Secondary metabolites and nutrient balance in casuarinas: An insight into Protein Competition Model (PCM)
The total phenolics, total condensed tannins (TCT), nitrogen (N) and total protein (TP) in needles of Casuarina equisetifolia and Casuarina junghuhniana were studied to understand the carbon-nutrient balance (CNB) and the growth-differentiation balance (GDB) hypotheses. The carbon-nutrient balance (CNB) hypothesis postulates that phenolic levels in plants are determined by the balance between carbon and nutrient availability1. The growth-differentiation balance (GDB) hypothesis2 considers factors that limit growth and differentiation. The production of phenolics dominates when factors other than photosynthate supply are suboptimal for growth (e.g., under nutrient limitation). Resource-based theories assume that the synthesis of defensive compounds is constrained by the external availability of resources and internal trade-offs in resource allocation between growth and defense. It is stated that growth processes dominate over the production of defensive compounds and that more carbon is left for defensive compounds only when plant growth is restricted by a lack of mineral nutrient (emphasized by the CNB hypothesis) or by any factor (according to the GDB hypothesis). Jones and Hartley3 presented a protein competition model (PCM) for predicting total phenolics allocation and content in leaves of higher plants. Protein competition model (PCM) stated that “protein and phenolics synthesis compete for the common, limiting resource phenylalanine,” so nitrogen (N) rather than C is the limiting resource for synthesis of phenolics. In our study, the contents of Total Phenolics, and Total Condensed Tannin (TCT) in needles of C. equisetifolia were higher than the C. junghuhniana. However, Total protein and nitrogen (N) contents were higher in C. junghuhniana than C. equisetifolia. There was a significant negative correlation between Total phenolics, TCT and Total Protein, N contents. Therefore, it is found from the present investigation that C. equisetifolia follows CNB hypothesis. However, C. junghuhniana follows GDB hypothesis, since it contains low defense chemicals viz., phenolics & TCT and high nitrogen and protein contents. Hence, the adaptability of C. equisetifolia in coastal areas and C. junghuhniana in drier inland condition is realized.
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