Is the abaxial palisade parenchyma in phyllaries of the sunflower (Helianthus annuus L.) capitulum a missing trait in modern genotypes

Abstract

The involucral bracts (IB or phyllaries) of the sunflower(Helianthus annuus L.) capitulum constitute an example of photosynthesizingorgans that contribute to the photosynthesis budget duringthe generation of crop yield. The anatomy of IB was analyzed in twodomesticated primitive sunflower genotypes, Havasupai and Hopi, inthe sunflower line HA89B, in the wild Helianthus annuus ssp. annuus,in the sunflower male-fertile line R013 and in the commercial hybridDKOP3845. Stomata and trichomes were counted on the adaxial andabaxial epidermis. In all cases, the IB showed a one-layered adaxialand abaxial epidermis, secretory ducts and parenchymatic cells withabundant chloroplast. The vascular system was similar to that of thenomophylls; however, their bundles were smaller, with an abaxial surfaceshowing abundance of glandular and non-glandular trichomesand stomata. IB of Havasupai, Hopi and the male-fertile line strainshowed higher number of adaxial hypodermic strata than those ofHA89B, DKOP3845 and wild sunflower (2-3 vs. 1), and one mesophyllwith inverted polarity with respect to a foliage leaf: the presenceof a spongy parenchyma on the adaxial side was observed with arudimentary palisade parenchyma on the abaxial side. Stomatal densityof the IB was significantly higher in Hopi and Havasupai than inHA89B and DKOP3845, with values ranging from 132 to 156 vs 73to 110 stomata/mm2, respectively. Like the modern commercial hybrid,the IB of male-fertile line showed lower stomatal density (83stomata/mm2) and scarce abaxial trichomes. The anatomical studies ofphyllaries in Asteraceae remain relatively poor in the literature. Fromthe functional point of view, sunflower breeding produced undesiredchanges in the IB anatomy. Future studies for comparing the physiological(photosynthesis and respiration) and biochemical activities ofthe RuBisCO among the IB of the primitive (Havasupai and Hopi),male-fertile lines and modern sunflower genotypes will help to definethe magnitude of their importance as a morphological trait to be consideredin future plans of sunflower breeding.