Impact of drying and cooling rate on the survival of the desiccation-sensitive wheat pollen

Daniela Impe,Daniel Ballesteros,Manuela Nagel

doi:10.1007/s00299-021-02819-w

Abstract

Key messageFast-drying and cooling induce fast intracellular water loss and reduced ice-crystal formation, which may promote the formation of intracellular glasses that might improve the likelihood of wheat pollen survival.Long-term storage of pollen is important for the fertilization of spatially or temporally isolated female parents, especially in hybrid breeding. Wheat pollen is dehydration-sensitive and rapidly loses viability after shedding. To preserve wheat pollen, we hypothesized that fast-drying and cooling rates would increase the rate of intracellular water content (WC) removal, decrease intracellular ice-crystal formation, and increase viability after exposure to ultra-low temperatures. Therefore, we compared slow air-drying with fast-drying (dry air flow) and found significant correlations between pollen WC and viability (r = 0.92, P < 0.001); significant differences in WCs after specific drying times; and comparable viabilities after drying to specific WCs. Fast-drying to WCs at which ice melting events were not detected (ΔH = 0 J mg−1 DW, < 0.28 mg H2O mg−1 DW) reduced pollen viability to 1.2 ± 1.0%, but when drying to 0.39 mg H2O mg−1 DW, some viable pollen was detected (39.4 ± 17.9%). Fast cooling (150 °C min−1) of fast-dried pollen to 0.91 ± 0.11 mg H2O mg−1 DW induced less and a delay of ice-crystal formation during cryomicroscopic-video-recordings compared to slow cooling (1 °C min−1), but viability was low (4.5–6.1%) and comparable between cooling rates. Our data support that the combination of fast-drying and cooling rates may enable the survival of wheat pollen likely due to (1) a reduction of the time pollen would be exposed to drying-related deleterious biochemical changes and (2) an inhibition of intracellular ice-crystal formation, but additional research is needed to obtain higher pollen survival after cooling.

Highlights

Preservation of pollen from flowering plants is an important way to complement plant biodiversity conservation efforts and to widen genetic diversity in breeding programs
The viability of wheat pollen is affected by the drying rate and water content Immediately after shedding, pollen of the wheat lines ‘Ferrum’ and TRI 9102 had a steady high WC of 1.72 ± 0.01 mg H2O mg−1 dry weight (DW) (63.3% of FW) and 1.57 ± 0.10 mg H2O mg−1 DW (61.1% of FW), respectively
We demonstrated that depending on the drying rate, pollen reduced the fraction of freezable water in less than 30 min, but viability was severely impaired, when pollen dried to the freezable WC at < 0.28 mg H 2O mg−1 DW, ~ 13%

Summary

Introduction

Preservation of pollen from flowering plants is an important way to complement plant biodiversity conservation efforts and to widen genetic diversity in breeding programs. Wheat (Triticum aestivum), the second most produced crop (http://www.fao.org/faostat/), develops desiccation-sensitive pollen which has completed second pollen mitosis and is tricellular before shedding. This pollen type is ready to germinate upon landing on the stigma (Brewbaker 1967; Franchi et al 2002) and loses its viability when exposed to ambient laboratory conditions for 60 min or to field conditions for 30 min (DSouza 1970). Studies on long-term storage of viable wheat pollen would be of high interest to support preservation, accelerate breeding programs, and to promote new breeding options

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