Variations of L. ruthenicum leaf functional traits in the lower reaches of Heihe River
In the long process of evolution, plants interact with the environment and gradually form adaptation strategies for internal physiology and external morphology to minimize the adverse effects of the environment. For example, leaf succulent, the so-called succulent plant refers to the proliferation of parenchyma cells in organs such as leaves and stems, the increase in the number of cells, the increase in volume, and the absorption and storage of large amounts of water, resulting in a significant increase in water content per unit weight or volume of tissue. These adapting plants to a phenomenon of poor environment and intra-plant traits vary with the environment, makes the plant can survive in a new environment . This study showed that the desert halophyte L. ruthenicum was characterized by low leaf SLA, LDMC, C, N and N:P levels, high LT, Suc, P content and C:N performance. SLA is one of the key leaf traits of plant carbon uptake strategy , it can reflect the distribution of plants and their adaptation to habitats . LDMC mainly reflects the ability of plants to retain nutrients . In addition, SLA and LDMC are the best variables for classifying plant species on the plant resource utilization classification axis . This paper showed that L. ruthenicum was a resource reservation species for its low SLA, N content yet high C:N, this also indicated that L. ruthenicum is in "slow-return" end of the spectrum: Plants that invest in high LMA have a slower photosynthetic rate, but the leaf life is longer, so their slower income (carbon absorption) rate can be compensated by a longer income stream [6,46]. Furthermore, SLA and LDMC are two important soil-fertility predictors as well as leaf N, P nutrient contents and N:P [15,47-49]. The combination of these predictors indicates that soil fertility is lacking in the Ejina desert area in the lower reaches of Heihe River and the growth of L. ruthenicum is mainly restricted by N. Prior studies that have noted the importance of C:N and C:P ratios can effectively reflect the balance between competitive and defensive strategies . When N and P contents were high, C:N and C:P ratios were comparatively low. Plants will apply competitive strategies at high photosynthetic rates. Conversely, when C content was high, C:N and C:P ratios were high. This moment plants adopted a strong defensive strategy under low photosynthetic rates [50-51], The results of this study indicated that L. ruthenicum had a defensive strategy under desert saline habitats. Leaf thickness (LT) is generally considered to be a very important leaf trait characteristic, which may be related to leaf life span, stress tolerance, and litter decomposition rate [52-53]. Osmond et al  found that plant leaves are thicker in nutrient-poor environments. This article LT pattern was consistent with previous researches. In order to adapt to the poor environment, the succulent plants proliferate in a large number of parenchyma cells, such as leaves and stems. In eight different habitats, L. ruthenicum showed a significant succulence (Suc) used to store moisture in arid less rainfall environments of Ejina desert areas. Eight L. ruthenicum communities had higher P content, may suggest that local minerals decompose faster meanwhile ensure that enough young leaves are produced to reduce the persecution of salt toxic ions in the soil. The leaves of L. ruthenicum belong to succulent foliage, and the higher the water content (TWC) of the succulent plant, the stronger the degree of tolerance to water stress and the more drought-tolerant . SLV is an important leaf trait introduced according to the leaf characteristics of desert plants. RWC reflects the resistance of plants to dehydration. The higher RWC leads to stronger resistance to dehydration and leaves have higher osmotic adjustment function.
Trade-off strategies among functional traits of L. ruthenicum
The existence of a fundamental trade-off between the rapid acquisition and the efficient conservation of resources has been discussed in the ecological literature for more than forty years . However, it is only over the course of the last two decades that the availability of large data sets has allowed for its precise quantification and for the identification of the trait syndromes that can be used to characterize trade-offs for a wide variety of plants [4,6,57]. Those strategies that have been proven include: Species with small SLA have thicker leaves or denser tissues , have been explained to allow maintenance of leaf function or delayed leaf death under very dry conditions . Some fundamental relationships found in leaf economics spectrum work: A significantly positive correlation was found between LT and Suc, which confirmed that succulent plant water conservation strategy. While a significantly negative correlation was found between LT and C content, this is related to the thicker LT causing a decrease in the specific leaf area and thus affecting carbon acquisition . SLA is a combination of leaf tissue density (LD) and leaf thickness (LT): leaf tissue density is significantly positively correlated with leaf dry matter content (LDMC), leading to the equation: SLA = 1/(LD×LT)≈1/(LDMC×LT) . This paper did not show a significant relationship between SLA and LT, but proved that SLA has a strongly negative correlation with LDMC and LD, the significantly negative correlation of LT and C as well as SLA (SLV) and LD (LDMC) indicated a trade-off between resource acquisition and resource conservation under drought and saline environment. LDMC and LD are positively correlated, both of which were significantly negatively correlated with TWC. Negative correlation of TWC, RWC and LDMC expressed as another trade-off between the intracellular water content and nutrient accumulation due to photosynthesis. Leaf water content is a useful indicator of plant water balance, Suc was significantly positively correlated with TWC, RWC and P content, but strongly negatively correlated with C. This study confirms that leaf succulence can improve the energy returns from leaf investment by replacing expensive C structures with water .
Does soil moisture and salinity affect plant functional traits?
In contrast to significant trait correlation patterns, there were only a few significant changes in the leaf morphological traits and C:N:P stoichiometry of desert halophytes with different salinity and moisture habitats. The distribution of salt in different soil layers might explain these results. In this paper, shallow soil layer (0-40 cm) water content significantly affected leaf P content but not C and N contents, also significantly affected leaf N:P, C:P because of higher leaf P content, this result supports evidence from previous observations, the effect of shallow soil moisture on leaf stoichiometry may be interpreted as rich applicability of soil N, while P nutrient element is unstable. In addition, shallow soil layer water content significantly affected the leaf RWC trait at the negative direction, and shallow soil water significantly promoted the leaf C:P ratio showed L. ruthenicum had a defensive life strategy. However, soil salinity significantly affected leaf LT, C, N contents and C:N ratio, positively with LT, C:N, negatively with leaf C and N contents. This may be a morphological and physiological evolutionary strategy for plants to cope with salt stress. In the deeper soil layer (40-80 cm), HCO3- significantly influenced leaf functional traits, deeper soil layer water contents significantly reduced leaf C and N contents, highly positively with C:N, but not obviously influenced leaf P content and other functional traits, while soil salinity only significantly reduced leaf N contents and highly positively with C:N and leaf morphological trait SLV. Thus, the hydraulic properties required for plant safety at high salinity are at the expense of lower growth rates . In general, salt stress causes the normal growth of plants to be inhibited to varying degrees, mainly because salt stress inhibits plant photosynthesis by reducing soil water potential and reducing leaf stomatal conductance [63-64]. Therefore, the fixed C ability of the blade will also be reduced significant, this was consistent with the low leaf C phenomenon in this study. Many studies have confirmed that under salt stress, especially the higher Cl- content in the soil will inhibit the plant's absorption of NO3-, so the NO3- content in the leaves of the plants will decrease [65-66], from this point of view, the N content of halophytes decreases as well as the C content. However, some other studies have shown that the N content of succulent plants becomes larger as the salinity increases , this controversy will require more research in the future to prove.
Salt stress limits the growth of halophytes through adverse effects on various physiological and biochemical processes. Conversely, halophytes shows a response to increased salinity by diversity growth . Salinisation consists of an accumulation of water soluble salts in the soil that include the ions of K+, Mg2+, Ca2+, Cl−, SO42−, CO32−, HCO3− and Na+. We tried to analysis this process with salt ions of different depths of soil. The RDA results showed that SWC, HCO3-, Cl-1 and CO32- can explain the variation of functional traits well. Surprisingly, Na+ was not in this rank. Because the importance of Cl- and Na+ has been mentioned in much salinity stress studies[67-69]. It was found that Na+ is more effective than K + and Cl- in cell swelling, leaf succulence and shoot development. According to our knowledge, the soluble salt in the lower reaches of the Heihe River Basin dominated by Na+, HCO3-, SO42-and Ca2+ , the above study may explain the results of our soil ion effects on functional traits.