Nitrogen-15 remobilisation in Saccharum spp. at the vegetative growth under drought stress and rehydration conditions

Abstract

Drought stress is a major constraint on sugarcane (Saccharum spp.) productivity, negatively affecting nitrogen uptake and assimilation. Under such conditions, internal nitrogen remobilisation becomes essential for sustaining plant growth and development. This study investigated the effects of drought stress and subsequent rehydration on nitrogen remobilisation in sugarcane (variety CTC9001bt), with a specific focus on the interaction between nitrogen and carbon metabolism. Sugarcane plants were cultivated under controlled greenhouse conditions and subjected to well-watered (~90% water holding capacity, WHC) and water- deficient (~20% WHC) treatments. 15N-labelled ammonium sulphate was applied to the +1 leaf at 25 days after transplanting (DAT) to trace nitrogen remobilisation patterns. Physiological, biochemical, and isotopic analyses were performed at maximum stress (38 DAT) and following rehydration (43 DAT). Drought stress significantly reduced plant height, leaf water potential, stomatal conductance, photosynthetic rate, and transpiration. These physiological changes were accompanied by a decline in carbon isotope discrimination (Δ13C), particularly in roots, leaves, and 15N-labelled leaves. In contrast, Δ13C values in stalks increased under drought. Nitrogen remobilisation was enhanced under water deficit conditions, with preferential allocation to roots and sheath leaves, contributing to improved nitrogen use efficiency (NUE). The accumulation of starch and osmoprotectants, such as proline, soluble sugars, and carotenoids, supports photoprotection and metabolic balance. Following rehydration, nitrogen was redistributed to the leaf blades, promoting partial recovery of chlorophyll content and photosynthetic performance. However, gas exchange parameters remained below those of well-watered plants, indicating incomplete recovery. Key enzymes involved in nitrogen metabolism, including glutamine synthetase (GS) and nitrate reductase (NR), were modulated by water availability. Changes in the carbon-to- nitrogen ratio reflected dynamic resource allocation in response to stress and recovery. These findings underscore the importance of integrated nitrogen and carbon metabolic responses for drought resilience in sugarcane. Importantly, understanding nitrogen remobilisation dynamics has practical agronomic implications, including the potential to optimise fertiliser management, reduce nitrogen losses, and enhance yield stability under drought conditions. Moreover, these insights can support the development of precision agriculture strategies and inform breeding programmes aimed at improving NUE and environmental sustainability in sugarcane production systems.