Heat and Drought Co-Stress During Microgametogenesis Altered Pollen Development, Fertility and Grain Production in Two-Rowed Winter Barley

The effects of concurrent heat and drought on phenology, antioxidant enzyme activity, anther and pistil development, anatomy, pollen function and grain production were studied to characterize stress responses in two barley varieties with contrasting tolerance. Plants were grown under optimal environmental conditions until the mid-uninucleate stage of microspore development, then subjected to water withholding and temperatures 10 °C above the optimum during microgametogenesis. HD co-stress shortened the duration of pollen development and tapetum degradation by 3 days. In Lambada leaves, increased GST activity, together with maintained CAT, APX, and GR activities and flavonoid content, was associated with higher pollen viability, fertility, and grain production under HD co-stress, suggesting that enhanced antioxidant capacity may contribute to improved stress tolerance. Compared with Spinner, HD-stressed Lambada pollen maintained higher starch accumulation and better-preserved ultrastructure, whereas Spinner pollen contained smaller starch granules and exhibited swollen mitochondria with ruptured cristae, curved endoplasmic reticulum with dilated cisternae, and numerous autophagosomes and autophagic vacuoles throughout the cytoplasm. Spatiotemporal variation in pollen viability, fertility, and grain production was associated with both the developmental stage of microspores and pollen cells and the position of florets along spikes under HD co-stress.