A Proposed Method for Simultaneous Measurement of Cuticular Transpiration From Different Leaf Surfaces in Camellia sinensis

Revisiting plant cuticle biophysics

The plant cuticle is located at the interface of the plant with the environment, thus acting as a protective barrier against biotic and abiotic external stress factors, and regulating water loss. Additionally, it modulates mechanical stresses derived from internal tissues and also from the environment. Recent advances in the understanding of the hydric, mechanical, thermal, and, to a lower extent, optical and electric properties of the cuticle, as well as their phenomenological connections and relationships are reviewed. An equilibrium based on the interaction among the different biophysical properties is essential to ensure plant growth and development. The notable variability reported in cuticle geometry, surface topography, and microchemistry affects the analysis of some biophysical properties of the cuticle. This review aimed to provide an updated view of the plant cuticle, understood as a modification of the cell wall, in order to establish the state-of-the-art biophysics of the plant cuticle, and to serve as an inspiration for future research in the field.

Amphistomy: stomata patterning inferred from 13C content and leaf-side-specific deposition of epicuticular wax

BACKGROUND AND AIMS

The benefits and costs of amphistomy (AS) vs. hypostomy (HS) are not fully understood. Here, we quantify benefits of access of CO2 through stomata on the upper (adaxial) leaf surface, using 13C abundance in the adaxial and abaxial epicuticular wax. Additionally, a relationship between the distribution of stomata and epicuticular wax on the opposite leaf sides is studied.

METHODS

We suggest that the 13C content of long-chain aliphatic compounds of cuticular wax records the leaf internal CO2 concentration in chloroplasts adjacent to the adaxial and abaxial epidermes. This unique property stems from: (1) wax synthesis being located exclusively in epidermal cells; and (2) ongoing wax renewal over the whole leaf lifespan. Compound-specific and bulk wax 13C abundance (δ) was related to amphistomy level (ASL; as a fraction of adaxial in all stomata) of four AS and five HS species grown under various levels of irradiance. The isotopic polarity of epicuticular wax, i.e. the difference in abaxial and adaxial δ (δab - δad), was used to calculate the leaf dorsiventral CO2 gradient. Leaf-side-specific epicuticular wax deposition (amphiwaxy level) was estimated and related to ASL.

KEY RESULTS

In HS species, the CO2 concentration in the adaxial epidermis was lower than in the abaxial one, independently of light conditions. In AS leaves grown in high-light and low-light conditions, the isotopic polarity and CO2 gradient varied in parallel with ASL. The AS leaves grown in high-light conditions increased ASL compared with low light, and δab - δad approached near-zero values. Changes in ASL occurred concomitantly with changes in amphiwaxy level.

CONCLUSIONS

Leaf wax isotopic polarity is a newly identified leaf trait, distinguishing between hypo- and amphistomatous species and indicating that increased ASL in sun-exposed AS leaves reduces the CO2 gradient across the leaf mesophyll. Stomata and epicuticular wax deposition follow similar leaf-side patterning.

Lipids: A noteworthy role in better tea quality

New shoots from tea plants (Camellia sinensis) are changed into finished tea after the process, which endows the products with a characteristic flavor. Tea quality is reflected in all aspects, from new shoots to the finished tea that are affected by cultivar, cultivation condition, harvest season, manufacturing methods, and quality of fresh tea leaves. Lipids are hydrophobic metabolites connected with tea flavor quality formation. Herein, we emphasize that the lipids composition in preharvest tea leaves is crucial for materials quality and hence tea flavor. The characterization of lipids in preharvest tea leaves provides a reference to obtain better tea quality. Lipids transformation in postharvest stages of tea leaves differs from varieties of tea types, and lipid oxidations functions in the tea flavor formation. A comprehensive overview of the lipids in tea leaves of preharvest and postharvest stages is necessary to improve tea quality.

Measuring residual transpiration in plants: a comparative analysis of different methods

Residual transpiration (RT) is defined as a loss of water through the leaf cuticle while stomata are closed. Reduced RT might be a potentially valuable trait for improving plant performance under water deficit conditions imposed by either drought or salinity. Due to the presence of stomata on the leaf surface, it is technically challenging to measure RT. RT has been estimated by the water loss through either astomatous leaf surface or isolated astomatous cuticular layers. This approach is not suitable for all species (e.g. not applicable to grasses) and is difficult and too time consuming for large-scale screening in breeding programs. Several alternative methods may be used to quantify the extent of RT; each of them comes with its own advantages and limitations. In this study, we have undertaken a comparative assessment of eight various methods of assessing RT, using barley (Hordeum vulgare ) plants as a model species. RT measured by water retention curves and a portable gas exchange (infrared gas analyser; IRGA) system had low resolution and were not able to differentiate between RT rates from young and old leaves. Methods based on quantification of the water loss at several time-points were found to be the easiest and least time-consuming compared to others. Of these, the 'three time-points water loss' method is deemed as the most suitable for the high throughput screening of plant germplasm for RT traits.

Sustained Agricultural Spraying: From Leaf Wettability to Dynamic Droplet Impact Behavior

Crop production and quality safety system have the potential to nurture human health and improve environmental sustainability. Providing a growing global population with sufficient and healthy food is an immediate challenge. However, this system largely depends on the spraying of agrochemicals. Crop leaves are covered with different microstructures, exhibiting distinct hydrophilic, hydrophobic, or even superhydrophobic wetting characteristics, thus leading to various deposition difficulties of sprayed droplets. Here, the relationship between wettability and surface microstructure in different crop leaves from biological and interfacial structural perspectives is systematically demonstrated. A relational model is proposed in which complex microstructures lead to stronger leaf hydrophobicity. And adding surfactant with a faster dynamically migrating velocity and reducing droplet size can improve agrochemical precise deposition. These contribute toward highly accurate and efficient targeted applications with fewer agrochemicals use and promote sustainable models of eco-friendly agriculture systems.

Massive increases in C31 alkane on Zygophyllum xanthoxylum leaves contribute to its excellent abiotic stress tolerance

BACKGROUND AND AIMS

Desert plants possess excellent water-conservation capacities to survive in extreme environments. Cuticular wax plays a pivotal role in reducing water loss through plant aerial surfaces. However, the role of cuticular wax in water retention by desert plants is poorly understood.

METHODS

We investigated leaf epidermal morphology and wax composition of five desert shrubs from north-west China and characterized the wax morphology and composition for the typical xerophyte Zygophyllum xanthoxylum under salt, drought and heat treatments. Moreover, we examined leaf water loss and chlorophyll leaching of Z. xanthoxylum and analysed their relationships with wax composition under the above treatments.

KEY RESULTS

The leaf epidermis of Z. xanthoxylum was densely covered by cuticular wax, whereas the other four desert shrubs had trichomes or cuticular folds in addition to cuticular wax. The total amount of cuticular wax on leaves of Z. xanthoxylum and Ammopiptanthus mongolicus was significantly higher than that of the other three shrubs. Strikingly, C31 alkane, the most abundant component, composed >71 % of total alkanes in Z. xanthoxylum, which was higher than for the other four shrubs studied here. Salt, drought and heat treatments resulted in significant increases in the amount of cuticular wax. Of these treatments, the combined drought plus 45 °C treatment led to the largest increase (107 %) in the total amount of cuticular wax, attributable primarily to an increase of 122 % in C31 alkane. Moreover, the proportion of C31 alkane within total alkanes remained >75 % in all the above treatments. Notably, the water loss and chlorophyll leaching were reduced, which was negatively correlated with C31 alkane content.

CONCLUSION

Zygophyllum xanthoxylum could serve as a model desert plant for study of the function of cuticular wax in water retention because of its relatively uncomplicated leaf surface and because it accumulates C31 alkane massively to reduce cuticular permeability and resist abiotic stressors.

KLU/CYP78A5, a Cytochrome P450 Monooxygenase Identified via Fox Hunting, Contributes to Cuticle Biosynthesis and Improves Various Abiotic Stress Tolerances

Acquired osmotolerance after salt stress is widespread among Arabidopsis thaliana (Arabidopsis) accessions. Most salt-tolerant accessions exhibit acquired osmotolerance, whereas Col-0 does not. To identify genes that can confer acquired osmotolerance to Col-0 plants, we performed full-length cDNA overexpression (FOX) hunting using full-length cDNAs of halophyte Eutrema salsugineum, a close relative of Arabidopsis. We identified EsCYP78A5 as a gene that can confer acquired osmotolerance to Col-0 wild-type (WT) plants. EsCYP78A5 encodes a cytochrome P450 monooxygenase and the Arabidopsis ortholog is known as KLU. We also demonstrated that transgenic Col-0 plants overexpressing AtKLU (AtKLUox) exhibited acquired osmotolerance. Interestingly, KLU overexpression improved not only acquired osmotolerance but also osmo-shock, salt-shock, oxidative, and heat-stress tolerances. Under normal conditions, the AtKLUox plants showed growth retardation with shiny green leaves. The AtKLUox plants also accumulated higher anthocyanin levels and developed denser cuticular wax than WT plants. Compared to WT plants, the AtKLUox plants accumulated significantly higher levels of cutin monomers and very-long-chain fatty acids, which play an important role in the development of cuticular wax and membrane lipids. Endoplasmic reticulum (ER) stress induced by osmotic or heat stress was reduced in AtKLUox plants compared to WT plants. These findings suggest that KLU is involved in the cuticle biosynthesis, accumulation of cuticular wax, and reduction of ER stress induced by abiotic stresses, leading to the observed abiotic stress tolerances.

The Tea Plant Leaf Cuticle: From Plant Protection to Tea Quality

Camellia sinensis (tea tree) is a perennial evergreen woody crop that has been planted in more than 50 countries worldwide; its leaves are harvested to make tea, which is one of the most popular nonalcoholic beverages. The cuticle is the major transpiration barrier to restrict nonstomatal water loss and it affects the drought tolerance of tea plants. The cuticle may also provide molecular cues for the interaction with herbivores and pathogens. The tea-making process almost always includes a postharvest withering treatment to reduce leaf water content, and many studies have demonstrated that withering treatment-induced metabolite transformation is essential to shape the quality of the tea made. Tea leaf cuticle is expected to affect its withering properties and the dynamics of postharvest metabolome remodeling. In addition, it has long been speculated that the cuticle may contribute to the aroma quality of tea. However, concrete experimental evidence is lacking to prove or refute this hypothesis. Even though its relevance to the abiotic and biotic stress tolerance and postharvest processing properties of tea tree, tea cuticle has long been neglected. Recently, there are several studies on the tea cuticle regarding its structure, wax composition, transpiration barrier organization, environmental stresses-induced wax modification, and structure-function relations. This review is devoted to tea cuticle, the recent research progresses were summarized and unresolved questions and future research directions were also discussed.

Water Sorption and Desorption of Isolated Cuticles From Three Woody Species With Focus on Ilex aquifolium

The cuticle is a lipid-rich layer that protects aerial plant organs against multiple stress factors such as dehydration. In this study, cuticle composition and structure in relation to water loss are examined in a broad ecophysiological context, taking into consideration leaf age and side from Ilex aquifolium (holly) in comparison with Eucalyptus globulus (eucalypt) and Prunus laurocerasus (cherry laurel). Enzymatically isolated cuticular membranes from holly leaves were studied under three treatment conditions: natural (no chemical treatment), after dewaxing, and after methanolysis, and the rate of water loss was assessed. Structural and chemical changes were evaluated using different microscopy techniques and by Fourier transform infrared (FTIR) spectroscopy. The potential mechanisms of solute absorption by holly leaves were additionally evaluated, also testing if its prickly leaf margin may facilitate uptake. The results indicate that the treatment conditions led to structural changes, and that chemical composition was hardly affected because of the occurrence of cutan. Structural changes led to more hydrophilic adaxial surfaces, which retained more water and were more efficient than natural cuticles, while changes were not significant for abaxial surfaces. Across natural cuticles, age was a significant factor for eucalypt but not for holly. Young eucalypt cuticles were the group that absorbed more water and had the lowest water loss rate. When comparing older leaf cuticles of the three species, cherry laurel was found to absorb more water, which was, however, lost more slowly, compared with the other species. Evidence was gained that holly leaves can absorb foliar-applied solutes (traced after calcium chloride application) through the adaxial and abaxial surfaces, the adaxial mid veins, and to a lower extent, the spines. In conclusion, for the species examined, the results show variations in leaf cuticle composition and structure in relation to leaf ontogeny, and water sorption and desorption capacity.

Leaf Cuticular Transpiration Barrier Organization in Tea Tree Under Normal Growth Conditions

The cuticle plays a major role in restricting nonstomatal water transpiration in plants. There is therefore a long-standing interest to understand the structure and function of the plant cuticle. Although many efforts have been devoted, it remains controversial to what degree the various cuticular parameters contribute to the water transpiration barrier. In this study, eight tea germplasms were grown under normal conditions; cuticle thickness, wax coverage, and compositions were analyzed from the epicuticular waxes and the intracuticular waxes of both leaf surfaces. The cuticular transpiration rates were measured from the individual leaf surface as well as the intracuticular wax layer. Epicuticular wax resistances were also calculated from both leaf surfaces. The correlation analysis between the cuticular transpiration rates (or resistances) and various cuticle parameters was conducted. We found that the abaxial cuticular transpiration rates accounted for 64-78% of total cuticular transpiration and were the dominant factor in the variations for the total cuticular transpiration. On the adaxial surface, the major cuticular transpiration barrier was located on the intracuticular waxes; however, on the abaxial surface, the major cuticular transpiration barrier was located on the epicuticular waxes. Cuticle thickness was not a factor affecting cuticular transpiration. However, the abaxial epicuticular wax coverage was found to be significantly and positively correlated with the abaxial epicuticular resistance. Correlation analysis suggested that the very-long-chain aliphatic compounds and glycol esters play major roles in the cuticular transpiration barrier in tea trees grown under normal conditions. Our results provided novel insights about the complex structure-functional relationships in the tea cuticle.

An improved theory for calculating leaf gas exchange more precisely accounting for small fluxes

The widely used theory for gas exchange proposed by von Caemmerer and Farquhar (vCF) integrates molar fluxes, mole fraction gradients and ternary effects but does not account for cuticular fluxes, for separation of the leaf surface conditions or for ternary effects within the boundary layer. The magnitude of cuticular conductance to water (g_cw) is a key factor for determining plant survival in drought but is difficult to measure and often neglected in routine gas exchange studies. The vCF ternary effect is applied to the total flux without the recognition of different pathways that are affected by it. These simplifications lead to errors in estimations of stomatal conductance, intercellular carbon dioxide concentration (C_i) and other gas exchange parameters. The theory presented here is a more precise physical approach to the electrical resistance analogy for gas exchange, resulting in a more accurate calculation of gas exchange parameters. Additionally, we extend our theory, using physiological concepts, to create a model that allows us to calculate cuticular conductance to water.

Foliar water and solute absorption: an update

The absorption of water and solutes by plant leaves has been recognised since more than two centuries. Given the polar nature of water and solutes, the mechanisms of foliar uptake have been proposed to be similar for water and electrolytes, including nutrient solutions. Research efforts since the 19th century focussed on characterising the properties of cuticles and applying foliar sprays to crop plants as a tool for improving crop nutrition. This was accompanied by the development of hundreds of studies aimed at characterising the chemical and structural nature of plant cuticles from different species and the mechanisms of cuticular and, to a lower extent, stomatal penetration of water and solutes. The processes involved are complex and will be affected by multiple environmental, physico-chemical and physiological factors which are only partially clear to date. During the last decades, the body of evidence that water transport across leaf surfaces of native species may contribute to water balances (absorption and loss) at an ecosystem level has grown. Given the potential importance of foliar water absorption for many plant species and ecosystems as shown in recent studies, the aim of this review is to first integrate current knowledge on plant surface composition, structure, wettability and physico-chemical interactions with surface-deposited matter. The different mechanisms of foliar absorption of water and electrolytes and experimental procedures for tracing the uptake process are discussed before posing several outstanding questions which should be tackled in future studies.

Plasticity of the Cuticular Transpiration Barrier in Response to Water Shortage and Resupply in Camellia sinensis: A Role of Cuticular Waxes

The cuticle is regarded as a non-living tissue; it remains unknown whether the cuticle could be reversibly modified and what are the potential mechanisms. In this study, three tea germplasms (Wuniuzao, 0202-10, and 0306A) were subjected to water deprivation followed by rehydration. The epicuticular waxes and intracuticular waxes from both leaf surfaces were quantified from the mature 5th leaf. Cuticular transpiration rates were then measured from leaf drying curves, and the correlations between cuticular transpiration rates and cuticular wax coverage were analyzed. We found that the cuticular transpiration barriers were reinforced by drought and reversed by rehydration treatment; the initial weak cuticular transpiration barriers were preferentially reinforced by drought stress, while the original major cuticular transpiration barriers were either strengthened or unaltered. Correlation analysis suggests that cuticle modifications could be realized by selective deposition of specific wax compounds into individual cuticular compartments through multiple mechanisms, including in vivo wax synthesis or transport, dynamic phase separation between epicuticular waxes and the intracuticular waxes, in vitro polymerization, and retro transportation into epidermal cell wall or protoplast for further transformation. Our data suggest that modifications of a limited set of specific wax components from individual cuticular compartments are sufficient to alter cuticular transpiration barrier properties.