Temperature dependence of pollen germination and tube growth in conifers relates to their distribution along an elevational gradient in Washington State, USA.

Abstract

Background and aims: Pollen germination and tube growth are essential processes for successful fertilization. They are among the most temperature-vulnerable stages and subsequently affect seed production and determine population persistence and species distribution under climate change. Our study aims to investigate intra- and interspecific variations in the temperature dependence of pollen germination and tube length growth and to explore how these variations differ for pollen from elevational gradients.

Methods: We focused on three conifer species, Pinus contorta, Picea engelmannii and Pinus ponderosa, with pollen collected from 350 to 2200 m elevation in Washington State, USA. We conducted pollen viability tests at temperatures from 5 to 40 °C in 5 °C intervals. After testing for 4 d, we took images of these samples under a microscope to monitor pollen germination percentage (GP) and tube length (TL). We applied the gamma function to describe the temperature dependence of GP and TL and estimated key parameters, including the optimal temperature for GP (Topt_GP) and TL (Topt_TL).

Key results: Results showed that pollen from three species and different elevations within a species have different GP, TL, Topt_GP, and Topt_TL. The population with a higher Topt_GP would also have a higher Topt_TL, while Topt_TL was generally higher than Topt_GP, i.e. a positive but not one-to-one relationship. However, only Pinus contorta showed that populations from higher elevations have lower Topt_GP and Topt_TL and vice versa. The variability in GP increased at extreme temperatures, whereas the variability in TL was greatest near Topt_TL.

Conclusions: Our study demonstrates the temperature dependences of three conifers across a wide range of temperatures. Pollen germination and tube growth are highly sensitive to temperature conditions and vary among species and elevations, affecting their reproduction success during warming. Our findings can provide valuable insights to advance our understanding of how conifer pollen responds to rising temperatures.