Ovary abortion in field-grown maize under water-deficit conditions is determined by photo-assimilation supply

Ovary development is important for kernel setting, hence a crucial factor determining maize (Zea mays L.) yield. The supply of photo-assimilation to ovaries plays a significant role in the overall grain development. However, whether ovary abortion induced by soil water deficit (SWD) during ear differentiation is correlated with photo-assimilation supply is not clear. This study investigated if occurrence of SWD before silking suppresses ovary development and photo-assimilation supply. In 2018 and 2019, water deficit effects on controlled field-grown maize plants were surveyed from jointing to silking (ear differentiation stage) stages on ovary development, anthesis and silking interval (ASI), leaf and canopy photo-assimilation including SPAD, photosynthetic index, net photosynthetic rate, leaf area index (LAI), net assimilation rate (NAR), and yield and its components. Four SWD treatments were used: control, mild, moderate, and severe. SWD suppressed plant growth and photo-assimilation supply and increased ASI. Water-deficit significantly decreased the canopy photosynthetic characteristics, while severe SWD additionally decreased LAI and NAR at silking stage by 20 % and 58 % in 2018, and by 17 % and 65 % in 2019. Under water-deficit conditions, the temporal and spatial variation of carbon metabolism in the maize ovaries revealed significantly reduced soluble sugar and starch contents at 20 days after fertilization. Thus, the grain-filling rate was suppressed during initial grain growth stage, and the degree of decrease was positively related with the magnitude of water-deficit stress severity. The soluble sugar and starch contents in the apical ovaries were lower than in ovaries in the middle and basal positions. In 2019, highest ovary abortion frequency at the apical position was recorded, followed by the basal and middle positions, while in 2018 90 % abortion frequency was observed in all the ear positions with severe stress. Under mild, moderate, and severe water-deficit stress conditions, grain number per ear decreased by 26 %, 61 %, and 93 % in 2018, and 9 %, 14 %, and 41 % in 2019, respectively. To address more frequent and intense water-deficit stress events under future changing climate, maize breeding needs to focus on developing drought-tolerant maize varieties with minimal ovary abortion.