Circadian Clocks in Crop Productivity: Mechanisms, Breeding Strategies, and Chrono-Agricultural Applications

Circadian clocks are endogenous timing systems that coordinate plant physiology, metabolism, development, and stress responses with daily and seasonal environmental cycles. In crops, circadian and photoperiodic pathways influence agronomically important traits including photosynthesis, carbon allocation, flowering time, growth, stress resilience, and nutritional quality. Although flowering time and photoperiod response pathways have long been indirectly exploited during domestication and breeding, the broader potential of circadian regulation for crop improvement and time-sensitive management remains only partially developed. This review examines the role of plant circadian clocks in crop productivity, with emphasis on molecular mechanisms, crop-specific clock-associated loci, breeding strategies, and chrono-agricultural applications. We summarize conserved and divergent features of the plant clock, including transcriptional repression and activation modules, environmental entrainment, and post-transcriptional regulatory layers. We then discuss how circadian regulation shapes productivity traits and highlight examples from rice, wheat, barley, maize, soybean, sorghum, tomato, and other crops. These examples show that agricultural adaptation often involves fine-tuning or rewiring circadian and photoperiodic outputs rather than maintaining a universal optimal clock state. Finally, we evaluate chrono-agriculture as an emerging framework for aligning management practices with biological timing. While controlled-environment agriculture and high-value horticultural systems are currently the most practical settings for testing chrono-agricultural strategies, open-field applications require careful consideration of environmental variability, sensor limitations, labour, machinery logistics, economic feasibility, and multi-environment validation. Integrating circadian biology with crop genetics, phenotyping, modelling, and agronomy may provide new opportunities to improve productivity, resilience, resource-use efficiency, and quality traits in sustainable agricultural systems.