Some Often Overlooked Plant Characteristics as Determinants of Plant Growth: A Reconsideration

Plant primary production is ultimately dependent on photosynthetic CO2 uptake. However, photosynthate production also depends on the amount of photosynthetically active tissue constructed by the plant and on many aspects of carbon use (expenses and investments). Over almost a century, physiological ecology of gas exchange has dominated causal explanations of plant growth. As gas exchange studies became easier to do, and thus more common over the last decade, it also became clear that their contribution towards a deeper understanding of plant growth has been greatly overestimated. When I was asked to outline some of the future needs for a better understanding of plant growth, my feeling was that other aspects of plant growth control, aside from gas exchange, have already been highlighted sufficiently by several authors. However, most of the concepts discussed still have penetrated neither textbooks nor the entire spectrum of related research. Thus, the only justification for reconsidering this field of plant biology (at the risk of being repetitive) may be the persistent and widespread ignorance of the multitude of plant attributes, other than leaf photosynthesis, that affect plant growth. Recent conceptual advances resulting from detailed investigations of plant growth underline the great significance of dry matter partitioning, respiration, life-spans and 'costs' of leaves, developmental processes, herbivore-defence and canopy structure as growth determinants. These characteristics often completely mask variation in predictions of growth, based on leaf photosynthetic capacity. A particularly nice example is the study on photoperiodism in grasses by Heide and co-workers (e.g. Heide, Bush & Evans, 1985). They showed that dosis-neutral manipulations of daylength can increase relative growth rate by 40%, despite a decline in unit-leaf-area-based rate of photosynthesis. The growth of lettuce (Lorenz & Wiebe, 1980) and tomato (Smeets & Garretsen, 1986) are two other examples where the unsuitability of photosynthesis as a criterion for growth control has been demonstrated. Mooney & Chiariello (1984) showed that 5 weeks of synchronous growth is sufficient for wild radish to reach twice the total mass of cultivated radish, just because a greater fraction of photosynthate is allocated to new leaf tissue rather than storage tissue. Poorter (1989) has examined and reviewed the reasons for the widespread discrepancy between relative growth rate and net assimilation rate, and concludes that leaf weight ratio and specific leaf area are of paramount significance. Both of these plant characteristics are often controlled by cost/benefit requirements of the plant growing under a particular set of environmental conditions (e.g. water or nitrogen supply, or herbivory). However, there are situations, where none of these mechanistic explanations apply, because developmental constraints as, for example, the number and size of primordia or flowering induction, are determining growth (Korner & Pelaez Menendez-Riedl, 1989). Hence, there is no a priori reason to assume that any of the aforementioned growth determinants is 'the' key parameter that controls the translation of leaf photosynthesis into increments of total plant dry matter. It should suffice here to refer to some particularly instructive texts, published within the last few years, for further reading: Mooney & Chiariello (1984); Bloom, Chapin & Mooney (1985); Bazzaz et al. (1987); Chapin et al. (1987); Lambers (1987); Pearcy et a]. (1987); Schulze & Chapin (1987); Lambers et al. (1989); Williams, Field & Mooney (1989). If we are to understand growth, we are left with the problem that a multitude of variables must be studied simultaneously. When conducting a comprehensive growth analysis there is no way to avoid this complexity. Equal growth may be achieved with various combinations of leaf photosynthetic capacity, leaf area per total plant mass,