Gas Exchange of Mimosa tenuiflora (Willd.) Poiret Under Water Deficit and Rewatering

This research aimed to evaluate the physiological responses of Mimosa tenuiflora plants submitted to variable water availability conditions during the nursery stage. Twelve-month-old plants kept in plastic pots containing 5 kg of the substrate composed of the subsoil soil mixture and bovine manure (2:1) were submitted to two treatments: irrigated (control) and water stress, which was imposed through the suspension of irrigation, rewatering after seven days of stress. The relative water content (RWC) and stomatal parameters were evaluated. The M. tenuiflora plants responded quickly to the irrigation suspension, promoting the closure of the stomata, occurring reduction in stomatal conductance, transpiration rate and photosynthesis. The instantaneous efficiency in water use of plants under water deficit remained high only until the middle of the period when irrigation was suspended, and then declined until the last day of the water deficit. After rehydration, the plants showed recovery in all evaluated parameters, indicating that the level of stress imposed did not cause irreversible damages in the cells and tissues.


Introduction
Mimosa tenuiflora (Willd.) Poiret.), popularly named 'jurema-preta', is one of the most commonly woody species in the Brazilian semiarid, belonging to the Mimosaceae family. It is native to the Caatinga Biome, but not endemic to the region, and can also be found in northern Venezuela and Colombia and in dry regions of southern Mexico, Honduras and El Salvador (Azevê do et al., 2012).
It is a species that tolerates droughts well, with a large annual regrowth capacity, and is used as an energy source due to the excellent quality of the wood. In addition, it can favor the regeneration of plants that grow under its canopy, thus contributing to the regeneration of eroded soils, as well as being indicated for ecological restoration programs (Santos et al., 2011, Paterno et al., 2016.
In tropical regions, droughts are often associated with high solar radiation (Gouvê a and Marenco, 2018) which combined with the low capacity of Caatingaś soils, reduce the water storage capacity of this biome. This situation tends to worsen in the face of global climate change that our planet is going through. In their review of climate projections on the Brazilian Northeast, citing some authors, Guimarã es et al. (2016) report that this is a region with considerable vulnerability to climate change and is subject to changes in the distribution of extreme events, summer events, spatial distribution of rainfall, etc. Hence the need for a better understanding of the physiological strategies developed by the plants of this region to survive under these conditions. Plant species that inhabit semiarid climate regions respond to water deficit by developing strategies and/or mechanisms for escape or drought tolerance (Araújo, 2005). Because they are subject to long periods of drought, changes occur in physiological, biochemical, molecular and morphological processes due to reduced leaf water potential. Among these physiological modifications, stomata closure assumes a prominent position, causing reduction in stomatal conductance, transpiration and photosynthesis rate, causing a decrease in plant growth (Chaves et al., 2003, Fernandes et al., 2015. In addition, there is a decrease in protective protein synthesis (Choudhury et al., 2017), and as a survival strategy plants can promote osmotic adjustment by accumulation of osmotically compatible solutes, ensuring the maintenance of cell turgidity, avoiding tissue death (Sircelj et al., 2007, Nio et al., 2011.
Several researchers have developed research to evaluate the effects of water deficit on gas exchange in forest species (Costa et al., 2015, Oliveira et al., 2016, Moura et al., 2016, Oliveira and Gualtieri, 2017. However, there is little research with native plants of Caatinga on this subject, and more research is needed to provide this information.
In this sense, this aimed to evaluate the physiological and biochemical responses of black jurema plants submitted to progressive water deficit and subsequent rewatering.
Seeds of M. tenuiflora were subjected to integumentary dormancy break according to Bakke et al. (2006) and then placed to germinate in conical tubes (125 cm 3 ) containing the substrate composed of subsoil and cattle manure mixture, 2: 1 ratio. The soil used was analyzed both chemical and physical properties (Table 1). Thirty days after emergence, the seedlings were transferred to plastic pots containing 5 kg of the above substrate. Irrigation was performed daily, trying to keep the substrate humidity close to 70% of the retention capacity, determined by weighing.
The plants were kept in this condition until they reached one year of age, when were separated into two groups, in which one group continued to be normally irrigated (irrigated treatment) and the other had suspended irrigation (water deficit treatment).
The treatment of water stress persisted until it was realized that the photosynthesis (A) reached values close to zero, which happened on the seventh day after irrigation interruption. After this, the water supply to the plants resumed, until the photosynthesis rate of the water stress treatment plants reached values close to those observed in the control treatment plants, which occurred three days after rehydration.

Water Relation and Gas Exchange Measurements
Throughout the periods of water deficit and rewatering, relative water content (RWC), photosynthesis (A), transpiration rate (E), stomatal conductance (gs), intercellular CO 2 concentration (ci) were measured. The water use efficiency (WUE) was obtained by the ratio A/gs, and ci/ca was calculated by ratio in ci and CO 2 atmosphere concentration (ca).
Plant water status was determined by measurements of relative water content (RWC). In order to obtain the RWC values, leaves were weighed for fresh mass (FM) determination and then placed in glass dishes for 48 h. After measurement of saturated mass (SM), the plant samples were dried at 70 º C for 72 h to determine dry mass (DM). The RWC values were evaluated as RWC (%) = [(fw−dw)/(tw−dw)]×100 (Weatherley, 1950), in which fw, dw and tw refer to fresh weight, dry weight and turgid weight, respectively.
Gas exchange measurements were taken from entirely unfolded and healthy leaves (two leaves per plant, in four plants per treatment), exposed and perpendicularly oriented to the ISSN 2166-0379 2019 sun, from young branches in the middle third of the plant, between 10:00 and 11:00 am, using a portable photosynthesis analyzer LCpro-SD (ADC Bio Scientific Ltd.). All gas exchange measurements were recorded during the typical diurnal range of air temperature, and humidity under our field conditions, and PAR flux was adjusted to 1 200 μmol m -2 s -1 .

Data Analysis
All experiments were performed with four replicates, and five plants per replication. Data were subjected to one-way ANOVA, and significant differences determined by the Scott-Knott test (p <0.05), using the ASSISTAT software (Silva and Azevedo, 2002).

Relative Water Content and Gas Exchange
There was a significant difference in RWC between treatments from the second day of water deficit imposition, with gradual reduction until the seventh day ( Figure 1). While irrigated plants maintained a high RWC (90%), plants under water deficit reached critical levels on the fourth day, with 55%. This value falls to 48% on the seventh day, corresponding to almost half of the RWC of irrigated plants. One day after rewatering, the plants recovered turgidity, reaching 73% RWC indicating that there was no damage to cells under low tissue hydration condition, with rapid rehydration with the resumption of water supply. Following the reduction in RWC, stomata were affected on the first day of water deficit, with progressive reduction of E (Figure 2A), gs ( Figure 2B), A ( Figure 2C), and increase ci ( Figure 2D).
On the seventh day after the beginning of the treatments, irrigated plants presented E equal to 4.48 mmol m -2 s -1 , while in plants under water deficit was 0.043 mmol m -2 s -1 (Figure 2A ), representing a 99% difference. On the day of irrigation suspension, the E value was 3.28 ISSN 2166-0379 2019 mmol m -2 s -1 , reaching 0.043 mmol m -2 s -1 , with 98.6% reduction. Despite the rapid recovery in RWC (Figure 1), only on the second day after irrigation that there was an increase in E, but much lower than the treatment irrigated. On the last day of the evaluation, three days after the resumption of water supply, there was a total recovery in plant's transpiration.

Journal of Agricultural Studies
The gs in plants under water deficit decreased progressively as the irrigation suspension period increased, reaching a value equivalent to 94% of the irrigated plants, on the last day after irrigation suspension. After resumption of irrigation, the recovery of this parameter followed the same behavior in E.
Regarding A in irrigated plants the values ranged from 24.2 to 34.2 µmol m -2 s -1 , while in plants under deficit the variation ranged from 21.1 to 0 µmol m -2 s -1 , respectively on the first and seventh day of the beginning of the treatments ( Figure 2C). On the fourth day, theoretically under moderate stress, the plants showed reductions of 54%, 76% and 42% in gs, E and A, indicating that, despite the reduction in stomatal conductance, there was little influence on CO 2 assimilation ( Figure 2). However, as the water deficit period increased, stomata closure increased, causing total inhibition in photosynthesis. As with gs and E, on the third day after rehydration there was a complete recovery in A, reaching levels equal to those of irrigated plants, indicating that the water stress imposed was not enough to cause irreversible damage to the stomatal function and the photosynthetic apparatus.  Vol. 7,No. 4 Irrigated plants maintained WUE between 0.1 and 0.17 µmol mol -1 ( Figure 3A), while plants under water deficit increased from the second to the fourth day, and decreased until 0 µmol mol -1 on the seventh day without irrigation.
Due to the behavior observed in ci ( Figure 2D), the ci/ca ratio ( Figure 3B) of irrigated plants remained almost constant, whereas in plants under water deficit increased, reaching a maximum of 1.0 on the seventh day after the start of treatments, decreasing after rewatering.

Discussion
According to Pardo (2010), RWC 50% is critical to plants, and the adequate value for well-hydrated tissues ranges from 85 to 95%. However, some species adapted to arid and semiarid climate regions can reach this value without tissue death, which may be the case of M. tenuiflora plants in this research, since after the resumption of the supply of adequately restored their water status. These results agree with those found in the literature, in Handroanthus impetiginosus (Pessoa et al. 2017), Myracrodruon urundeuva (Costa et al., 2015), Hevea brasiliensis (Chen et al., 2010), and Quercus pubecens (Gallé et al. ., 2007).
The speed of response to the reduction in water availability varies by species, and the degree of this response directly affects the maintenance of plant tissue water balance. In H. impetiginosus seedlings, a 15% reduction in RWC was observed on the second day after the water supply was suspended (Pessoa et al. 2017). These researchers also reported that at 10 days after irrigation suspension, RWC in irrigated and under water deficit plants were 83% and 26%, respectively, resulting in a 69% reduction. In M. urundeuva irrigated plants, Costa et al. (2015) observed a variation of 82 to 91%, while in plants under water deficit, the RWC progressively reduced until reaching the value of 70% on the 12th day of evaluation.
The recovery of RWC in M. tenuiflora plants after rewatering indicates that stress did not cause irreversible damage to the hydraulic conductivity of the root xylem vessels. Among the ISSN 2166-0379 2019 effects of water stress are cavitation (Holbrook et al., 2001, Otieno et al., 2005 and consequent decrease in hydraulic conductivity (Chen et al., 2010). However, the phenomenon of cavitation can be immediately neutralized after the resumption of water supply to plants, with recovery of root hydraulic conductivity (Holbrook et al., 2001, Otieno et al., 2005.

Journal of Agricultural Studies
The rapid reduction in E and gs of plants under water deficit demonstrate the sensitivity of the stomata of this species to water deficit, constituting a survival strategy under adverse water conditions, avoiding excessive loss of tissue water and consequently death by dehydration. The fact that the stomatal mechanism was recovered indicates that the time elapsed from the imposition of water restriction did not cause irreversible damage to the stomata, enabling the recovery of its functioning after plant rehydration. When water supply decreases, guard cells respond to leaf water potential and control their water loss by decreasing transpiration (Chaves et al., 2010). These authors add that partial pressure of intercellular CO 2 can control stomatal opening by supplying CO 2 to chloroplast or by CO 2 demand for photosynthesis. The physiological mechanisms of stomata response to water deficit are complex and not yet fully understood (Patakas et al., 2005), since they can respond to a series of stimuli, and not all of them are directly related to soil water content, as stomatal sensitivity to vapor pressure deficit (VPD) (Habermann et al., 2003). Another stimulus that may regulate the stomatal response is increased cellular abscisic acid (ABA) under water stress conditions (Harris and Outlaw, 1991) or apoplastic ABA on the surface of guard cells, which supposedly cause a reduction in turgor pressure (Assmann, 2003), which in turn leads to stomatal closure (Sousa et al., 2006). These factors make stomatal control independent or indirectly related to soil water content.
Decreased photosynthesis under water deficit conditions may occur due to stomatal factors, due to reduced stomatal conductance, restricting gas exchange between the plant and the external environment (Flexas and Medrano, 2002, Flexas et al., 2004, Farooq et al., 2009, or by non-stomatal factors, due to damage caused to the photosynthetic apparatus (Fu and Huang, 2001) as well as by decreased activity of Calvin Cycle enzymes (Monakhova and Chernyadév, 2004). Zhou et al. (2013) and Osakabe et al. (2014) related a close correlation between water stress and gs, which constitutes is one of the most important factors that leads to a decline in A during drought conditions (Peeva and Cornic 2009;Zhou et al. 2013).
Under mild to moderate water deficits, stomatal closure is one of the first responses of plants, restricting water loss and carbon assimilation (Chaves et al., 2003). However, if the deficit condition persists, acclimatization responses such as growth inhibition and osmoregulation are triggered, which are key elements for maintaining the water status of the plant and therefore assimilation of the plant carbon under water scarcity (Chaves et al., 2010). In grapevine, maintenance of Calvin Cycle enzymes activity and maximum carboxylation and electron transport rates have been reported under low to moderate water availability (Souza et al., 2005). However, as stress intensifies these parameters decline especially in electron transport (Maroco et al., 2002, Souza et al., 2005, possibly as a result of decreased ATP production (Chaves et al., 2010).
The reduction in WUE here observed contradicts the results obtained in other studies, where Journal of Agricultural Studies ISSN 2166-0379 2019 there was an increase in WUE under water stress conditions (Diaz-López et al., 2012, Dombroski et al., 2014. While stomatal conductance is more affected than photosynthesis under moderate water stress until the fourth day (Figures 2A and 2B), WUE has increased ( Figure 3A). Elevation in WUE even before complete stomatal closure is a common phenomenon in plants under water stress (Pou et al. 2008, Chen et al., 2010. Similar to what happened in ci ( Figure 2D), in the middle of the water deficit period, between days three and four, the values of ci/ca were low, meaning that there was low accumulation of CO 2 in the intercellular spaces, probably because it is being used for photosynthesis, even in smaller quantities ( Figure 2C). Thus, non-stomatal factors are believed to play an important role in limiting photosynthesis under water deficit conditions in black jurema plants, as gs decreased and the ci/ca ratio increased, as reported (Bacelar et al., 2007). However, some authors state that ci values should be carefully analyzed, since stomata opening does not occur evenly due to irregularities in the leaf surface (Bacelar et al., 2007). In addition, ci calculated from gas exchange measurements can be overestimated and lead to erroneous conclusions of non-stomatal limitation of photosynthesis (Downton et al., 1988).

Conclusion
-Mimosa tenuiflora plants respond quickly to irrigation suspension, promoting stomata closure, occuring reduction in stomatal conductance, transpiration rate and photosynthesis.
-The water use efficiency of M. tenuiflora plants under water deficit remains high for a short period after irrigation water restriction.
-The stress level imposed did not cause irreversible damage to cells and tissue of plants.