Nutrient, water, and their interactions influence the allocation of investment by plants to resistance and tolerance traits. We used a completely crossed randomizedblock design experiment to examine the independent and interactive effects of nutrients and water availability on tannin production of C. equisetifolia seedlings. The results showed that nitrogen and phosphorus fertilizer have significant effects on total phenolics (TP) and extractable condensed tannins (ECT) concentrations of branchlets. TP and ECT concentrations decreased with fertilizer addition and increased in arid condition. This pattern lends to support source-sink hypothesis such as the carbon-nutrient balance (CNB) hypothesis and the growth-differentiation balance (GDB) hypothesis. Soluble sugars or starch concentrations were both inversely related to TP concentrations. However, there was no significant correlation between them and ECT concentrations. In addition, chlorophyll concentration had a positive linear correlation with TP and no significant correlation with ECT. On the contrary, chlorophyll a/b ratios were negatively correlated with TP and positively correlated with ECT. The discrepancy of relationship between carbohydrates and TP or ECT showed that the biosynthetic routes of different tannins were different. In this study, no significant correlation between TP and N, or ECT and N, did not support protein competition model (PCM). TP:N and ECT:N ratios were higher in nutrient deficiency and arid conditions, which were one of the important nutrient conservation strategies for C. equisetifolia.
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