Gas exchange , Chlorophyll Fluorescence and Pigments of Noni ( Morinda citrifolia L . ) under Salt Stress

Noni is a fruit crop well adapted to different soil and climatic conditions. Aiming to evaluate the physiological responses to salinity, noni seedlings were grown in two levels of NaCl (0 and 100 mM) in nutrient solution and the effects of salt stress on gas exchange, chlorophyll a fluorescence, photosynthetic pigments, relative water content and membrane integrity were assessed after 1, 10, 20, 30 and 40 days of salt stress. The experimental design was a completely randomized in 2 × 5 factorial arrangement with four replications. Salinity did not affect the intrinsic efficiency of water use, but reduced net assimilation of CO2, stomatal conductance, transpiration, carboxylation efficiency and contents of chlorophyll a, b, and total carotenoids. Salinity caused no major changes in chlorophyll fluorescence, however the stressed plants showed a decrease in photoprotection capacity by the cycle of xanthophylls. Salinity did not affect the water status of the leaves, but damages to the integrity of the membranes were observed due to duration of salt exposure. The data indicate that noni presents stomatal closure as a mechanism of salinity tolerance, reducing water loss by transpiration and maintaining the water status.


Introduction
The increase in salinity of the soil solution beyond the tolerance capacity of the plants is an obstacle to agricultural production, particularily in irrigated soils of arid and semi-arid regions.In Brazil, the semi-arid region occupies an area of 969,589.4km², located mainly in the Northeast part of the country (Brazil, 2007).In these areas, the scarce availability of water associated with use of brackish groundwater in irrigation, and climatic conditions, decreases the agricultural production, which is one of the main environmental obstacles to agricultural development in the region.
Crops in arid and semi-arid regions are often irrigated with poor quality water at certain time of the year, since in most areas it is the only source of water.However, across most of the countries in the world, it has been successfully used with adoption of appropriate management practices of soil, water and the use of plants tolerant to salinity (Bezerra, Lacerda, Hernandez, Silva, & Gheyi, 2010).
The salinity may decrease the absorption of water by plants, and provoke unbalanced nutrition, as well as modify the metabolic process and decrease the photosynthetic efficiency of crops (Munns & Tester, 2008;Iqbal, Umar, N. Khan, & M. Khan, 2014).Thus, plants need to use physiological and biochemical mechanisms to face the osmotic and specific ion effects of salts.Therefore, it is necessary to understand the mechanisms of tolerance of plant to salinity for economical agricultural production.
The study of physiological variables such as stomatal conductance and chlorophyll a fluorescence are important to explain the osmotic effects and water conditions imposed by salinity on photosynthetic efficiency of plants.In this context, some gas exchange and chlorophyll a fluorescence studies have been used as a nondestructive method for evaluating the plant's tolerance to salinity (Silva, Ribeiro, Ferreira-Silva, Viégas, & Silveira, 2011;Azevedo Neto, Perreira, Costa, & Santos, 2011;Silva et al., 2014).These variables are of fast and non-invasive determination, which allow to monitor the development of the plant, providing qualitative and quantitative information on the physiological conditions of photosynthetic processes (Silva et al., 2011;Kalaji et al., 2014).
The chlorophyll a fluorescence determines the state of energy distribution in the thylakoid membrane, the quantum efficiency of photosystem II (PSII) and the extent of photoinhibition (H.Wang, F. Wang, G. Wang, & Majourhat, 2007).Thus, the qualitative and quantitative information about the photosynthetic processes in chloroplasts show the functioning of the photosynthetic apparatus under different internal and external conditions (Roháček & Barták, 1999).Silva et al. (2011) reported that the salinity strongly reduces the gas exchanges and the photochemical activity in Jatropha curcura, caused by ionic stress.Therefore, evaluations of exchanges and chlorophyll a fluorescence can be utilized as tools to diagnose the integrity of photosynthetic apparatus under adverse environmental conditions (Gonçalves et al., 2010).
Morinda citrifolia Linn, popularly known as noni, belongs to Rubiaceae family and is a medicinal plant used over 2000 years by the Polynesians (Chan-Blanco et al., 2006).The leaves and especially the fruits are consumed in different ways by many communities around the world due to the effects related to antioxidant, anti-inflammatory, analgesic, immunomodulatory, antibacterial, antitumor activity, sources of vitamins, among others (Chan-Blanco et al., 2006;Costa, Oliveira, Silva, Macini-Filho, & Lima, 2013).
Noni cultivation can be a viable alternative for the arid and semiarid regions.The plant acclimatizes to diverse environmental conditions (Nelson & Elevitch, 2006), though some studies have shown that irrigation with saline water reduces its growth (Souto et al., 2015a;Souto, Cavalcante, Lima Neto, Mesquita, & Santos, 2016).
Considering the medicinal importance of noni, its adaptive capacity, and the lack of information on its growth in saline environment, this study aimed to evaluate the gas exchange, the chlorophyll a fluorescene, and the pigment contents in noni plants under stress, for a better understanding of the tolerance mechanisms to salt stress of this species.

Growth and Treatment Conditions
The experiment was carried out in a greenhouse, in a completely randomized design, by using the factorial scheme of five harvest times versus two NaCl levels of salinity in nutrient solution, with four replicates.The mean values of temperature, air relative humidity, and photosynthetic active radiation (at noon) were 25 °C, 81% and 1200 µmol m -2 s -1 , respectively.
Noni seedlings three months old and with four pairs of leaves were selected.Seedlings were transferred to containers with 12 L of Furlani (1998) nutrient solution, in a Floating hydroponic system.Noni seedlings remained in nutrient solution for four days for acclimation.After this period, the seedlings were submitted to the respective salt treatments (nutrient solution without NaCl -control or nutrient solution with 100 mM NaCl -salt stress).Sodium chloride was gradually added (25 mM day -1 ), to avoid osmotic shock.The volume of the solutions was completed daily with water and the renewal was performed weekly.The pH was maintained at 6.0±0.2 by adding NaOH or HCl.The system was maintained under intermittent aeration of 15 minutes every hour, using an air compressor coupled to a timer.
Plants from each treatment were harvested at 1, 10, 20, 30 and 40 days after the end of salt additions and shoot dry mass (SDM) was obtained after drying in an oven at 65 °C for 72 h.

Gas Exchange
Assimilation rate of CO 2 (A), stomatal conductance (g s ), transpiration (E), water use efficiency (WUE), carboxylation efficiency (A/Ci), and leaf temperature (T f ) were obtained with a LI-6400XT portable gas exchange measuring system (LI-COR Biosciences Inc., Lincoln, Nebraska, USA) containing a source of blue/red light.The measurements were performed every 10 days, from 8:00 a.m. to 10:00 a.m., under artificial saturating jas.ccsenet.light at 10 average w

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