Variations of L. ruthenicum leaf functional traits in the lower reaches of Heihe River
This study has showed that the desert halophyte L. ruthenicum is characterized by low leaf SLA, LDMC, C content, N content and N:P ratios, as well as high LT, Suc, P content and C:N ratios. SLA is one of the key leaf traits related to plant carbon uptake strategy , it could reflect the distribution of plants and their adaptation to different habitats . LDMC mainly reflects the ability of plants to retain nutrients . In addition, SLA and LDMC are proved to be the best variables for classifying plant species on the plant resource utilization classification axis . This paper showed that L. ruthenicum is a resource reservation species due to its lower SLA and N content, and higher C:N ratio, which also indicates that L. ruthenicum is in the "slow-return" end of the spectrum. Plants that invest in high LMA (Leaf mass per area) have a slower photosynthetic rate, but a longer leaf life. Therefore, their slower income (carbon absorption) rate can be compensated by a longer income stream [6,40]. Furthermore, SLA and LDMC are two important soil-fertility predictors in addition to leaf N and P contents and N:P ratios [15,41-43]. The combination of these predictors indicates that soil fertility is lower in the Ejina desert area in the lower reaches of the Heihe River and that the growth of L. ruthenicum is mainly restricted by N content. Prior studies have demonstrated the importance of C:N and C:P ratios, which play an important role in effectively reflecting the balance between competitive and defensive strategies . When N and P contents are higher, C:N and C:P ratios are comparatively lower. Plants will subject to competitive strategies at high photosynthetic rates. Conversely, high C content leads to high C:N and C:P ratios, showing how plants adopt a strong defensive strategy under low photosynthetic rates [44-45]. Results of this study indicate that L. ruthenicum has a flexible adaption strategies in different desert saline habitats: when soil salinity is higher, foliar N is lower, and the C: N ratio is large, a defensive strategy is adopted; when N contents are higher and the C:N ratio is lower, a competitive survival strategy is adopted. Leaf thickness (LT) is generally considered to be a very important leaf trait characteristic, which may connect with leaf life span, stress tolerance, and litter decomposition rate [46-47]. Osmond et al. found that plant leaves are generally thicker in nutrient-poor environments, the LT pattern presented by Osmond et al. is consistent with previous research . In order to adapt to harsh environments, succulent plants produce a large number of parenchyma cells, in organs such as the leaves and stems. In eight different habitats, L. ruthenicum shows a significant amount of succulence (Suc) used to store moisture in the arid and low-rainfall environments of the Ejina desert. The P content of all eight L. ruthenicum populations were higher than that of the 753 terrestrial plant species in China [13,24], showing a fast decomposition of local minerals to ensure sufficient production of young leaves thus to reduce toxic salt ions accumulation of each leaf. Leaves of L. ruthenicum belongs to the succulent foliage group, which shows enhanced drought-tolerance when the water content (TWC) of a succulent gets higher . SLV is an important leaf trait according to the leaf characteristics of desert plants. RWC reflects the resistance of plants towards dehydration: higher RWC leads to stronger resistance to dehydration, since the leaves have higher osmotic adjustment functions.
Trade-offs between 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 over forty years. However, it was only over the course of the last two decades that the availability of large data sets has allowed for the precise quantification and identification of the trait syndromes that can be used to characterize trade-offs for a wide variety of plants. For example, species with small SLA have thicker leaves or denser tissues, which allows for the maintenance of leaf function or the delaying leaf death under very dry conditions.
Some fundamental relationships found in leaf economics spectrum research include a significantly positive correlation between LT and Suc, which confirms that succulent plants employ a water conservation strategy . While a significantly negative correlation has been found between LT and C content, this can be related to the fact that thicker leaves cause a decrease in the SLA which affects carbon acquisition . Some literatures report that SLA is actually a combination of leaf tissue density (LD) and leaf thickness (LT), since leaf tissue density is significantly positive correlated with leaf dry matter content (LDMC), leading to a equation: SLA = 1/(LD×LT)≈1/(LDMC×LT) . This paper did not show a significant relationship between SLA and LT, but demonstrated that SLA had a strongly negative correlation with LDMC and LD. The significantly negative correlation between LT and C content, as well as between SLA (SLV) and LD (LDMC), indicates a trade-off between resource acquisition and resource conservation under drought and saline conditions.
LDMC and LD are positively correlated, with both being significantly negative correlated with TWC. Negative correlation of TWC, RWC and LDMC expresses another trade-off between the intracellular water content and nutrient accumulation due to photosynthetic CO2 assimilation, showing that leaf water content is a useful indicator of plant water balance. Suc is significantly positive correlated with TWC, RWC and P content, but strongly negatively correlated with C content. This confirms that leaf succulence can improve the energy returns from leaf investment by replacing expensive carbon structures with water .
To what extent does soil moisture and salinity affect leaf functional traits?
In contrast to significant trait correlation patterns, there are only a few significant differences in the leaf morphological traits and C:N:P stoichiometry of desert halophytes with different salinity and moisture habitats. In this paper, we found that SWC and HCO3- in shallow soil layers is a good predictor of leaf traits. Between them, SWC has larger contributions to leaf P content, N:P ratios and C:P ratios while HCO3- has the greatest impact on LDMC, these can be inferred from previous research: in desert ecosystems, lower SWC coupled with higher soil alkalinity acts to decrease both soil N and P availability . Due to this, SWC has a great impact on the levels of leaf P and N:P, and HCO3- affects the production of leaf dry matter content. The result was supported by other observations .The changing C:P pattern along environmental gradients suggested that L. ruthenicum had a flexible life strategy under different environments. In the deeper soil layer, HCO3-, followed by SO42-, mainly influences leaf functional traits. In the RDA diagram, deep soil SWC has a negative effect on leaf N content and N:P, but has a positive effect on leaf C:N. SWC does not obviously influence other functional traits. At the same time, the effects of soil salinity also converged with SWC. It can be concluded the hydraulic properties required for plant safety at higher salinity are at the expense of lower growth rates . People already know a lot about the effects of salt stress on plants. The common sense is that salt stress reduces some transaminase activities, reduces plant N content, and damages plant growth . Therefore, the carbon fixation ability of the blade will also be reduced significantly, which is consistent with the low leaf C content phenomenon shown in this paper. Many studies have confirmed that salt stress, especially chloride salt stress, will inhibit plant's NO3- absorption, so the NO3- content in a plant’s leaves will decrease during salt stress [56-57]. However, some other studies have shown that the N content of succulent plants becomes larger as the salinity increases . This discrepancy will require additional research in the future to resolve.
Salt stress limits the growth of halophytes through adverse effects on various physiological and biochemical processes. Conversely, halophytes respond to increased salinity by expanding in diversity . Salinization consists of an accumulation of water-soluble salts in the soil, including the ions of K+, Mg2+, Ca2+, Cl−, SO42−, CO32−, HCO3− and Na+. We tried to analyze this process using salt ions at different depths of soil. The RDA results show that SWC, HCO3-, CO32-, SO42- and Cl- can explain the variation of functional traits well. Surprisingly, Na+ content could not explain the variation significantly, despite the importance of Cl- and Na+ as mentioned in many salt stress studies [58-60]. According to our current knowledge, the soluble salts in the lower reaches of the Heihe River Basin are mainly Na+, HCO3-, SO42-and Ca2+ . However, there are few studies showing how these ions affect leaf functional traits and trade-off strategies, which may become our future research focus.