1 INTRODUCTION

Plants usually face acute desiccation in arid environments that are characterized by few rainfall events and massive circadian fluctuations in temperature (Brodribb et al., 2020; Choat et al., 2012; Schreel & Steppe, 2019; Yao et al., 2020, 2021). Therefore, forms of occult precipitation, such as dewfall, which naturally condenses on leaf surfaces in the form of atmospheric moisture at dawn and occurs frequently during the growing season (Agam & Berliner, 2006; Hao et al., 2012; Schreel & Steppe, 2019), are viable water resources for plant survival in these harsh environments (Jacobs et al., 1999; Schreel & Steppe, 2019; Zhang et al., 2015). Accordingly, plants have evolved unique structures to retain and absorb dew (Berry et al., 2019; Malik et al., 2015), such as the water-collecting leaf hairs (awns) of Syntrichia caninervis (Pan et al., 2016), the spines of the cactus Opuntia microdasys (Ju et al., 2012) and the leaf trichomes of Combretum leprosum (Pina et al., 2016). These specific tissues are regarded as morphological structures adapted to absorb dew that might decrease the probability of the leaf water potential (Ψ_leaf) reaching the lower threshold value and contribute to plant survival in arid environments (Eller et al., 2016; Kim et al., 2017; Schreel et al., 2020). Therefore, illustrating the effects of foliar dew uptake by trichomes on leaf hydraulic conductivity and gas exchange would provide insight into plant drought resistance.

It has been shown that under drought conditions, leaf hydraulic conductance (K_leaf) is lost either from xylem tissue cavitation and embolus formation (embolism) (Brodribb et al., 2014, 2016) and/or from tissues outside of the xylem (Scoffoni et al., 2017; Yao et al., 2021). A decrease in K_leaf results in a decrease of photosynthetic rate and ultimately plant growth and survival, and the Ψ_leaf at 50% K_leaf loss (K_leaf P₅₀) is regarded as an essential indicator of species resistance to drought (Blackman et al., 2014; Scoffoni & Sack, 2017). Indeed, the K_leaf P₅₀ has been shown to be closely related to the mean annual precipitation (MAP), with species growing in the arid environments usually exhibiting stronger drought resistance (more negative values of K_leaf P₅₀) than species growing in humid environments (Blackman et al., 2012, 2014; Nardini & Luglio, 2014; Yao et al., 2021). Moreover, atmospheric moisture absorption from aerial organs may alleviate drought stress. Indeed, dew absorption by leaf trichomes is widely observed in species growing in arid environments (Berry et al., 2019; Limm et al., 2009; Schreel & Steppe, 2019). Thus, foliar dew absorption may prevent a rapid decrease in Ψ_leaf, K_leaf, and gas exchange and contribute to plant drought resistance by inducing a lower relative soil water content (RSWC) at 50% loss of leaf hydraulic conductance (K_leaf RSWC₅₀) during dehydration without changing the K_leaf P₅₀.

The genus Caragana comprises deciduous perennial shrubs or small trees that are widely distributed across Eastern Europe and temperate Asia (Fang et al., 2014; Yao et al., 2021). They have important ecological and economic value, including playing a key role in vegetation succession from shifting dunes to sandy grasslands, helping to restore degraded land by fixing atmospheric nitrogen, forming shelterbelts for crops and pastures, serving as supplemental livestock forage (leaves and flowers), providing fuel (re-sprouted shoots) for local farmers and aiding in preventing desertification and soil erosion (Fang et al., 2008, 2013). C. korshinskii, one of the most important drought-resistant species in this genus, is widely distributed in desert areas where the MAP is less than 200 mm and has long, dense trichomes on both leaf sides. Its congener, C. sinica, which has no leaf trichomes, is distributed in humid environments, where the MAP is more than 1400 mm (Fang et al., 2011, 2017). In this study, both species were studied to gain insight into the role of leaf trichomes in absorbing dew and ameliorating water relations, K_leaf and gas exchange. We hypothesized that the species that grow in arid environments (C. korshinskii) would use leaf trichomes to absorb dew efficiently to prevent a rapid decrease in Ψ_leaf, K_leaf, and gas exchange, which would lower the soil water content level at which 50% K_leaf (K_leaf RSWC₅₀), photosynthesis (A RSWC₅₀), and stomatal conductance (g_s RSWC₅₀) loss occurred in response to dehydration. We also hypothesized that a closely related species that grows in humid environments (C. sinica) would have higher values of K_leaf RSWC₅₀, A RSWC₅₀, and g_s RSWC₅₀.

2 MATERIALS AND METHODS

2.1 Experimental site and plant materials

The experiment was conducted at Lanzhou University, Lanzhou, Gansu Province, China (Yuzhong campus, 35°51′N, 104°07′S, altitude 1620 m) in naturally lit glasshouses with average environmental conditions (temperature, 23 ± 2°C; humidity, 40–60%). We selected two Caragana species that occur in regions with dramatically different MAP. C. korshinskii grows widely in arid environments. It has pinnate compound leaves with 12–16 leaflets (Figure 1), and its leaves are covered with dense trichomes on both the adaxial (upper) and abaxial (lower) surfaces. C. sinica grows in humid environments and has four leaflets and no trichomes (Figure 1).

In June 2017, seeds of C. korshinskii were collected from wild populations at physiological maturity when the pods changed color from green to brown. After collection, ripe pods were spread out in the laboratory at 20°C until they opened, and the seeds were removed. The seeds were stored at 4°C until the start of the experiment. In February 2018, seeds were placed in Petri dishes lined with wet filter paper for germination at 20°C. After germination, when the seedling roots were approximately 0.3 mm long, three seedlings were transferred into each of 100 plastic pots for each species, that were 270 mm high and 170 mm in diameter and contained 3.6 kg of a 1:1 (v:v) mixture of sieved peat soil and Perlite. C. sinica does not form seeds, so C. sinica tissue was cultured from more than 50 individuals as described by Yao et al. (2021). Then, the tissue-cultured seedlings of C. sinica were moved to pots (one individual per pot, 100 pots). All pots received ample amount of water and nutrient supply. Eighty to ninety percent of RSWC was maintained by weighing the pots and replacing the lost water every 2 days.

3 RESULTS

There was no significant difference in trichome length between the well-watered and drought-stressed plants of C. korshinskii, but trichome density on both the adaxial and abaxial leaf surfaces in drought-stressed plants increased by approximately 50 and 25% compared with that in well-watered plants, respectively (Figure 2). To test whether the trichomes absorbed dew, we spread dew on C. korshinskii leaf surfaces and observed dew absorption using digital microscopy. The observations revealed that the dew drops were absorbed rapidly by the C. korshinskii leaf trichomes; the large water droplets became smaller quickly and absorbed, within approximately 20 s (Figure 3).

In well-watered C. korshinskii plants, Ψ_pd, Ψ_mor, and Ψ_md were approximately −0.5, −1.1, and − 1.8 MPa, respectively. Similar values were also observed in C. sinica (Figure 4). With the decrease in RSWC, the Ψ_leaf dropped gradually, but the Ψ_pd decreased more slowly in plants with dew than in plants without dew in C. korshinskii. Thus, the Ψ_pd in plants with dew was about −2.6 MPa higher than that in plants without dew when the RSWC decreased below 5% (Figure 4, Table 1). A similar effect was also observed in the Ψ_mor (−3.5 MPa in the plants with dew vs. −5.9 MPa in the plants without dew) and in the Ψ_md (−5.3 MPa in the plants with dew vs. −6.2 MPa in the plants without dew) (Figure 4, Table 1). The generalized linear model demonstrated that the difference between the regression lines of plants with dew (DS & D) and those without dew (DS & ND) was significant (Figure 4). In C. sinica, dew improved the Ψ_pd in plants, but the degree of increase was far less than that observed in C. korshinskii, and the Ψ_mor and Ψ_md were similar in C. sinica plants with dew and without dew with decreasing RSWC (Figure 4, Table. 1).

In C. korshinskii, the K_leaf declined more slowly in plants with dew than in those without dew, and a significant difference was observed between the regression lines. When the RSWC was less than 5%, the K_leaf was about 8 mmol m⁻² s⁻¹ MPa⁻¹ in plants with dew and approximately zero in plants without dew (Figure 5, Table 1). However, the effect of dew on the K_leaf in C. sinica was not observed, and the K_leaf was almost the same between C. sinica plants with dew and without dew (Figure 5, Table. 1). The dew also markedly ameliorated the gas exchange parameters of C. korshinskii, with A of 7.4 μmol m⁻² s⁻¹ and g_s of 0.15 mol m⁻² s⁻¹ at 5% RSWC, while those in plants with no dew decreased to approximately zero. In C. sinica, no significant difference in A and g_s was observed between the DS & D and DS & ND treatments, and both A and g_s decreased to zero. Thus, the K_leaf RSWC₅₀, A RSWC_50, and g_s RSWC₅₀ were 7.6, 13.5, and 19.4% in DS & D and 20.5, 26.5, and 35.5% in DS & ND in C. korshinskii, respectively_. In C. sinica, both A and g_s were more sensitive to decreasing RSWC than in C. korshinskii, and the K_leaf RSWC₅₀, A RSWC₅₀ and g_s RSWC₅₀ were almost the same between DS & D and DS & ND (Figure 5, Table 2). The K_leaf P₅₀ was about −2.7 and − 1.7 MPa in C. korshinskii and C. sinica, respectively, in DS & ND treatment, and dew absorption had no effect on the K_leaf P₅₀ in either species (Figure 6; Table 2).

4 DISCUSSION

In arid environments, plants consistently face environmental challenges, such as water scarcity (Brodribb et al., 2020; Choat et al., 2012; Yao et al., 2021) Therefore, plants have evolved diverse coping strategies to adapt to these conditions, including developing deeper root architecture to maximize water absorption from soil (Gibbens & Lenz, 2001), having smaller leaf surface area to minimize water loss through transpiration and exhibiting rapid stomatal closure (Ahammed, Li, Mao, et al., 2020; Ahammed, Li, Wan, et al., 2020; Ahammed, Li, Yang, et al., 2020; Li et al., 2019; Li et al., 2020; Yao et al., 2020). Other structural modifications in the form of epidermal appendages designed to collect atmospheric water, such as the unique conical spines on cactus stems, the awns of desert mosses (Ju et al., 2012; Pan et al., 2016), and the leaf trichomes in numerous xeromorphic plant species (Huttunen et al., 2010; Ning et al., 2016; Schreel et al., 2020). The species inhabiting arid environments usually have dense foliar trichomes, as evident by the leaf trichomes of Phlomis fruticosa, Plectranthus australis, Acacia verniciflua, Solanum pennellii, Combretum leprosum, Arabidopsis lyrata, Arabidopsis kamchatica, Caragana korshinskii, and Conocarpus lancifolius (Huttunen et al., 2010; Ning et al., 2016; Pina et al., 2016; Redha et al., 2011; Sletvold & Agren, 2012). The same results were also observed in the Caragana genus in this study; C. korshinskii, which inhabits regions where precipitation is less than 200 mm/year, exhibited dense trichomes on both its adaxial and abaxial leaf surfaces.

Trichomes can be regarded as one of the superficial evolutionary structures with which plants contend with hostile environments because of their numerous ecophysiological functions, such as protecting the plant from ultraviolet (UV) radiations (Karabourniotis & Bornman, 1999), reducing heat loading (Skelton et al., 2012), discharging salts in saline environments, acting as water storehouses and producing hydrophilic substances (Paiva & Martins, 2011). Apart from these functions, there is evidence of foliar water uptake in an increasing number of species, for example, conifers (Breshears et al., 2008; Simonin et al., 2009), herbaceous vegetation (Zhuang & Zhao, 2010), broadleaf trees (Yan et al., 2015), and diverse species in multiple biomes, from tropical montane cloud forests to mangrove forests (Steppe et al., 2018) and desert ecosystems (Yan et al., 2015). In this study, we found that the leaf trichomes of C. korshinskii facilitate foliar water uptake as an adaptive strategy for acquiring water from alternative sources in arid environments. Similar results have also been documented in Tillandsia ionantha, Syntrichia caninervis, and Combretum leprosum, which generally dominate arid and semiarid biomes (Ohrui et al., 2007; Pan et al., 2016; Pina et al., 2016).

Foliar water absorption not only contributes to maintaining the leaf water status but also allows substantial water to pass through the branches and stems towards the belowground components through water-conducting xylem (Burgess & Dawson, 2004; Eller et al., 2013), forming an upside-down water transport system. The results of this study show a close relationship between absorption of dew and a slow decrease in Ψ_leaf similar to that reported in other plant species (Breshears et al., 2008; Pina et al., 2016; Zhuang & Zhao, 2010). Our results, in combination with previously published results, emphasize that absorption dew is a viable water resource for plant survival in harsh environments. In beech (Fagus sylvatica), foliar trichomes, which exhibit strong hygroscopic properties as a result of their structural and chemical design, constitute a major foliar water uptake pathway, while foliar water uptake is less driven by cuticular uptake and does not occur through the stomata (Schreel et al., 2020). In this study, we suggest that dew is mainly absorbed by trichomes rather than by cuticles or stomata due to (1) the rapid absorption of dew drops by C. korshinskii leaf trichomes observed under the digital microscope (Figure 2); (2) the amelioration of K_leaf and gaseous exchange with decreasing RSWC under drought and dew treatments in C. korshinskii but not in C. sinica, which does not have leaf trichomes (Figures 4 and 5); these results suggest that water was mainly absorbed via the trichomes, not via the stomata or cuticles; and (3) xerophytes being more likely than species in humid environments to have thick cuticles, which suggests that the chance of dew absorption by cuticles is relatively low in C. korshinskii growing in arid environments. Indeed, some experimental studies have explored how water is absorbed into the trichomes by capillary action, diffuses into leaf tissues with the aid of several aquaporin (water channel) proteins (Schreel et al., 2020), and moves intracellularly through apoplastic or symplastic pathways (Buckley, 2015; Chrispeels & Agre, 1994). In drought-stressed C. korshinskii plants, trichome density on both the adaxial and abaxial leaf surfaces increased greatly compared with that in well-watered plants, which might arise from drought-induced leaf shrinkage (Scoffoni et al., 2014), would facilitate foliar water uptake greatly and efficiency.

Intensifying soil water deficits induce a more negative Ψ_leaf, which causes a deterioration in xylem tension that leads to hydraulic dysfunction and consequent decreases in K_leaf (Brodribb et al., 2014; Scoffoni & Sack, 2017; Yao et al., 2021). Plant hydraulic failure acts as a primary physiological driver of drought-driven plant mortality by completely disconnecting the pathway between soil water and leaves (Brodribb et al., 2020; Choat et al., 2012). Recently, some studies showed that rehydration through water uptake from the shoot surface induced hydraulic recovery after drought-induced embolism repair (Earles et al., 2016; Fuenzalida et al., 2019; Mayr et al., 2014); these findings suggest that the role of foliar water absorption in contributing to the maintenance of hydraulic conductivity, which, in turn, mediates the functionality of gas exchange. Here, the outcomes of our investigation revealed that dew absorption through trichomes changed the Ψ_leaf in C. korshinskii and that slowly decreasing Ψ_leaf had a positive influence on the plant hydraulic system by slowing K_leaf loss as well as by inducing a lower K_leaf RSWC₅₀ during soil water deficiency (Figure 7). According to the vulnerability segmentation hypothesis, the distal plant parts (leaves) are more vulnerable to hydraulic dysfunction than the basal parts (stem and root) (Creek et al., 2018; Scholz et al., 2014; Tyree et al., 1993). Here, we also exposed another function of leaves, that is, reducing the risk of stem hydraulic dysfunction under severe drought stress by absorbing dew through trichomes to improve the leaf water status. In addition, foliar water uptake may also mitigate the greater evaporative loads induced by the high vapor pressure deficits that occur in a daily cycle in arid environments (Scoffoni et al., 2011).

It has been shown that the ability of plants to withstand leaf hydraulic dysfunction shapes species distributions across precipitation gradients (Blackman et al., 2012, 2014; Nardini & Luglio, 2014; Yao et al., 2021). Our current study showed that Caragana species growing in arid environments achieved more negative K_leaf P₅₀ values than Caragana species growing in humid regions (Yao et al., 2021). The results were consistent with findings that K_leaf P₅₀ values are more negative in species growing in lower-rainfall environments (Blackman et al., 2009, 2012, 2014). However, none of the studies considered leaf dew absorption, which occurs widely in arid environments. Indeed, with regard to the role of dew in leaf hydraulics, as leaf wetting events occur >100 days/year in various ecosystems (Dawson & Goldsmith, 2018), the importance of foliar water uptake in the rehydration of leaves cannot be ignored (Breshears et al., 2008; Schreel & Steppe, 2019). Here, we clearly observed that dew absorption amends leaf hydraulic functions during dehydration. It has been shown that stems and roots can be rehydrated by the redistribution of water absorbed by the leaves and that this redistributed water can relieve tension on the water column (Steppe et al., 2018) and may coincide with whole-plant xylem cavitation recovery. The dew absorption maintains leaf hydraulic function during dehydration and does not induce a more negative K_leaf P₅₀. Indeed, it induces lower K_leaf RSWC₅₀ as well as A RSWC₅₀ and g_s RSWC₅₀ by decoupling the leaf water status from soil water availability. The results suggest that leaf water uptake in C. korshinskii can alleviate drought stress, which broadens the distribution of this species and allows it to grow in more arid regions that experience non-rainfall events such as fog and dew rather than relying on soil water availability alone. However, C. sinica, which grows in humid areas and has no trichomes, showed poor foliar water uptake. The collection of occult precipitation during water shortages is an influential mechanism enabling plants to escape drastic hydraulic stress during the dry season; therefore, we should consider foliar water uptake when testing plant drought tolerance in arid environments.

5 CONCLUSIONS

The ability of C. korshinskii trichomes to absorb dew slowed the decrease in Ψ_leaf, K_leaf^, and gas exchange with decreasing soil water content and resulted in lower K_leaf RSWC₅₀, A RSWC₅₀^, and g_s RSWC₅₀ than were observed in the absence of dew. Its congener C. sinica, which does not have trichomes, displayed little ability to absorb dew under drought stress and did not show improvement in the above parameters. The results here provide new insights into the adaptability of species to arid environments. Further work is needed to elucidate the water pathway from the epidermal trichomes of C. korshinskii to the entire plant, and dew absorption should be tested in additional species growing along precipitation gradients.

AUTHOR CONTRIBUTIONS

XWF designed experiments, XWF and YX revised manuscript. GQY and MMH performed the data analysis. ZFN partially involved in field experiments. MW had primary responsibility for experiment conduction and writing the manuscript. All authors have read and approved the final manuscript.