Fast Drying Combined With Rapid Cooling Can Enhance The Survival of The Desiccation Sensitive Wheat Pollen After Ultra-Low Temperature Storage
Abstract Long-term storage of pollen is important for the fertilization of spatially or temporally isolated female parents, especially during hybrid breeding. Wheat pollen is dehydration-sensitive and rapidly loses viability after shedding. To preserve wheat pollen, we hypothesized that fast-(flash)-drying and fast cooling (150°C min-1) compared to slow-(air)-drying and slow cooling (1°C min-1) would increase the rate of intracellular water content (WC) removal, decrease intracellular ice crystal formation, and increase viability after exposure to ultra-low temperatures. High correlations were found between pollen WC and viability analyzed by impedance flow cytometry (IFC viability: r=0.92, P<0.001) and pollen germination (r=0.94, P<0.001). After 10 min of air-drying, 66% WC was lost and pollen germination was at 12.2±12.3%. After 10 min of flash-drying, WC of pollen reduced by 74%. IFC viability decreased from 90.2±6.7 to 39.4±17.9%, and pollen germination dropped from 33.7±16.9 to 1.9±3.9%. After 12 min of flash-drying, WCs decreased to <0.34 mg H2O mg-1 DW, ice crystal formation was completely prevented (ΔH=0 J mg-1 DW), and pollen germination reached 1.2±1.0%. After slow and fast cooling, flash-dried pollen (WC 0.91±0.11 mg H2O mg-1 DW) showed less ice crystal formation during cryomicroscopic-video-recordings and had IFC viability of 4.5±7.0% (slow) and 6.1±8.8% (fast), respectively, compared to air-dried pollen which lost all viability. Generally, fast-(flash)-drying and increased cooling rates may enable the survival of wheat pollen likely due to (1) a fast rate of intracellular WC loss that reduces deleterious biochemical changes associated with the drying process and (2) a delay and reduction in intracellular ice crystal formation.
