| Resumo: |
The relationships between (1) leaf biomass and morphology (lamina area and petiole and lamina inclination), (2) petiole's mechanical and structural properties, and (3) the vertical light gradient inside the crop's canopy were studied in field grown sunflower (Helianthus annuus L.) plants, maintained at optimum soil water and mineral levels. At flowering, incident photosynthetic active radiation (PAR) was measured at the top of the canopy and on individual leaves using a quantum sensor. The fraction of direct incident radiation which passed through the canopy reaching each individual leaf was then calculated. Individual petiole and lamina inclination angles (iaPetiole and iaLamina, respectively) were measured from sequential digital images. These were taken rotating plants and using a stationary digital photographic camera. Petiole length and lamina area were measured after detaching leaves from each plant. Leaves were separated in petiole, lamina and main veins, and their dry mass obtained. Petiole transverse cuts stained with acid fluoroglucinol were used to measure the relative area occupied by lignified and fibrous tissues. The petiole's structural Young's modulus (E Petiole) for different leaves was calculated from a three-point bending test. This was performed in petiole segments 4.0 to 8.0 cm length. Petiole flexural stiffness (EI Petiole) was calculated using elementary beam theory for homogenous materials. Intercepted PAR in the canopy for individual leaves decreased basipetally. The iaPetiole increased acropetally from -9.0° to +60.0°, while the iaLamina increased basipetally from +1.0° to -60.0° in concordance with increments in the intercepted PAR. Petiole specific weight per tranverse sections (g/cm²) did not change with leaf position whilst lamina specific weight decreased acropetally. Main veins dry weight increased basipetally. E Petiole and EI Petiole increased acropetally. There was a positive relationship between intercepted PAR and the ratio dry weightPetiole/ dry weightLamina. The relative area occupied by supporting tissue was significantly higher in upper petioles than in lower ones. These results suggest that, in order to optimize incident PAR interception, sunflower plants invest more energy in differentiating supporting tissues in the petioles of the upper canopy. This determines that the higher canopy strata has a preferentially planophyllous/erectophyllous leaf architecture.
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