Mechanisms Of Plant Leaves Based On Leaf Surface Wettability On Rainfall Interception And Dust-capturing

Leaf surfaces represent the key interfaces between plants and their environmentwhich affect substances and energy exchanging processes. Leaf surface wettability,indicating the affinity for water on the leaf surface, is a common phenomenon for plantsin a wide variety of habitats, and directly affects the microhabitat and microclimateavailable for dry and wet deposition, canopy interception, leaf photosynthesis andmicrobes colonization. The researches concentrated on mechanisms of plant leavesbased on leaf surface wettability on rainfall interception and dust-capturing have beenconsidered to be of great theoretical and academic importance.Some important problems relating to leaf surface wettability were investigatedincluding: Leaf surface wettability characteristics of plant species and related affectingfactors; The seasonal changes of leaf surface wettability; Patterns of leaf surfacewettability along a moisture gradient on the Loess Plateau; Effects of leaf surfacewettability, surface free energy and water drop shape on rainfall interception; Effects ofleaf surface wettability and surface free energy on dust-capturing capacity, and seasonalchanges of leaf dust-capturing capability; Leaf surface wettability under different urbanatmospheric environments and the potential of leaf surface wettability for bimonitoringof urban habitat quality.The major conclusions are as follows:(1) There were significant differences in contact angle among species and betweenadaxial and abaxial leaf surfaces for the investigated species in the study areas, and thecontact angles ranged widely from40o to145o, with a mean value of103.4°. Leaf waterrepellency was significantly greater on the abaxial surface than on the adaxial surface for41species, twenty-eight species had greater leaf contact angle on the adaxial surfacethan on the abaxial surface, and twenty-six species had no significant differencebetween adaxial and abaxial surfaces. Leaf contact angle of shrubs and herbs weresignificantly greater than that of trees, which relate to the difference in leaf epidermalwax, leaf water status.(2) Leaf contact angle increased significantly with increasing epidermal waxcontent when wax contents were lower than0.75g/m~2. In this condition, the absoluteamount of epidermal wax was the critical factor. When wax contents were higher than0.75g/m~2, leaf contact angle could be classified into two types: leaves covered byconvex epidermal cells and wax crystal with contact angle kept around120°; leaveswith smooth surface and covered by eax membrane, the contact angles kept around80°.For this instance, the roughness created by the complexity of wax structure was themain factor. The amount, distribution, and morphology of trichomes had great influenceon leaf contact angle, and different types of action pattern may lead to different wettingcharacteristics. The type, size, and amount of stomata all have influence on leaf surfacewettability, and the effects are complicated.(3) During the growing season, leaf contact angle of Cercis chinensis Bunge,Sophora japonica Linn., Berberis thunbergii cv. atropurpurea, Viburnum odoratissimum,Ginkgo biloba linn., Buxus sinica, Ligustrum lucidum Ait., Ligustrum quihoui Carr.decreased significantly. But, leaf contact angle of Trifolium repens Linn. kept constant.Decreases of40°for the hydrophobic species (Sophora japonica Linn., Berberisthunbergii cv. atropurpurea, Ginkgo biloba Linn.) were observed. Whereas, forhydrophilic species (Cercis chinensis Bunge, Viburnum odoratissimum, Buxus sinica,Ligustrum lucidum Ait., Ligustrum quihoui Carr.), decreases of10°-20°could be found.(4) Contact angle did not change significantly with the decrease of leaf relativewater content for7hydrophobic surfaces (adaxial and abaxial surfaces of Ginkgo bilobaLinn., Sophora japonica Linn., Berberis thumbergii cv. atropurpurea, and the adaxialsurface of Trifolium repens) till apoplastic water content (B, the lower limit of plantuseable water). However, for9hydrophilic surfaces (adaxial and abaxial surfaces ofPopulus Canadensis Moench, Euonymus japonius Thunb., Prunus serrulata Lindl.,Buxus sinica, and the abaxial surface of Trifolium repens linn.), the variation of contact angle during dehydration process could be classified into three types:(1) Contact anglesteadily decreased for the adaxial surface of Buxus sinica;(2) Contact angle dropped toa lower value and recovered later for abaxial surface of Prunus serrulata Lindl. beyondapoplastic water content;(3) Contact angle kept constant when relative water contentwas larger than apoplastic water content for the other7hydrophilic surfaces.Only contact angle on the adaxial surface of Prunus serrulata Lindl., Euonymusjaponius Thunb., and abaxial surface of Buxus sinica among eight species showedobvious diurnal variation with the same contact angle in the morning and at noon andhigher contact angle at night, suggesting diurnal changes of contact angle were mainlydecided by other factors than leaf water status.(5) There were significant variations in leaf contact angle and roughness ofLigustrum lucidum Ait. and Viburnum odoratissimum during the growing season. ForLigustrum lucidum Ait., old leaves had much rougher surfaces than did young leaves.For Viburnum odoratissimum, the adaxial surfaces of young leaves were rougher thanthose for old leaves, but the abaxial surfaces were opposite in roughness. The trend inleaf wettability was that young leaves had higher, and old leaves lower. The variationsof leaf contact angle related to the changes of roughness and the chemical componentsof plant leaves. There existed many papillae and hollows, radii of about10μm, onadaxial surfaces of these two plants. Such structure was advantageous for capturingPM10(particle matter with aerodynamic diameter less than10μm), which might be themost important of the commonly occurring air pollutants.(6) The general trend in leaf surface wettability along the moisture gradient on theLoess Plateau was from high contact angles and spherical droplets for leaf surfaces inShenmu to relative lower contact angles in Yichuan and Chunhua. Sixteen speciesdistributed in the three or two sites, and the variations of leaf contact angle weredependent on plant species. Contact angle did not change significantly with the decreaseof precipitation for hydrophobic surfaces (the adaxial surface of Lespedeza bicolorTurcz., and adaxial and abaxial surfaces of Sophora davidii (Franch.) Skeels). However,the variations of contact angle for hydrophilic surfaces could be classified into twotypes:(1) Contact angle significantly increased in a drier site for the adaxial and abaxialsurfaces of Armeniaca sibirica (Linn.) Lam., Lonicera hispida Pall. ex Roem. et Schult., and the abaxial surface of Hippophae rhamnoides Linn.;(2) Contact angle kept constantfor the adaxial and abaxial surface of Elaeagnus pungens Thunb., Viburnum dilatatumThunb.In addition, the incidence of adaxial surface with higher contact angle than abaxialsurface in Shenmu were common than that in Yichuan and Chunhua. Moreover, theincidence of amphistomatous and pubescent leaves was much large in Shenmu than inYichuan and Chunhua. A significant increase in wax content and stomatal density onadaxial surface was found in a drier site. However, a decrease in stomatal density,stomatal pore and guard cell on abaxial surface were observed, the difference was notsignificant.(7) Leaf maximum water storage capacities of twenty one plant species in Xi’anwere investigated employing two different methods-submerging of and spraying atphytoelements, respectively. Average leaf maximum water storage capacities covered awide range from29.4to180.0g/m~2, and94.1to278.3g/m~2, using the submerging andspraying method, respectively. The spraying method yielded significantly higher valuesas compared to the submerging method, the overestimation ranging from9.3%to87.2%.For spraying method, the correlations between leaf maximum water storagecapacities and leaf surface free energy, its polar and dispersive component, leaf contactangles were not significant. As to submerging method, however, significant positivecorrelations between leaf maximum water storage capacities and leaf surface free energy,its dispersive and polar component could be found. And the results showed asignificantly negative correlation between leaf maximum water storage capacities andleaf contact angles.(8) The maximum dust-capturing capacities ranged from0.8to38.6g/m~2usingartificial dust-deposition method for21representative urban greening species in Xi’an,and there were significant difference among plant species with the greatestvariation up to forty times. The amount, distribution, and morphology of trichomes hadgreat influence on dust-capturing capability of leaves, which might be due to thedifferent action pattern between trichomes and particulate matters. The contact angles ofplant leaves were negatively correlated with maximum dust-capturing capability except for the four species which had trichomes. The correlation between maximumdust-capturing capacity and surface free energy and its dispersive component wassignificantly positive, but the positive correlation between maximum dust-capturingcapability and polar component was not significant.(9) Plant leaves showed significant difference in dust-capturing capacity during thewhole growing season for the six investigated species including Sophora japonica Linn.,Platanus acerifolia (Ait.) Willd., Ginkgo biloba Linn., Cedrus deodara (Roxb) Loud,Pinus tabulaeformis Carr., Ligustrum lucidum Ait. The general trend in leafdust-capturing capacity was that young leaves had lower, and mature leaves higher. Theturning point from lower to higher dust-capturing capacity was dependent on plantspecies. The average dust-capturing capacity for Sophora japonica Linn., Platanusacerifolia (Ait.) Willd., Ginkgo biloba Linn., Cedrus deodara (Roxb) Loud, Pinustabulaeformis Carr., Ligustrum lucidum Ait. was1.44,3.24,1.34,1.68,4.47,3.61g/m~2,respectively. Most probably, the lack of epicuticular wax that, after erosion, provided afavourable substrate for adhesion, seemed to be the key factor leading to seasonalvariations in leaf dust-capturing capacity.(10) Leaf surface wettability of Ligustrum lucidum Ait. and Ligustrum quihoui Carr.showed significant differences under different urban atmospheric environment, whichwas highest in comparatively pollution-free area (CPFA), and followed by residentialand educational area (REA), commercial and service area (CSA), heavy-traffic area(HTA) and industrial area (IA). Contact angle of adaxial and abaxial surfaces ofLigustrum lucidum Ait. for REA, CSA, HTA and IA were1.3%,14.3%,7.5%,21.4%;6.1%,13.7%,12.4%,35.1%lower, respectively, than CPF. However, for Ligustrumquihoui Carr., decreases of1.0%,1.4%,6.7%,9.0%;0.0%,4.8%,2.9%,5.7%wereobserved for the adaxial and abaxial surfaces, respectively. Leaves are the main placeof interaction between plants and the environment, and are continuously exposed tohigh levels of different types of air pollutants. The variations in leaf surface wettabilityclosely related with the urbanization and industrialization level, which could indicatedistrict character. Various pollutants emitted from industry and traffic couldaccumulated in leaves and foliar dust, and had great influence on leaf characteristics.Therefore, leaf surface wettability can be regarded as a potentially good bioindicators for urban habitat quality.The innovations of this study represent in the following four points:(1) For hydrophobic surfaces, contact angle did not change significantly with thedecrease of leaf relative water content above the apoplastic water content (B, the lowerlimit of plant useable water). However, for hydrophilic surfaces, the variations ofcontact angle with relative water content could be classified into three types:(1) Contactangle steadily decreased;(2) Contact angle dropped to a lower value and recovered later;(3) Contact angle kept constant.(2) Leaf epidermal wax content had great influence on leaf contact angle. Whenwax contents were lower than0.75g/m~2, leaf contact angle increased significantly withincreasing epidermal wax content. In this condition, the absolute amount of epidermalwax was the critical factor. While, wax contents were higher than0.75g/m~2, the positiverelationship between between leaf contact angle and wax content was not obvious. Forthis instance, the roughness created by the complexity of wax structure was the mainfactor.(3) The general trend in leaf surface wettability along the moisture gradient on theLoess Plateau was from high contact angles and spherical droplets for leaf surface inShenmu to relative lower contact angles in Yichuan and Chunhua.(4) Leaf contact angle was sensitive to surrounding environment, and decreasedsignificantly in parallel with pollution level, and can be regarded as a potentially goodbioindicator for urban habitat quality.