Flora (2003) 198, 349–357 http://www.urbanfischer.de/journals/flora
Patterns of leaf surface wetness in some important medicinal and aromatic plants of Western Himalaya1 Subedar Pandey & Pramod Kumar Nagar* Division of Biotechnology, Institute of Himalayan Bioresource Technology, Palampur – 176 061, (HP) India Submitted: Nov 20, 2002 · Accepted, in revised form: Feb 25, 2003
Summary The present study aims at investigating the role of leaf morphological features and their relation to leaf surface wettability for important medicinal and aromatic plants of Western Himalaya. The surface features related to leaf wettability were studied in 30 plant species representing 21 different families growing under open and shade conditions. The leaf surfaces with the highest density of trichomes and stomata per unit area were found to be the least wettable, regardless of condition type. Most of the species of both conditions were hypostomatic, and per unit area concentration of stomata contributes more than stomatal size to stomatal area index. Leaf surfaces of open condition species were more water repellent with higher stomatal density, and had lower water droplet retention than shade species. It is suggested that leaf morphological features (stomata and trichome) had a strong influence in reducing the leaf area with surface moisture, which could be correlated with the frequency and duration of leaf wettability in a given condition. Key words: Leaf wetness, Leaf morphology, medicinal and aromatic plants, stomata, trichome, Western Himalaya
Introduction Leaves can be considered, functionally, as iterated green antennae specialized for trapping light energy, absorbing CO2, transpiring water, and functioning as a sensitive organ to monitor the environment. Size, shape and morphological features of leaves are to a large extent genetically controlled, implying that these are adaptive features lending advantage to plants in specific habitats. However, developmental flexibility exists even within an individual plant, with leaf size, shape and surface features depending on environmental circumstances prevailing during leaf formation (Volkenburgh 1999). Leaf surface features such as a waxy cuticle, dense surface trichomes and specific surface roughness have been postulated as adaptations to prevent excessive wetting of leaves (Kaul 1976). 1
IHBT communication number: 2214
Most plant species experience a period of leaf wetness during the entire period of their growth and development as a result of fog, rain, dew or mist. Leaf wetness can be a factor of ecophysioloical importance because the stomata of wet leaves are always occluded by water droplets or a water film, at least in part (Ishibashi & Terashima 1995). Since diffusion of CO2 is 10000 times slower in water than in air (Nobel 1991), leaf wetness greatly reduces the rate of photosynthetic gas exchange. In special cases, leaf surface moisture has been suggested as an important factor for plant growth due to the possibility of absorption of water (Schmitt et al. 1989). Excess leaf wetness may promote pathogen infection of native and agricultural species (Evans et al. 1992). Pollutant deposition and foliar nutrient leaching also are affected by leaf surface wetness (Massman et al. 1994; Cape 1996), and differences in leaf surface micromorphology can be decisive for both, pollutant damage (Pal et al. 2002) and susceptibility of leaves to prolonged wetness. Evidence suggests that photosyn-
* Corresponding author: Pramod Kumar Nagar, Division of Biotechnology, Institute of Himalayan Bioresource Technology, Palampur – 176 061, (HP) India, e-mail:
[email protected] 0367-2530/03/198/05-349 $ 15.00/0
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thesis and growth in many species may be much reduced because of wet leaf surfaces (Brewer & Smith 1995). Ishibashi & Terashima (1995) reported that leaf wetness causes not only instantaneous suppression of photosynthesis but also chronic damage to the photosynthetic apparatus, which can have a major impact on production in nature. Water repellency is the tendency for a water droplet to bead off a leaf as a spherical droplet rather than remaining on the leaf surface (Neinhuis & Barthlott 1997). The pronounced water repellency of leaf surfaces is of great ecological importance as it reduces leaching of substances from the interior of the leaves and helps to prevent the growth of epiphyllic micro-organisms (Preece & Dickinson 1971). Contact angle measurement is a very sensitive indicator for the repellency of a surface and has been applied in various biological areas, since the degree of surface wetting by water gives information on the characteristics of the outermost layer of the interphase between the liquid water and the atmosphere (Holmes-Farley & Whitesides 1987). Few studies have addressed the relationship of leaf morphological features with leaf surface wettability of cultivated and native plants (Smith & Mcclean 1989; Pandey & Nagar 2002). Different plant species show a broad range of leaf wettability from being covered by a film of water to completely water repellent. The purpose of the present study was to investigate the role of leaf morphological features affecting the surface moisture by evaluating leaf wettability of some important medicinal and aromatic plant species of Western Himalaya under the influence of open and shaded grow conditions.
Materials and methods Location Thirty plant species representing 21 different families (Tab. 2) growing in the Institute’s Experimental Farm at Palampur (1300 m asl, 32°6N, 76°33E), occurring in open field (full sunlight) and under shade of nylon net (50% irradiance) were selected for the study. All the observations were recorded during 2000–2001. The mean monthly weather data for the period are presented in table 1.
Leaf surface characteristics and leaf wettability Measurements made on both adaxial and abaxial leaf surfaces included contact angles (θ) of water droplets on the leaf surface (leaf wettability), water droplet retention (angular value), trichome density, stomatal density, guard cell and pore length and stomatal area index. All measurements were made on six randomly selected healthy leaves from three different plants per condition with five replications per leaf. The degree of water repellency of the leaf surface was determined by measuring the contact angle (θ) of a 2 mm3 water droplet placed by micropipette on each leaf disc mounted on glass slides using double sided tape. The angle (θ) of a line tangent to the droplet through the point of contact between the droplet and the leaf surface was measured according to Brewer et al. (1991). The criteria for judging surface wettability were based on those of Crisp (1968), where θ 130° was non wettable. For all leaves, θ was measured relative to the
Table 1. Periodical variation in mean weather data (temperature, wind velocity, relative humidity, bright sun shine, rainfall and rainy days) for the year 2000–2001. Months
Temperature (°C) Wind velocity Max. Min. (km h–1)
Relative Humidity (%)
Bright Sun Shine (hr.)
Rainfall (mm)
Rainy days
April, 2000 May June July August September October November December January, 2001 February March
29 31 29 27 26 26 27 21 19 16 21 22
40 56 74 85 81 70 50 51 41 52 47 48
10 8 6 3 4 7 10 6 8 7 8 8
3 15 71 100 126 22 0 2 0.3 15 14 9
1 2 4 5 5 3 0 1 1 1 3 2
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16 20 19 20 19 17 18 11 7 5 8 10
6 5 5 3 3 3 4 3 4 4 5 6
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Acanthaceae Barleria cristata L. (Herb) Apocynaceae Catharanthus roseus G. Don. (Herb) Vinca major L. (Herb) Araceae Acorus calamus L. (Herb) Asteraceae Artemisia parviflora Buch.-Ham. ex Roxb. (Herb) Erigeron canadensis L. (Herb) Solidago canadensis L. (Herb) Berberidaceae Berberis lycium Royle (Shrub) Buxaceae Sarcococca saligna D. Don. Muell. Arg. (Shrub) Caprifoliaceae Lonicera caprifolium L. (Climber) Hypericaceae Hypericum choisianum Wall. (Shrub)
Family/Species (Open condition)
128 ±2 129 ±1 83 ±1 114 n. a. 146 ±1 86 ±2 122 ±1 129 ±1 94 ±2
121 ±3
134 ±1
134 ±1 90 ±2 84 ±1 108 ±1 141 ±2 76 ±1 121 n. a. 124 ±1 93 ±1
96 ±3
107 ±1
25 ±1
33 ±1
30 ±1
47 ±1
24 ±1
38 ±2
21 ±1
34 n. a.
40 ±1
36 ±1
29 ±1
25 ±1
27 ±1
32 ±1
46 ±1
23 ±3
37 ±1
15 ±1
20 ±1
46 ±1
27 ±1
25 ±1
AB
AD
AD
AB
Retention degree
Contact angle (θ)
a
a
a
a
a
80 ±4
206 ±15
123 ±9
a
35 ±6
a
AD
222 ±21
174 ±4
86 ±6
192 ±12
165 ±13
92 ±5
254 ±13
130 ±11
83 ±4
312 ±12
291 ±4
AB
Stomatal density (mm–2)
a
a
