Globally recurrent extreme high temperature (HT) events severely limit rice production. This study investigated whether a controlled moderate soil drying (MD) could replace the conventional well-watered (WW) regime to more effectively mitigate HT stress on pistil fertilization in photo-thermosensitive genetic male-sterile (PTGMS) rice, and examined the role of brassinosteroids (BRs). Two PTGMS rice varieties were cultivated under normal temperature (NT) and HT conditions, paired with WW and MD strategies during anthesis. In the conventional WW regime, waterlogging reduced BR levels in roots and pistils due to excessive decomposition, weakening active water uptake driven by root activity and failing to alleviate transpiration-pulled passive water extraction hampered by restricted stomatal openings. Thereby, it caused water imbalance in plants and weakened pistil function due to a suppressed ascorbate-glutathione (AsA-GSH) cycle and hyperactive nicotinamide adenine dinucleotide phosphate oxidase (NOX) activity. This exacerbated pistil fertilization impairment and hybrid seed yield loss under HT stress. Conversely, by promoting BR synthesis and inhibiting its decomposition in roots and pistils, the MD strategy enhanced root activity and transpiration-driven water uptake. It maintained plant water balance and supported pistil function by suppressing NOX activity and enhancing AsA-GSH cycle-driven redox homeostasis. Thus, it mitigated HT-induced pistil fertilization impairment and hybrid seed yield loss. The precise function of BRs in moderating the protective effects of MD against the detrimental impacts of HT stress on pistil fertilization in PTGMS rice was confirmed through genetic and chemical approaches. Consequently, a controlled MD method proved more effective than the conventional WW regime in alleviating HT stress on pistil fertilization in PTGMS rice by promoting BR enhancement.

This study examined the involvement of cytokinins in the process by which moderate water limitation (MWL) mediates nitrogen (N) remobilization from source to sink during the grain-filling phase in wheat. Field experiments were performed using N application rates of low (LN), medium (MN), and high (HN). Two soil moisture regimes were implemented for each N rate: conventional well-watered (CWW) and MWL post anthesis. The MWL application optimized N, total free amino acids (FAA), and trans-zeatin (Z)+trans-zeatin riboside (ZR) reallocation from the source organs (stems and leaves) to the sink organ (spikes) in wheat. Compared to those in the CWW regime, the activities of proteolytic enzymes, including endopeptidase, carboxypeptidase, and aminopeptidase within stems and leaves, and the expression levels of total FAA transporter genes in spikes were significantly elevated in the MWL regime, showing a close correlation with the Z+ZR levels in the spikes. Application of kinetin to stems and leaves significantly inhibited proteolytic enzyme activities, promoting N retention in stems and leaves, decreasing N accumulation in the sink organ, and reducing the N harvest index. In contrast, applying kinetin to spikes significantly upregulated expression levels of FAA transporter genes, reducing N retention in stems and leaves, increasing N accumulation in the sink organ, and raising the N harvest index. Such facilitation induced by the MWL in the remobilization of N from source to sink was greater at HN than at LN or MN. Results demonstrate that post-anthesis MWL can significantly intensify the remobilization of N from source to sink, while also synergistically enhancing grain yield and N use efficiency through strategically redistributing cytokinins (Z+ZR) between source and sink in wheat.