Leaf inclination and light interception in the sunflower (Helianthus annuus L.). Importance of the petiole's mechanical and structural properties

The relationships between leaf biomass and morphology (lamina area and petiole and lamina inclination), petiole’s mechanical and structural properties and the vertical light gradient inside the crop’s canopy were studied in field grown sunflower ( Helianthus annuus L.) plants, maintained at an optimum soil water and mineral status. The objective of this work was to study the role of petiole’s mechanical properties on foliar inclination, and the relationships between leaf and petiole angle variation patterns along the canopy and leaf and petiole biomass partitioning and morphology. 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 that passes through the canopy reaching each individual leaf was then calculated. Individual petiole and lamina inclination angles ( ia Petiole and ia Lamina respectively) were measured from sequential digital images taken from rotated plants and a stationary camera. Petiole length and lamina area were measured after detaching the leaves from each plant. Leaves were separated in petiole, lamina and main veins, and their dry biomass obtained. Petiole transverse cuts stained with acid fluoroglucinol were used to measure the relative area occupied by lignified and fibrous tissue. The petiole’s structural Young's modulus ( EPetiole ) for different leaves was calculated from a threepoint bending test performed in petiole segments about 4.0 to 8.0 cm long. 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 ia Petiole increased acropetally from -9.0 o to +60.0 o while the ia Lamina increased basipetally from +1.0 o to -60.0 o in concordance with increments in the intercepted PAR. Petiole specific weight (g/cm 2 ) did not change with leaf position whilst lamina specific weight decreased acropetally. Main veins dry weight increased basipetally. EPetiole and EI Petiole increased acropetally. The relationship between intercepted PAR and the ratio biomass dry weight Petiole/biomass dry weight Lamina was positively correlated. 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 the interception of incident PAR, the sunflower plant invests more energy in differentiating supporting tissues in the petioles of the upper canopy resulting the higher canopy strata in a preferentially planophyllous/erectophyllous leaf architecture.