请润色这句话Increased heatwaves have already exceeded many crop tolerance thresholds with global warming and greatly threatened food security It becomes increasingly urgent to identify and develop heat tole

As global warming continues to cause increased heatwaves, many crops are facing threats to their tolerance thresholds, greatly endangering food security. It is now of utmost importance to identify and develop heat-tolerant germplasm in major food crops. To this end, we conducted field trials in north and southwest China in 2021 and 2022, using 306 elite maize inbred lines from both China and the United States and six different sowing dates. The newly released inbred lines exhibited higher seed set, kernel/ear, and yield compared to early released lines, primarily due to a shorter anthesis-silking interval, longer anthesis-silking overlap duration, and more emerged silk number. However, we found that the newly released germplasm is more susceptible to high temperature (HT) stress than early released germplasm, attributed to limitations in tassel morphology and improvements in ear development made to compensate for this. We identified a key threshold for spikelet/tassel (~700) over which tassel can produce enough viable pollen grains for emerged silk/ear of most modern maize hybrids, even under HT stress. Additionally, we noted that spikelet opening angle, a novel phenotype in coping with HT stress, decreased with breeding eras, further reflecting the higher sensitivity of newly released maize germplasm to HT stress. Our work provides crucial information for the breeding and selection of HT tolerant maize hybrids, revealing the responses of breeding-era maize germplasm to growing environments and quantifying the crucial traits of tassel.