ISHS Acta Horticulturae 705: V International Walnut Symposium
LEAFLET HETEROGENEITY IN JUGLANS REGIA: AN UN-ADVERTED BIAS IN ASSIMILATION MODELS |
| Authors: | G. Bongi, P. Paris |
| Keywords: | Juglans regia cv. Feltrina, leaf porosity, 13C discrimination, stomata density |
| Abstract:
A number of models uses extensive properties of walnut crown, like leaf area density, as input, and detailed information of leaf position, for producing a model of tree photosynthesis and growth; leaf biochemistry is determined as an intensive property, specific for the population (Balandier et al., 2000). In walnut the terminal or distal leaflet is usually the more extended, followed by 3, 4 or 5 laterals (Fig. 1). The proximal ones are smaller but with an almost double stomatal density in our case. It results that leaflet area, an extensive property, is associated with stomatal density, an intensive property. In C3 leaf biochemistry current models stomatal density is assumed to be constant to some degree within a canopy and leaf porosity to be CO2-regulated by a feedback on stomatal opening by sub-stomatal CO2 partial pressure, but this assumption is not valid in this experience. Leaf stable carbon isotope discrimination  of cellulose is a compound effect of carboxylation preference for 12CO2 during photosynthesis and diffusion limitations for 13CO2 across walls, solvation, boundary layer and stomata; in a population of leaves taken from the same environment is an integral, reverse index of water use, the quotient between photosynthesis and transpiration (Farquhar et al., 1989). The ratio of CO2 partial pressure in the stroma, relative to ambient, regulates directly and inversely water quotient.
In our samples we found smaller in the distal leaflets, and the difference with the basal leaflet exceeded the limits of confidence of single leaf models.
Leaflet shape was already analyzed in 21 walnut populations (Malvolti et al., 1994) where leaf morphological variations were found correlated with genetic distances but with a great variability of the same genotype among different places; phenotypic plasticity was hypothesized.
An analysis of stomatal density within a leaf confirms this, and casts doubts about the functional value of aggregate indexes like leaf area density in walnut photosynthetic modelling.
At any given condition the leaves with smaller CO2 ratios, contribute less photosynthesis and transpiration than expected from its nitrogen content.
| |
Download Adobe Acrobat Reader (free software to read PDF files) | |
