Reproductive Cold Stress in Contrasting Sorghum Genotypes: Is Pollen Fertility Really the Crucial Trait?

Abstract

The influence of cold stress during the reproductive phase can lead to substantial yield losses in sorghum. In order to extend cultivation into temperate regions, a better understanding of reproductive cold tolerance is essential for breeding progress. To further elucidate the mechanisms responsible for cold tolerance, a cold-tolerant and a cold-sensitive parental line, along with their reciprocal F1 hybrids, were subjected to cold stress at various stages of reproductive development, with a focus on pollen fertility and receptivity of female floral organs. For this purpose, pollen measurements were conducted using impedance flow cytometry, and the panicle harvest index was determined post-maturation. While existing literature primarily attributes reduced pollen fertility as the cause of decreased seed set, this study provides evidence that female floral organs might be more affected than previously assumed. We found that the onset of generative tissue formation until BBCH39 (flag leaf visible) is the most cold-sensitive developmental stage and that there is no predominance of maternal or paternal effects associated with the inheritance of cold tolerance in reciprocal F1 hybrids. These findings offer valuable insights for the development of cold-tolerant sorghum varieties to enable cultivation in colder regions and enhance yield stability in temperate climates. Further studies should aim at validating and expanding these findings from the limited number of representative genotypes analyzed in the present manuscript to global sorghum diversity.

Keywords: Sorghum bicolor; climate adaptation; cold sensitivity; pollen fertility; reproductive cold tolerance; sorghum hybrid breeding; spikelet fertility.

FIGURE 1

Cold‐tolerant parental line SB14011 and cold‐sensitive parental line SC1056 and their reciprocal F1 hybrids (SB × SC and SC × SB) (grown under optimal conditions [see Table 1]).

FIGURE 2

(a) BBCH scale for sorghum in reference to the cereal scale developed by Zadoks et al. (1974). (b) Overview of the developmental stages to be examined—BBCH32: flag leaf not yet visible, seat low down; BBCH35: flag leaf not yet visible, shortly before the appearance of the leaf tip; BBCH39: ligule of the flag leaf visible; BBCH51: emergence of the panicle.

FIGURE 3

Left: Pollen fertility experiment—individual sorghum plant bagged prior to anthesis to ensure self‐pollination; right: Pollen receptivity experiment—stressed sorghum plant (“female plant”) bagged together with an unstressed pollinator plant (“male plant”) of the same genotype.

FIGURE 4

Box plots depicting the panicle harvest index (PHI) and seed yield of a cold‐tolerant (SB14011) and a cold‐sensitive (SC1056) sorghum inbred line subjected to cold stress at developmental stages BBCH32, BBCH35, BBCH39, and BBCH51 and under optimal conditions (control); p‐values in the Supporting Information (Table S2).

FIGURE 5

Panicles of the cold‐sensitive genotype (SC1056) and the cold tolerant genotype (SB14011) after cold stress from BBCH32, BBCH35 BBCH39, and BBCH51 and under control conditions.

FIGURE 6

Boxplots depicting the panicle harvest index (PHI) and seed yield of the reciprocal hybrids SC1056 × SB14011 and SB14011 × SC1056 subjected to cold stress at different developmental stages (BBCH32, BBCH35, BBCH39, and BBCH51) and under optimal conditions (control); p‐values in the Supporting Information (Table S2).

FIGURE 7

Boxplots depicting the proportion of fertile pollen, cell concentration, and total number of fertile pollen of a cold‐tolerant (SB14011), a cold‐sensitive (SC1056) sorghum inbred line and their reciprocal F1 hybrids (SB × SC and SC × SB) subjected to the different treatments; p‐values in the Supporting Information (Table S2).

FIGURE 8

Representation of the genotype × treatment interaction for the examined features panicle harvest index (PHI) and seed yield (g); Treatment means cold stress after BBCH32, BBCH35, BBCH39, and BBCH51 and under controlled conditions. Genotypes means the cold tolerant genotype SB14011 and the cold‐sensitive genotype SC1056 as well as their reciprocal hybrids SB × SC and SC × SB.

FIGURE 9

Representation of the genotype × treatment interaction for the examined features proportion of fertile pollen (%), cell concentration (cells/mL), and total number of fertile pollen (cells/mL); treatment means cold stress after BBCH32, BBCH35, BBCH39, and BBCH51 and under controlled conditions. Genotypes means the cold tolerant genotype SB14011 and the cold‐sensitive genotype SC1056 as well as their reciprocal hybrids SB × SC and SC × SB.

FIGURE 10

Heatmap illustrating Pearson's correlations among the considered traits: panicle weight (g), seed yield (g), panicle harvest index (PHI), cell count, fertile pollen (%), and cell concentration (cells/mL); strong positive correlations (1) are depicted in red, while strong negative correlations (−1) are represented in blue (significance level: ***0.001; **0.01; *0.05).

FIGURE 11

Boxplots showing the seed yield of the cold‐tolerant parental line SB14011 and the cold‐sensitive parental line SC1056 and their reciprocal F1 hybrids in the different treatment stages (cold stress after BBCH32, BBCH35, BBCH39, and BBCH51); p‐values in the Supporting Information (Table S1).

FIGURE 12

Boxplots showing the panicle harvest index (PHI) after cross‐pollination and self‐pollination of a cold‐tolerant (SB14011) and a cold‐sensitive inbred line (SC1056) and their reciprocal hybrids (SB14011 × SC1056 and SC1056 × SB14011) exposed to cold stress at different developmental stages; The significance levels refer to the differences between the pollination forms of a genotype within a treatment stage (significance level: ***0.001; **0.01; *0.05).