The Ecophysiology of the New Zealand Mangrove, Avicennia marina (Forssk.) Vierh. subsp. australasica (Walp.) J. Everett
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The availability of water is an important environmental factor determining plant growth, survival, and distribution globally. From a physiological viewpoint, non-structural carbohydrates (NSCs) are necessary for plants to cope with abiotic factors such as drought and may allow us to understand how plants like mangroves are able to survive in what is seemingly an inhospitable habitat. Furthermore, the metabolic profiles of these plants may also provide relevant information as to how routine metabolic processes contribute to their success in estuarine conditions. In this thesis, I studied the role of NSCs in species distribution and survival of the New Zealand mangrove, Avicennia marina subsp. australasica. In Chapter 2, I discuss the methods of NSC quantification used in this study. I also investigate latitudinal NSC dynamics in New Zealand mangroves. NSC quantification using enzymatic hydrolysis did not yield interpretable results for the species studied. So I show that near infrared spectroscopy is a useful method to estimate NSC content in the species. My results show that total carbon content was significantly affected by season (summer vs. winter) across the latitudinal gradient. Total carbon content was increased in the southernmost sites in summer. In summer, only leaf total NSC content was increased in the southernmost sites in comparison to the northern sites. Whereas in stem cores, total NSC content was neither affected by season nor latitude. In Chapter 3, I elucidate the role of different NSC levels in the physiological responses of the New Zealand mangroves to drought and salinity. Plants with different NSC levels were obtained through a light swapping treatment. Low NSC (L-NSC) plants were grown in low light conditions during the second half of the light swapping treatment, whereas high NSC (H-NSC) plants were those that were grown in high light conditions in the second half of the manipulation. I show that during drought and saline conditions, higher NSC levels help in maintenance of physiological functioning, however, growth parameters remained unaffected in both L-NSC and H-NSC plants. My results show that most variables such as stem water potential and hydraulic conductivity were affected by three-way or four-way interactions along with main effects of either initial NSC level, salinity, drought, and time. Plants with high initial NSC had higher water potential and conductivity under high drought and salinity. Plants with high NSC also had higher survival rates under drought and elevated salinity levels. In Chapter 4, when describing metabolomic responses to drought and salinity, I show that high NSC plants have lower abundance of α-ketoglutarate than the low NSC plants under high drought and salinity conditions. H-NSC plants also had higher abundance of soluble sugars under high drought and salinity aiding osmotic adjustment. In conclusion, this thesis contributes to our knowledge of NSCs in mangrove ecophysiology and improves our general understanding of NSC dynamics in plants. My work extends the established paradigms of plant physiological responses of terrestrial tropical and temperate tree species under abiotic stress, especially drought and salinity, to mangroves. The results from my work can be further explored and incorporated into vegetation modelling, useful in prediction of future mangrove or other tree species distribution.