Modelling leaf expansion in a fluctuating environment: are changes in specific leaf area a consequence of changes in expansion rate?

Abstract

Leaf expansion rate varies with leaf temperature, photon flux density (PPFD), evaporative demand and soil water status. In most simulation models, it is calculated every day by multiplying the amount of carbohydrate available to leaves by specific leaf area (SLA). However, leaf expansion rate is considerably reduced by mild water deficits which do not affect photosynthesis, and is not affected by a reduction in the PPFD intercepted during rapid leaf expansion. Specific leaf area undergoes a several-fold variability depending on PPFD, soil water status and time of day. It is increased when environmental conditions have a greater depressive effect on expansion rate than on photosynthesis, and is decreased in the opposite case. It is therefore appropriate to model leaf expansion independently of the plant carbon budget. Consistent characteristics can be deduced from a series of experiments, allowing a model of leaf expansion to be proposed. (i) Time courses of relative leaf expansion rate and of epidermal cell division rate are well conserved within a plant and across a large range of environmental conditions, provided that durations and rates are expressed in thermal time. Maximum relative rates are common to all zones of a leaf and to all leaves of a plant, in maize and sunflower. (ii) A water deficit, or a reduction in intercepted PPFD, imposed in the first half of the period of leaf development affects the relative expansion rate in the deficit only, but permanently affects the absolute expansion rate. In contrast, a reduction in PPFD causes no effect on leaf expansion if imposed in the rapid expansion period when the leaf is autotrophic. (iii) Expansion rate is related to evaporative demand and to the concentration of ABA in the xylem sap with relationships that apply under both field and laboratory conditions. (iv) Tissue expansion and epidermal cell division behave as independent processes which determine epidermal cell area at each time.

Leaf area index simulation in soybean grown under near-optimal conditions

“…The approach used in this model is only valid when leaf growth is not source limited, which can only be achieved when agronomic management is optimized. Tardieu et al (1999) used a similar approach to simulate leaf area in sunflower (Helianthus annus L.) and maize (Zea mays L.).…”

“…SLA varies in different environments, developmental stages, and strata of leaves within the canopy (Gunn et al, 1999;Lieth et al, 1986;Lugg and Sinclair, 1979). The mechanism of such flexibility in SLA is still not known and thus the variable often becomes the source of uncertainty in models that rely on SLA in simulating LAI (Tardieu et al, 1999). Lugg and Sinclair (1979) showed that SLA of the uppermost fully expanded soybean leaf varied from 0.333 cm 2 mg −1 at R1 to 0.146 cm 2 mg −1 at R6.…”

“…Carbon accumulation is not directly a mechanism of leaf expansion and thus SLA should not be regarded as model parameter but rather as an output (Tardieu et al, 1999).…”

Specific leaf area in barley: individual leaves versus whole plants

“…SLA P is determined by the sum of the processes which determine the surface expansion and net weight gain of individual leaves. Surface expansion is considered by Tardieu (1999) and net weight gain wil be considered here. Net weight gain by a leaf is a complex of many processes.…”

Modelling kinetics of plant canopy architecture—concepts and applications

“…It may, though, be possible to account for the effect of stresses in early growth affecting later canopy characteristics by using time-delayed functions but introducing stress functions without describing the organs they actually apply to can be but empirical. The limitations in describing the effects of stresses early in crop/organ life may explain some of the difficulty in parameterisation of source-sink relationships through concepts such as specific leaf weight, specific leaf area, or harvest index (Birch et al 1998, Tardieu et al 1999.…”