J. Pesticide Sci. 16, 651-663 (1991)

Original Article

Wettability Characteristics of Crop Leaf Surfaces*

Tadakazu WATANABE and Isamu YAMAGUCHI

The Institute of Physical and Chemical Research, Hirosawa, Wako 351-01, Japan

(Received April 12, 1991)

To evaluate the wettability of plant leaves as a result of interactions between solution and leaf surface, the amount and shape of the retained solution on a specific area of the leaf surface were measured after dipping a leaf into immersion solutions (surface tension: 21. 5-63. 5 mN. m-1) containing a dye by 1. 0% and various surfactants 0. 2%, and micro- and macro-structures of the surface were observed. The transition pattern of shape corresponding to the transition of surface tension on the leaf surface was termed as wettability characteristics

pattern (WCP) and WCPs of leaves of 39 crops were categorized into three major groups: WCPs I, II and III including seven sub-WCPs. Also, the critical surface tension to wet each leaf surface completely was estimated. It was found that WCP and critical surface tension were specific to each leaf surface and the former was mainly related to the microstruc-tures of leaf surface, especially morphology and distribution of epicuticular waxes and rela-tively wettable veins. Comparison and evaluation of leaf-surface wettability are possible by WCP on a static basis.

INTRODUCTION

In an application of pesticidal solutions, the retained quantity and distribution of the ac-

tive ingredients on plant surface are largely influenced by wetting conditions.)-6) Some

distinguished studies have been done to evalu-

ate leaf-surface wettability, 4-10) but how to express wettability has not fully established

yet, and little is known of interrelationships between wettability and leaf-surface struc-tures. Reasons for this seem to be due to

particular properties of wettability, different from spreading properties of a droplet on leaf

surface, 6, 11, 12> and extreme complexity and heterogeneity of leaf-surface structure. 12-15)

The authors developed a leaf immersion meth-

od for integrated evaluation of leaf-surface wettability and suggested that the shape of

retained solution on a specific area of leaf sur-face could be a quantitative expression of the

wettability as a result of interactions between

the solution and the leaf surface, in the pre-

vious paper. l6>

This article deals with interrelationships be-

tween the wettability characteristics of leaf sur-

faces of 39 crops measured by the leaf immer-

sion method and their surface structures.

MATERIALS AND METHODS

1. Leaves of Crops Crops (39 species from 10 families) were

grown from seeds in the same growth condi-tions as in the previous paper. 16) Five of fully expanded leaves were employed for one measurement plot.

2. Measurements of the Specific Amount and Shape of Retained Solution

The specific amount and shape of retained immersion solution on leaf surface were meas-ured according to the method described in the previous paper. 16) Briefly, the amount and shape of retained immersion solution on a spe-cific area of leaf surface were measured after dipping a leaf vertically into a series of 20 immersion solutions (surface tension: 21. 5-

* Studies on Wetting Phenomena on Plant Leaf

Surfaces (Part 2). For Part 1, see Ref. 16).

652 日本農薬学会 誌 第16巻 第4号 平成3年11月

63. 5 mN. m-1) containing a dye (Direct Fast Scarlet 4BS, Colour Index; Direct Red 23, 29160) by 1. 0% and various surfactants 0. 2% under the conditions of leaf tip down and im-mersion lengths of 14 cm for Gramineae and 9 cm for others. All the measurements were done at 20-23C. The shapes of retained immersion solutions obtained were categorized into A-1

(continuous thin film; complete wetting), A-2 (flattened spherical segment or fragment of thin film), B-1 (lacked sphere or spherical seg-ment), B-2 (B-1 on veins), B-3 (B-1 as bridged between veins), C-1 (thin film on veins) and O (no retention).

3. Measurement of Leaf-Surface Structures 3. 1 Surface protuberancy and surface contour

line length as macrostructure Four test pieces, each ca. 2 mm wide, were

cut out traversially and longitudinally to mid-ribs, from the leaf area situated as that used for retention measurement. The density (per cm2) and average length (lam) of surface pro-tuberancies such as trichome, hair, papilla or the likes including on vein but not vein itself of more than ca. 20, am high on each section were measured with a stereomicroscope (Model Olympus PM-10-AK). The lengths of adaxial and abaxial surface contour lines at a rough-ness of more than ca. 20, am deep including vein but not surface protubers ancies and a supposed central line on each section were detected on

photographs with a digital curvimeter (Model Comcurve-8, Koizumi Seisakusho Co.). Both surface contour line lengths of each section were converted to indices (surface contour line index, SCI) to 100 of the supposed central line length. 3. 2 Microstructures Three pieces (ca. 5 mm X ca. 8 mm) of inter-

veinal areas from the leaf area situated as that used for retention measurement were fixed with 2 % osmic acid solution, Au-coated, and observed with a SEM17) (Model Mini-SEM 101, Akashi Seisakusho Co.). First, surface rough-ness at epidermal cell levels was observed at a 200- to 500-times magnification to be divided into four categories (very rugged, rugged, slightly smooth, very smooth) as a semimicro-structure. Next, the morphology and distri-bution state of epicuticular waxes on epidermal

surface were observed at a 2000- to 5000-times magnification as a microstructure. The mor-

phology observed was conventionally classified into eight categories; (1) filament, (2) coiled filament, (3) dendrite, (4) granule, (5) crust, (6) platelet, (7) erecting platelet and (8) amorphism, referring to the preceding stud-ies, 12, 15, 18, 19) and the distribution states into four categories (very dense, dense, slightly sparse, very sparse). In the case of amorphism, the distribution states were not noted because distinct discrimination between epicuticular waxes and cuticular proper surface was im-possible.

4. Extraction of Epicuticular Waxes Epicuticular waxes on leaves of more than 10, 000 cm2 for both leaf surfaces together were extracted by a three successive immersion into 500 ml chloroform for 10 sec. 2o)

RESULTS

1. Leaf-Surface Structures and Deposition of Epicuticular Waxes

Features of micro- and macrostructures and deposition of epicuticular waxes on leaf surface were shown in Table 1.

2. Wettability Characteristics of Leaf Surfaces The measured retention shape and volume

of immersion solution were plotted to surface tension, as shown in Fig. 1 (l)-(7). These figures showing the transition patterns of retention shape corresponding to the transition of sur-face tension were designed with the shapes A, B, C-1 and O. Since formation of a retention shape of solution on a solid surface is governed by the contact angles (advancing and receding) at an inclination13, 21-25) and these angles can be a final result of interactions among the sur-face tension, chemical constitutions and rough-ness of the leaf surface and its inclination, 22-25) the transition pattern is considered to be spe-cific to each leaf surface to show a character-istic in wettability. Hence, the transition pat-tern was termed as wettability characteristics

pattern (WCP), and WCPs of leaf surfaces of 39 crops used were roughly classified into three major WCPs including seven sub-WCPs, as in Table 2. If a leaf surface did not possess veins relatively wettable to interveinal areas, it re-

Journal of Pesticide Science 16 (4), November 1991 653

Table 1 Surface structures of crop leaves.

654日 本農薬 学会誌 第16巻 第4号 平成3年11月

Table 1 (Continued)

Journal of Pesticide Science 16 (4), November 1991 655

Table 1 (Continued)

656日 本農薬学会誌 第16巻 第4号 平成3年11月

suited in A- l, A-2, B-1 and/or O shapes of solution as in WCP-I and WCP-II. B-i is a lacked sphericai shape and generally tended to be larger at higher surface tensions and smaller at lower surface tensions to change to A-2, which was a transition further to A-1. A-2 did not appear in sub-WCP I-a but appeared in sub-WCP I-b in a narrow range of surface ten-sion, though WCP-I itself was characterized by having an O. A-2 diversified from flattened or deformed spherical shape to irregular fragment of thin film as surface tension lowered close to the critical surface tension (CST), 4, 26) at which complete wetting (A-1) initiated to occur. If relatively wettable veins existed on a leaf sur-face, it showed WCP-III consisting of B-2, B-3 and C-1 together with WCP-I or WCP-I I. B-2 and B-3 are usually in larger lacked spherical shape depositing only on wettable veins at rather higher surface tensions as in sub-WCP III-a, and C-1 is a continuous thin film only on veins at higher surface tensions as in sub-WCP III-b. The retained volume is thought to be

governed by the volume of a droplet and its number depositing on a unit area.

3. Interrelationships between WCP and Leaf-Surface Structures

Surface structures, deposition of epicuticular waxes and estimated CST values of the leaf surfaces used are arranged in Table 2. The

predominant features in surface structure to each WCP are considered to primarily depend on epicuticular waxes (morphology and distri-bution states) and relatively wettable veins. In detail and inclusively, the filament-erecting

platelet-rod-crust forms in dense-very dense states are for WCP-I (the filament-erecting

platelet-rod forms in dense-very dense states for sub-WCP I-a and filament-erecting plate-let-rod forms in less dense states and crust form in a denser state for sub-WCP I-b), the rod-crust-granule forms in very dense-very sparse states and amorphism for WCP-II (the rod-crust-granule forms in very dense-very sparse states and amorphism for sub-WCP II-

Table 1 (Continued)

a) Surface contour line length index, b) total epicuticular waxes on adaxial and abaxial surfaces

together, C) and d) density per cm2 (average length jcm) of surface protuberancies of more than 20, um, e) product of adaxial SCI and abaxial SCI, f) semimicro -structures, g) and h) morphology and distribu-

tion state of epicuticular waxes,i)no surface protuberancies of more than 20 jcm.

Journal of Pesticide Science 16 (4), November 1991 657

658 日本農薬 学会誌 第16巻 第4号 平成3年11月

Journal of Pesticide Science 16 (4), November 1991 659

Table 2 Classification of wettability characteristics patterns (WCP) and their relationships with leaf surface structures.

660日 本農薬学会誌 第16巻 第4号 平成3年11月

Table 2 (Continued)

Journal of Pesticide Science 16 (4), November 1991 661

a, the granule-crust forms in a sparser state and amorphism for sub-WCPs II-b and II-c), and a unique WCP by veins relatively wettable to interveinal areas for WCP-III (presence of easily wettable veins in hardly wettable inter-veinal areas at higher surface tensions for sub-WCP III-a and that of easily wettable veins in less wettable interveinal areas for sub-WCP III-b, though these interveinal areas could be-long to WCP-I and sub-WCPs II-a and II-b, respectively). Thus, it seems that building-up of WCP is governed primarily by the mor-

phology and distribution state of epicuticular waxes and presence of relatively wettable veins, though there are a few obscurities in the above classification of WCP to be further in-vestigated. Even wettability of veins could be decided by epicuticular waxes present on them. The semimicrostructures seem to influence supplementarily the building up of WCP with t} i i.:-waxes, but the macrostructures do not seem to play a significant role in WCP build-ing-up in this immersion method. However, the effect of surface protuberancies on WCP is still equivocal to be further investigated in terms of density, length, existing state, mor-

phology and their wettability.)2, 27, 28) The esti-mated CST values to initiate A-1 retention are commonly higher, i. e. ca. 41-50 mN Oml in WCP-I and much higher in sub-WCPs II-b and TI-c, but varied widely in other WCPs. A-1

retention formed at below CST values tended to fall in a narrow range of retained volume, i. e., ca. 0. 7-1. 1, cal/cm2 throughout all the leaves tested. As the thickness of A-1 reten-tion is calculated to be 7-11, um, A-1 retention itself can be more directly affected by a semi-microstructure of ca. 10, cam or so deep on leaf surface. Among the retention shapes, B tended to carry larger volumes of solution by forming a larger lacked spherical shape as the surface tension became higher.

DISCUSSION

The concept of WCP of a leaf surface was introduced to characterize the wettability of each leaf surface and three major WCPs in-cluding seven sub-WCPs were distinguished with leaves of crops. Important factors gov-erning the building-up of WCP were the sur-face tension of solution, microstructures byy epicuticular waxes and the presence of rela-tively wettable veins on leaf surface. In the case of WCP-I, the waxes in the filament-erect-ing platelet-rod-crust forms in a dense-very dense state tended to cause O shape by trap-ping an air film between the leaf surface and the solution at higher surface tensions, which is termed as a "composite wetting. "14, 29, 30) Although WCP-II has widely varied distribu-tion states of the waxes in various forms, sub-WCP II-a seemed to show a typical behavior

Table 2 (Continued)

a) Critical surface tension and retained volume, b) adaxial and abaxial surfaces, C) density (length), d) adaxial and abaxial SCIs, e) surface roughness at epidermal cell levels,f) morphology (distribu-

tion states) of epicuticular waxes, g) and/or, h) noted double,i) CST of more than 63. 5 mN Om-1.

662日 本農薬学会誌 第16巻 第4号 平成3年11月

of the retained solution on the smooth hydro-

phobic surface and sub-WCPs II-b and II-c that on the smooth and less hydrophobic or hydrophilic surfaces. Some obscurity in WCP classification can rise from the chemistry of waxes: the surfaces in WCP-I and sub-WCP II-a can be covered almost throughly by very hydrophobic constitutions of waxes but sub-WCPs II-b and II-c by less hydrophobic or rather hydrophilic constitutions or not to ex-pose originally wettable citicular proper sur-faces. 14, 22, 31)

Qualitative evaluation of the wettability of leaves with aqueous solutions has been adopted so far5, 6, 27) 32) and many factors af-fecting wettability were pointed out: rough-ness of cuticular surface and existence of trichome, 2, 3, 12) 30) constitutions and morphology of surface waxes, 12, 14, 17, 22) venation, topology of epidermal cell arrangement, surface ornamenta-tion and epicuticu ar waxes. " 2)3)6>1214 17 22 27>33) Among these, wettability is considered to be substantially governed by the presence of epicuticular waxes in this immersion method. The surface tension for initiation of A-1 retention was referred to as CST here, 4, 26) which is influenced by both chemical consti-tution and roughness of solid surface and the kind of liquids. 4, 34, 35) Since it is also shown that CST relates to the partial cohesive inter-action parameters of substances of solid sur-face and liquid35) and the interfacial tension between them decreases to almost zero mN O m-1 at its CST, 34) CST values obtained can be a relative indication of initiation of complete wetting but not absolute. Chemistry of epi-cuticular waxes is an important factor to be fully considered for wettability, which consist mainly of "very long chain fatty acid (VLCFA) derivatives" and triterpenoids. 12, 14, 15) These constitutions are generally very hydrophobic but their crystalline forms and orientation of hydrophilic groups could make up a fairly wide variation in leaf wettability12) and WCP of a leaf surface is not usually fixed but rather easily changes by many factors in production of waxes3, 12-15> and also by physical abrasion or wounds, parasitism, dust, chemicals and others. Thus, WCP can be specific to a combination

of leaf surface and solution and a useful cri-

terion to evaluate leaf-surface wettability on a static and quantitative basis.

ACKNOWLEDGMENTS

The authors thank Drs. Hiroyasu Watanabe and Kunihiko Matsumori of Ehime Univ. and Drs. Yasuo Homma and Yutaka Arimoto of this Institute for their helpful suggestions. Their thanks are also

given to Toho Chemical Ind. Co. and Mitsubishi Kasei Corp. for supplies of some surfactants and a dye, respectively.

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(1965) 8) R. M. Amsden & C. P. Lewins: World Rev.

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by W. V. Valkenburg, Marcel Dekker Inc., New York, pp. 343-385, 1973

10) G. Kadota & S. Matsunaka: J. Pesticide Sci. 11, 597 (1986)

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要 約

作 物 葉の 濡 れ 特 性*

渡部忠一, 山口 勇

植物 葉の濡れを 葉面 と溶液 との相 互作 用の結果 として

評価す るため, 39種 の作物 の成 葉を色素1. 0%お よび界

面活性剤0. 2%か ら成 る一連の浸漬液(表 面張力: 21.5

mN・m-1～63.5mN・m-1)に 浸漬 し, 葉面 に付着す る浸

漬液の固有の付着量 と液滴の形状を測定す るとともに,

葉面の ミクロおよびマ クロ構造を計測 した. 付着液滴の

形状は葉面 と溶液の相 互作用の表現 と考え られ, 表 面張

力の変化(63. 5mN・m-1→21.5mN・m-1)に 対応す る形

状の一 連の変化 のパ ター ンを濡れ特性パ ター ン(WCP)

と し, 主要3グ ル ー プ; WCPs I, IIお よびIII(7sub-

WCPsを 含む)に 分 類 した. また, 各葉面 の完全濡れを

開始す る臨界表面張力を求めた. WCPお よ び臨界表面

張力は各葉面 に固有であ り, 前者はお もに葉面 の ミクロ

構造, とくに, epicuticular waxesの 形態 と分布状態, お

よび葉脈間に対 し相対的に濡れやすい葉脈の存在に よっ

て決定 され ると考え られた. WCPに よって 各 葉面の濡

れを比較 し評価す ることが可能であ る.

*植 物葉の濡れ現象に関す る研究(第2報)