Low temperature in early spring impairs wheat growth and grain yield. However, little is known about the cytological and molecular mechanisms underlying low temperature regulation of wheat spike development. Microstructure observation and transcriptome sequencing of wheat spikes under low temperature were conducted. Low temperature slowed spike development, reduced the yield-component parameters of wheat spikes at the harvest stage, delayed the formation of lateral spikelets and tissue development, and induced the early differentiation of terminal spikelets. Low temperature increased the content of abscisic acid and caused the upregulation of genes in the abscisic acid signaling pathway, including those encoding PP2Cs, SnRK2s, and bZIP transcription factors. Low temperature also induced the upregulation of 33 cold-responsive genes involved in wheat response to low-temperature stress and regulation of abscisic acid biosynthesis and metabolism of other substances. The wheat spike adapted to cold conditions by changing the expression levels of genes involved in spike morphogenesis, including the transcription-factor genes MADS6, ERF4, ERF78, WOX6, and NAC48. These findings suggest that low temperature in early spring delays wheat spike development by increasing abscisic acid content or affecting the expression of genes involved in morphogenesis.

This study provided visual evidence of a nitrogen effect on starch granules (SGs) in wheat endosperm. Winter wheat (Titicum aestivum L.) cultivar Xumai 30 was cultured under no nitrogen (control) and 240 kg ha^-1 of nitrogen applied at the booting stage. The number, morphology, and size of A- and B-type SGs in subaleurone of dorsal endosperm (SDE), center of dorsal endosperm (CDE), modified aleurone (MA), subaleurone of ventral endosperm (SVE), and center of ventral endosperm (CVE) were observed under light and electron microscopes. (1) The distribution of SGs in SDE was similar to that in SVE, the distributions of SGs in CDE and CVE were similar, but the distribution of SGs in MA was different from those in the other four endosperm regions. The number of SGs in the five endosperm regions was in the order SDE > CDE > SVE > CVE > MA. (2) Nitrogen increased the number of A- and B-type SGs in SDE and SVE. Nitrogen also increased the number of B-type SGs but decreased the number of A-type SGs in CDE and CVE. Nitrogen decreased the numbers of A-type and B-type SGs in MA. The results suggest that increased N fertilizer application mainly increased the numbers of small SGs and decreased the numbers of large SGs, but that the results varied in different regions of the wheat endosperm.