Abstract
Drought stress triggers mature leaf senescence, which supports plant survival and remobilization of nutrients; yet leaf senescence also critically decreases post-drought crop yield. Drought generally results in carbon/nitrogen imbalance, which is reflected in the increased carbon:nitrogen (C:N) ratio in mature leaves, and which has been shown to be involved in inducing leaf senescence under normal growth conditions. Yet the involvement of the carbon/nitrogen balance in regulation of drought-induced leaf senescence is unclear. To investigate the role of carbon/nitrogen balance in drought-induced senescence, sorghum seedlings were subjected to a gradual soil drought treatment. Leaf senescence symptoms and the C:N ratio, which was indicated by the ratio of non-structural carbohydrate to total N content, were monitored during drought progression. In this study, leaf senescence developed about 12 days after the start of drought treatment, as indicated by various senescence symptoms including decreasing photosynthesis, photosystem II photochemistry efficiency (Fv/Fm) and chlorophyll content, and by the differential expression of senescence marker genes. The C:N ratio was significantly enhanced 10 to 12 days into drought treatment. Leaf senescence occurred in the older (lower) leaves, which had higher C:N ratios, but not in the younger (upper) leaves, which had lower C:N ratios. In addition, a detached leaf assay was conducted to investigate the effect of carbon/nitrogen availability on drought-induced senescence. Exogenous application of excess sugar combined with limited nitrogen promoted drought-induced leaf senescence. Thus our results suggest that the carbon/nitrogen balance may be involved in the regulation of drought-induced leaf senescence.
Highlights
Drought stress triggers various plant responses, which affect numerous plant systems ranging from gene expression patterns to physiological metabolism to growth and development
No significant difference in leaf relative water content (RWC) between treatments was seen until day 14, when a substantial drop occurred in the plants under drought conditions (Fig 2B)
There was no significant difference in leaf chlorophyll content between treatments until day 14, and a substantial decline occurred between days 14 and 16 in the plants under drought conditions (Fig 3C)
Summary
Drought stress triggers various plant responses, which affect numerous plant systems ranging from gene expression patterns to physiological metabolism to growth and development. The availability of carbon (C), especially in its carbohydrate form, and nitrogen (N) are important factors in the regulation of plant metabolism and development [6]. Analysis of growth at different C:N ratios in Arabidopsis has revealed that the C/N balance, rather than C or N alone, plays a predominant role in seedling growth regulation, storage lipid remobilization and photosynthetic gene expression [8]. Global gene expression analysis of Arabidopsis responses to a matrix of C:N treatments confirms the proposed importance of combined carbon and nitrogen (CN)-signaling in plants [9]. In addition to its role in early seedling growth, C/N balance plays an important role in regulating leaf senescence under normal conditions. Leaf senescence can be triggered by high C and low N availability (a typical C/N imbalance)[7,10]
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