TRANSPORT PROCESSES AND WATER RELATIONS

S ummary A cost‐benefit analysis of transport and of water relations in plants of a number of life forms is presented. For planktophytes, it appears that an increase in size above ∼ 50 μm diameter would restrict growth rate in oligotrophic environments owing to a greater increment of restriction by boundary layer of nutrient uptake than of maximum specific growth rate as organism size increases. For macroscopic aquatic haptophytes (mainly algae), even the most favourable combination of velocity of water flow and plant morphology cannot reduce these restrictions enough to make nutrient diffusion to the plant surface less limiting for growth in oligotrophic environments than is the case for planktophytes. Macroscopic aquatic rhizophytes, with their need for intraplant N‐ and P‐ flux from rhizoid or root to shoot, and for reduced C‐flux from shoot to rhizoid or root, involves cytoplasmic streaming in giant‐celled algae, and transport in the phloem and (probably) the xylem (using root pressure) in vascular plants. It is likely that the energetic running costs of nutrient transport (joules used per mol solute transported over a distance of 1 m) is higher for the giant‐celled algae than for the vascular plants. The predominant (in terms of global biomass) terrestrial plants are the rhizophytic, homoiohydric desiccation‐intolerant sporophytes of vascular plants. Such plants incur greater penalties, in terms of reduced specific growth rate under resource‐saturated or resource‐limited conditions, and of reduced resource use efficiency, with increased plant height. This is a result of diversion of resources to producing supporting tissue, and xylem and phloem, in larger amounts per unit biomass than is the case for smaller plants. By virtue of increased size, however, a plant can command a higher incident photon flux density as well as access to a greater depth of soil from which nutrients and water can be extracted. Other major categories of terrestrial rhizophytes are desiccation‐tolerant homoiohydric sporophytes of vascular plants, endohydric but poikilo hydric gametophytes of mosses, and ectohydric gametophytes of archegoniates and thalli of many algae and lichens. In the order in which they are listed, these plants have a smaller potential stature (none are more than 2 m high), and thus can command access to fewer resources than can taller plants with deeper root systems in the same community. However, the smaller plants have the potential for higher specific growth rates under resource‐saturated and resource‐limited conditions, and for higher resource use efficiencies, than do the homoiohydric, desiccation‐intolerant plants of larger stature, since they have a smaller diversion of resources to supporting and long‐distance transport tissues. The extreme of these trends is found in the haptophytic soil algae with no non‐green cells. Quantitation of the differences between life forms requires much more investigation, as do differences within life forms with respect to the handling of water and solute transport. Investigating the possible selective significance of these differences will be an even more challenging task.