Among-species differences in pollen quality and quantity limitation: implications for endemics in biodiverse hotspots

Abstract

Background and aims: Insufficient pollination is a function of quantity and quality of pollen receipt, and the relative contribution of each to pollen limitation may vary with intrinsic plant traits and extrinsic ecological properties. Community-level studies are essential to evaluate variation across species in quality limitation under common ecological conditions. This study examined whether endemic species are more limited by pollen quantity or quality than non-endemic co-flowering species in three endemic-rich plant communities located in biodiversity hotspots of different continents (Andalusia, California and Yucatan).

Methods: Natural variations in pollen receipt and pollen tube formation were analysed for 20 insect-pollinated plants. Endemic and non-endemic species that co-flowered were paired in order to estimate and compare the quantity and quality components of pre-zygotic pollination success, obtained through piecewise regression analysis of the relationship between pollen grains and pollen tubes of naturally pollinated wilted flowers.

Key results: Pollen tubes did not frequently exceed the number of ovules per flower. Only the combination of abundant and good quality pollen and a low number of ovules per flower conferred relief from pre-zygotic pollen limitation in the three stochastic pollination environments studied. Quality of pollen receipt was found to be as variable as quantity among study species. The relative pollination success of endemic and non-endemic species, and its quantity and quality components, was community dependent.

Conclusions: Assessing both quality and quantity of pollen receipt is key to determining the ovule fertilization potential of both endemic and widespread plants in biodiverse hotspot regions. Large natural variation among flowers of the same species in the two components and pollen tube formation deserves further analysis in order to estimate the environmental, phenotypic and intraindividual sources of variation that may affect how plants evolve to overcome this limitation in different communities worldwide.

Keywords: Co-flowering community; Mediterranean; Yucatan; piecewise regression; pollen limitation; pollen tubes; pollination; stigmatic pollen load.

Fig. 1.

Piecewise regression analyses of paired endemic (filled circles, solid lines) and non-endemic (open circles, dashed lines) species representative of the whole range of variation recorded. (A) The endemic Fumana paradoxa and the non-endemic Helianthemum cinereum (Cistaceae) in Andalusia (pair 2). (B) The endemics Thymus orospedanus and Sideritis incana, and the non-endemic Teucrium polium (Lamiaceae) in Andalusia (pair 4). (C) The endemic Triteleia peduncularis and the non-endemic Zigadenus venenosus (Liliaceae) in California (pair 5). (D) The endemic Cienfuegosia yucatanensis and the non-endemic Sida acuta (Malvaceae) in Yucatan (pair 8). Dotted lines indicate when the number of pollen tubes equals the average number of ovules per flower. Taxa codes are as in Table 1. Note that scales were adjusted to data ranges.

Fig. 2.

Initial linear relationship between number of pollen tubes per ovule and pollen grains in endemic (solid lines) and non-endemic (dashed lines) species studied in our three highly diverse communities, as indicated in the key.

Fig. 3.

Differences among species in the three study communities in the percentage of flowers that did not have any pollen grains, those that received pollen but developed less than one pollen tube per ovule (tube shortage) and those able to develop the same or more pollen tubes than ovules (tube surplus). Species were ordered by pairs from left to right as in Table 1.