Pollen production pattern in the capitulum of the cultivated sunflower (Helianthus annuus L.)

Abstract

The intra-plant pollen production in cultivated sunflower (Helianthus annuus L.) was quali- and quantitatively studied. Two modern self-pollinated, high oleic sunflower hybrids were grown during two consecutive seasons: 2009 in three planting dates, and 2010 in two planting dates. Two cross-pollinated varieties of sunflower, Hopi and Havasupai, were also studied to determine if an anomalous intracapitulum pollen gradient observed in preliminary studies could be a remnant character inherited from primitive genotypes. Pollen grains per flower (PGF), anther volume (AV), pollen grain volume (PGV) and pollen viability (PV) were measured in three capitulum sectors, external (ES), middle (MS) and internal (IS). At harvest, fully developed fruits (FDF) and incompletely developed fruits (IDF) per capitulum sector were counted. Based on seasonal averages of hybrids and planting dates, the capitulum IS showed the highest PGF number (40864 ± 6519) followed by the MS (35243 ± 5242) and the ES (28273 ± 3102). Anther length (AL) and width (AW) were higher in the IS (5.3 and 0.8 mm, respectively) compared with the ES (6.4 and 0.6 mm, respectively); ES and the MS had intermediate values (5.2 and 0.7 mm, and 4.9 and 0.6 mm, respectively). No significant differences (p=0.18) were observed in PGF per sector in capitula of Havasupai (average= 42679 ± 4107). On the other hand, Hopi flowers of the MS and IS produced a larger number of pollen grains (average = 53412 ± 3553) than those of the ES (37050 ± 5947). PGV showed no significant differences in both hybrids, capitulum sectors and growing seasons (average = 22168 ± 1042 μm3). A positive relationship between AL and AW and the total number of pollen grains was also observed. Pollen viability in both growing seasons was greater than 97% in all cases. At harvest, in all cases, the higher number of FDF was observed in the ES and MS (598 and 456 respectively) and the number of IDF was higher in the IS (132). Given that intra-plant pollen production of both, pollen quantity and viability were not limiting, the origin of IDF in different regions of the capitulum could not be explained by the lack of pollen.The finding of an a typical positive centripetal gradient of pollen grains in the capitulum as compared with other racemose inflorescences is interpreted as an adaptive response to the time sequence of anthesis in this species to ensure a homogeneous pollination in the capitulum.