Patterns of growth in relation to assimilation and translocation

Bryophytes

Growth and net assimilation Positive net photosynthesis tends to increase dry weight due to accumulation of assimilate. In this chapter we are concerned with the translation of this process into growth, in the sense of increase in plant size. Adaptations that permit positive net assimilation at substantial rates under severe environmental conditions are commonly viewed as the key to plant success in polar regions (e.g. Mooney, 1976), and considerable effort has been directed towards investigating environmental relationships of CO2 exchange in mosses and lichens. As discussed in Chapter 5, the results confirm that polar species in situ are able to photosynthesise at reasonable rates, but the assumption that assimilation would be maximised by a close correspondence between optimum conditions for net photosynthesis and the most frequently prevailing environmental conditions has not been fully substantiated (page 160; Lechowicz, 1981b). However, the parallel assumption, that survival is favoured by maximum rates of photosynthesis and growth, is not necessarily valid for plants with essentially opportunistic growth responses in environments where competition is not everywhere intense, and low stature may be advantageous.

Moreover, conditions promoting maximum NAR are often very different from those favouring growth in size. This is particularly true in polar species in which maximum NAR may occur at temperatures low enough to cause a severe depression of respiration, and probably other processes essential to growth. The latter may then be restricted more by direct limitation than by availability of assimilate, and adaptation further increasing net assimilation would be superfluous.