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ARTICLE

Leaf angle and light interception in sunflower (Helianthus annuus L.). Role of the petiole’s mechanical and anatomical properties

by Hernández LF

Laboratorio de Morfología Vegetal. Depto. de Agronomía, Universidad Nacional del Sur. Bahía Blanca, 8000, CIC Provincia de Buenos Aires, La Plata, 1900. Argentina.

* Corresponding Author:Address Correspondence to: Luis Hernández, e-mail: Fax: +54 - 0291 - 4595127

Phyton-International Journal of Experimental Botany 2010, 79(all), 109-115. https://doi.org/10.32604/phyton.2010.79.109

Abstract

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 (ia_Petiole and ia_Lamin, 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 ia_Petiole increased acropetally from -9.0° to +60.0°, while the ia_Lamina 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 weight_Petiole/ dry weight_Lamina. 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 the upper canopy. This determines that the higher canopy strata has a preferentially planophyllous/erectophyllous leaf architecture.

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