Department of Laboratory of Management and Valorization of Forest Resources, National Research Institute of Rural Engineering, Water and Forests (INRGREF), Tunisia
Received Date: Mar 27, 2017; Accepted Date: Apr 20, 2017; Published Date: Apr 24, 2017
Citation: Kachout SS, Rzigui T, Ennajah A, Baraket M, Baaziz KB, et al. (2017) Does Nitrogen Treatment Affect Leaf Photosynthetic Traits of Cork Oak (Quercus Suber L.) Populations?. J Fundam Renewable Energy Appl 7: 230. doi:10.4172/2090-4541.1000230
Copyright: © 2017 Kachout SS, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distributionand reproduction in any medium, provided the original author and source are credited.
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The present study was conducted to assess the impact of nitrogen treatment on the physiological aspects of cork oak (Quercus suber L.) seedlings from acorns collected from seven sources (Kroumiry and Cap-bon). Nitrogen fertilization increased significantly specific leaf area (SLA) and leaf mass area (LMA) of Q. suber L grown for 2 months at nitrogen concentration 1.5 g/l, respectively. Results indicate that leaf hydraulic conductance (KLeaf) increased significantly under nitrogen treatment, but no significant correlation was observed between KLeaf, A and gs. Stomatal conductance (gs), transpiration rate (E), net photosynthetic rate (A) and maximum efficiency of PSII (Fv/Fm) were investigated in seedlings of Q. suber L. This study validated that KLeaf, the stomatal conductance, transpiration rate and net photosynthetic rate of Q. suber were performed under increasing Nitrogen fertilizer. Stomatal conductance influences both photosynthesis and transpiration, thereby coupling the carbon and water cycles and affecting surface-atmosphere energy exchange. We found that the stomatal conductance, transpiration rate, net photosynthetic rate and maximum efficiency of PSII were showed variation in both photosynthetic traits due essentially to local genetic adaptation. However, the seven seedlings sources of cork oak (Q. suber L.) showed a different response of physiological aspects during nitrogen treatment. These findings suggest that nitrogen treatment affects gas exchange and the photosynthetic capacity of the cork oak. Based on the above results, we conclude that nitrogen fertilizer treatments could promote photosynthetic performance of Quercus suber by stimulating morphological and physiological responses.
Quercus suber ; Nitrogen treatment; Stomatal conductance; Transpiration rate; Net photosynthetic rate; Maximum efficiency of PSII; Leaf hydraulic conductance
Cork oak (Quercus suber L.) is a typical Mediterranean resprouting species, which presents great interest for restoration in fire-prone ecosystems  and great socio-economic importance in the Mediterranean region . Cork oak woodland (444 hectares, 70% of land utilization) is a major factor in Tunisia due to forestland with small treeless areas . Attempts at restoration of cork oak stands have been largely unsuccessful because of the poor natural regeneration of this species and the high seedling mortality after transplanting . In North Africa, cork oak forests extend from the Atlantic Coast in Morocco through the Algerian Coast to the North Western of Tunisia with a scattered distribution over the two Mountain ranges of Kroumirie and Mogod. Excessive human pressure is usually pointed as the main cause of the reduction in cork oak forest ecosystems .
As with most agricultural crop production the major driver of soil N2O emissions is likely to be nitrogen fertilizer, added to the soil to increase biomass production . Studies showed that nitrogen application exerted a significant increase in protein content and improved dough quality . Several studies have investigated nitrogen fertilizer uptake, assimilation and effects on plant development . Nitrogen is an essential component of chlorophyll that will reduce chlorophyll formation if lacking from the mineral nutrient supply of a plant, with concomitant effects on photosynthesis . Nitrogen fertilizer plays a significant role in soil carbon sequestration by increasing crop biomass and by influencing the microbial decomposition of crop residue [10,11]. Although applications of nitrogen fertilizer consistently increase crop biomass, its effect on soil carbon content varies with the type of soil , which affects the flux of CO2 into the atmosphere. Since nitrogen (N) is strongly limiting in Mediterranean forests , we analyzed the effects of nitrogen fertilizer on photosynthetic measurements of Q. suber L. Fertilizer management effects on plant water use and drought stress during production for many crops has not been evaluated fully . With some woody perennial plants it is possible that growth can be enhanced more by minimizing water stress than by increasing fertility . Nitrogen is a vitally important element for plant sand it profoundly influences leaf anatomical and functional traits . Previous studies have shown that leaf nitrogen promotes A by increasing Rubisco content and CO2 diffusion conductance . However, the correlation of leaf nitrogen content per leaf area with Kleaf remains to be investigated.
The increasing demand for cork and the low natural regeneration of this species clearly justify intensive planting with improved material. In the present study, we have investigated the possibility that nitrogen fertilizer might be beneficial for cork oak growth and development by analyzing physiological parameters including photosynthetic and gasexchange parameters in nursery seedlings from different provenances in Tunisia.
The study sought to compare leaf hydraulic conductance, transpiration rate, stomatal conductance, photosynthetic rate, maximum efficiency of PSII in response to nitrogen treatment on cork oak (Q. suber L.).
Experimental description and plant material
The study was carried out on cork oak (Q. suber L.) seedlings growing in the nursery located in the greenhouse of INRGREF (Tunis) under semi-arid bioclimate. Q. suber seedlings were collected from seven different locations in Kroumirie and Cap Bon in Tunisia (8 individuals from each populations of Sidi Zid (SZD), Kef Errand (KER), Djebel Serdj (JES), Djebel Chehid (JCH), Ain senoussi (AS), Feija (FEJ), Méjen essaf (MEJ)). The plants were growing 2 months at a nitrogen concentration of 1.5 g/l. The experiments use nitrogen fertilizer: Ammonium nitrogen (NH4-N).
The experimental design in the greenhouse was performed with a completely randomized design of 4 replicates per treatment. Plants of each population were fertilized twice a week from July 2015 to August 2015. The experiment was conducted on 6 month-year-old plants of Q. suber L.
The stomatal conductance, transpiration rate and net photosynthetic rate were measured on fully expanded leaves at similar development stages with portable open gas-exchange analysis systems with leaf chambers (LI-6400, Li-Cor, Inc., Lincoln, NE, USA).
The maximum quantum efficiency of photosystem II (Fv/Fm) was measured on the same leaves as above with a portable fluorometer. The leaves were dark-adapted with clips for 20 min. The minimal fluorescence (Fo) was measured under a weak pulse of modulating light over 0.8s and maximal fluorescence (Fm) was induced by a saturating pulse of light (8000 mmol m-2s-1) applied over 0.8 s. Then, Fv/Fm was calculated, where Fv was the difference between Fm and Fo.
Leaf hydraulic conductance (kLeaf; mmol m-2s-1 MPa-1) for leaves was measured using the in situ evaporative flux method, with kL calculated as :
kleaf = E/ΔΨstem-leaf
Where E is the transpiration rate (mmol m-2s-1) and ΔΨstem-leaf is the difference between stem xylem water potential (Ψstem; MPa) and leaf water potential (Ψleaf; MPa).
Effects of fertilization (N) on the morphological and physiological parameters were tested by ANOVA. Tukey’s test was used to evaluate differences between the means (P ≤ 0.05). The relationship between net photosynthesis (A), leaf hydraulic conductance (KLeaf) and stomatal conductance (gs) was tested by linear regression.
For above ground plant tissue, our results show that the specific leaf area (SLA), leaf mass area (LMA) of the seedlings subjected to nitrogen fertiliser significantly increased when compared with non-N fertiliser (Figure 1) . Contrary observed an exponential reduction of the total leaf area for Quercus canariensis. The specific leaf area and leaf mass area of Q. suber L. increased significantly in the nitrogen fertilization condition (Figure 1). The small effect of nitrogen on photosynthetic parameters suggests that the leaf photosynthetic rate remained constant, which is in agreement with studies of Lambers et al.  and McDonald et al.  showing little dependence of leaf photosynthesis on nitrogen supply. Increasing the rate of nitrogen supply increased the SLA and the organic nitrogen concentration in the leaf tissue. Nitrogen is a vitally important element for plants and it pro-foundly influences leaf anatomical and functional traits .
Leaf hydraulic conductance (Kleaf) is a major determinant of photosynthetic rate in plants. Results indicate that leaf hydraulic conductance increased significantly under nitrogen treatment (Figure 2), no significant correlation was observed between KLeaf, A and gs (Figure 3). Previous studies have shown correlations between A, gs and KLeaf across a wide range of species [22,23]. The coupling of stomatal conductance (gs) to CO2 and water vapour leads to strong coordination between gs and Kleaf [24,25]. In the present study, A and KLeaf were not correlated with gs in the cork oak (Figure 3).
Figure 3: Relationships between net photosynthesis (A), leaf hydraulic conductance (KL) and stomatal conductance (gs) of cork oak (Q. suber L.) in control and nitrogen treatment condition. The values shown are mean ± SDand data were fitted by linear regression. Regression coefficients and significance are shown when P<0.05.
E, gs, A and FV/Fm differed from cork oak between sites. Nitrogen fertilization and source of cork oak no significantly influenced E, gs, A and Fv/Fm. Indeed A, E, FV/Fm and gs showed similar trends for cork oak (Q. suber L.) from the seven sources (Kroumirie and Cap-bon). The highest A values were measured for cork oak (Q. suber L.) in the nitrogen fertilization condition. Maximum A values (7.87 μmol CO2 m−2s−1) were found in MEJ, while the lowest in JCH with 3.76 μmol CO2 m−2s−1, respectively (Figure 4). Similarly, the highest E values were measured in MEJ, 1.80 mol H2O m−2s−1, the lowest in JCH, 1.05 mol H2O m−2s−1 (Figure 5).
The highest gs in the nitrogen fertilization condition was in AS (0.16 mmol m−2s−1) and the lowest in SDZ (0.1 mmol m−2s−1). The lowest values were registered from SDZ (0.06 mmol m−2s−1), while in AS gs approached the highest values (0.14 mmol m−2s−1), mostly for the control (Figure 6).
Chlorophyll fluorescence (Fv/Fm) not differed among seedling sources as well as among nitrogen fertilization treatment. Fv/Fm represents the maximum photochemical efficiency of PSII, which is an important index of the degree of environmental stress. Fv/Fm of each treatment slightly decreased when compared with the control (Figure 7). For all cork oak (Q. suber L.) sources, nitrogen fertilization treatments showed no effect in Fv/Fm, however, there were no significative differences in the induction of Fv/Fm among the seven seedling sources (Figure 7). Under well watered condition, quantum efficiency of photosystem II (Fv/Fm) ratio in bioregulators treated plants was not showing significant variation. Plants treated with N fertilizer maintained higher quantum yield of photosystem II (Fv/Fm) than the control just in JCH and AS seedling sources. Chlorophyll fluorescence dynamics are an ideal internal measure of photosystem injury from various stresses [26,27] and can rapidly and sensitively reflect the relationship between the environment and the plant’s photosynthetic physiological state.
The above-mentioned photosynthetic changes may play an important role in determining how cork oak plants adjust their photosynthetic. Photosynthetic traits decreased due to a biochemical and morphological acclimation and environmental constraints, such as water and nutrient supply [28,29]. Those changes are associated with decreases in the nitrogen concentrations and the increase of LMA due to the assimilation of photosynthetic products . Plants are therefore expected to make an optimal nitrogen allocation into leaf photosynthetic proteins to achieve the maximum carbon gain in their own light environment . These physiological changes are also associated to morphological modifications during stress, as for example a reduced leaf area .
Moreover, photosynthesis is aptly regarded as the most important physiological process in plants which is particularly sensitive to effects of water deficiency . Even a small decrease in the water potential of a plant causes its stomata to closeand decreases the intensity of photosynthetic assimilation of CO2 . The reduction in photosynthetic activity under stress takes place due to decline in CO2 availability caused by the restriction of CO2 diffusion  and inhibition of ribulose-1,5-bisphosphate (RuBP) synthesis [33,34]. Our results suggest that more detailed anatomical and structural studies are needed to elucidate the impacts of leaf feature traits on Kleaf and gas exchange in cork oak.
The present study was motivated by the need to examine the nitrogen treatment and genetic sources influences on photosynthetic traits variation among seedlings of cork oak (Quercus suber L.) sources to assist reforestation decisions. Our study revealed that (i) The specific leaf area (SLA) and leaf mass area (LMA) increased significantly in the nitrogen fertilization condition (ii) the seedling sources of Q. suber L. differed in maximum efficiency of PSII (Fv/Fm), photosynthetic rate (Anet), transpiration rate (E) and stomatal conductance (gs) (iii) the seedling sources demonstrated different responses of photosynthetic traits under nitrogen treatment, suggesting that the observed variation in growth among seed sources resulted largely from genetic variation in functional traits rather than from photosynthetic traits. Present results may have practical applications to help maximize physiological performances and optimized resource use efficiency but the physiological mechanisms need further elucidation. The genetic basis of photosynthesis affected photosynthetic traits in various plants. However, the genetic relationship sources and photosynthesis remains to be elucidated.