap to recipient cells (Section 5.2). Phloem is responsible for redis-tributing nutrients, water and photoassimilates through plants along a living pathway (Section 5.3). The balance between supply and demand is integrated in phloem transport through sensitivity of conducting cells (Section 5.4), assimilate loading processes (Section 5.5) and unloading processes (Section 5.6) to developmental events and environmental constraints. Fine tuning of resource delivery to various organs is achieved through interchange of xylem and phloem contents, a process that has been quantified using lupin as a model plant.

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