Pollen Viability, Germination and Tube Elongation Responses to Heat Stress During the Reproductive Stage in Maize

ABSTRACT

Climate change‐induced heat stress increasingly threatens crop production, particularly when high temperatures occur during the sensitive flowering stage. In maize, successful fertilization depends on viable pollen, successful pollen germination and subsequent pollen tube elongation reaching the ovule. However, the extent to which these processes are influenced by heat stress remains unclear. This study investigated the effects of heat stress on pollen viability, pollen germination and pollen tube growth in the maize hybrid ZD 958 during the 2024 and 2025 growing seasons. At anthesis, tassels were collected and exposed in vitro to 3‐h daily heat treatments at 28°C, 31°C, 34°C, 37°C, 40°C and 43°C for three consecutive days. Pollen viability (TTC staining), germination rate and pollen tube elongation dynamics were assessed. While viability and germination declined only moderately up to 37°C, pollen tube elongation was severely inhibited, with final tube length decreasing to approximately 53% of the control (28°C) in both years. The estimated temperature for 50% inhibition ( T ₅₀ ) was significantly lower for pollen tube elongation (35.9°C) than for pollen viability (36.8°C) or pollen germination (37.5°C). Growth rate analysis revealed that pollen tube elongation declined by 5.75% per °C, compared to 5.13% for pollen viability and 4.39% for pollen germination. Although inter‐annual variation was observed, the hierarchical sensitivity of the three pollen traits remained consistent across both years. Pollen tube growth was the most heat‐sensitive trait. Our results suggest that pollen tube growth may represents a critical bottleneck in the fertilization process of this genotype under heat stress. Even a 1°C increase above 35°C severely compromise the ability of the pollen tube to reach the ovule, indicating that pollen tube thermotolerance is a promising target for breeding heat‐resilient maize varieties.