Factors controlling transpiration of mature field-grown
tea and its relationship with yield
A. Anandacoomaraswamy
a, W.A.J.M. De Costa
b,∗, H.W. Shyamalie
a, G.S. Campbell
c aTea Research Institute, St. Coomb’s Estate, Talawakelle, Sri LankabDepartment of Crop Science, Faculty of Agriculture, University of Peradeniya, Peradeniya 20400, Sri Lanka cDecagon, Pullman, WA, USA
Received 6 April 1999; received in revised form 11 January 2000; accepted 24 February 2000
Abstract
The objective of this experiment was to determine the factors influencing the transpiration rates of mature, clonal tea (Camellia sinensis L.) and estimate its transpiration efficiency. The heat pulse technique was used to measure transpiration rates of tea plants growing in the field as part of extensive canopies at Talawakelle, Sri Lanka during the period between 1 January and 19 February 1997. Irrigation and shading treatments were used to determine the influence of soil water content (S) and irradiance on transpiration rate. The transpiration rate declined only slightly when S decreased from field capacity (44%) to 33%. However, when S declined below 33%, the transpiration rate showed a rapid decline from 1.6 to 0.7 l per plant per day at 15% S. When S was near field capacity, maximum transpiration rates of 0.53–0.93 l plant−1h−1occurred between
1000 and 1500 h. The corresponding maxima when the S was near permanent wilting point (i.e. at−1.5 MPa matric potential) were 0.27–0.53 l plant−1h−1. Transpiration decreased linearly with decreasing irradiance throughout the range of radiation
levels tested (i.e. from 100 to 15% of full sunlight) at a rate of 0.031 l per plant per day per % reduction in solar irradiance. The daily transpiration rates of tea plants (0.42–1.07 l per plant per day) under the natural shade of Grevillea robusta were considerably lower than the value of tea plants in the open, 3.511 l per plant per day. Spraying of an antitranspirant, Kaolin, decreased canopy temperature by 2–4◦
C and especially around mid-day. Kaolin also decreased transpiration slightly during the period between 1000 and 1500 h. Transpiration efficiency (TE) was 9.637 kg ha−1(made tea) mm−1of water transpired.
The relationship between total dry matter yield and the ratio between transpiration and mean saturation vapour pressure deficit also was linear with a proportionality constant of 6.9 g kg−1kPa. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Tea; Transpiration; Heat pulse method; Irradiance; Soil water
1. Introduction
Tea is grown as a rainfed perennial crop in the humid regions of Sri Lanka at altitudes ranging from 0 to 2500 m a.s.l.. Tea requires a minimum rain-fall of 1200 mm per year, but 2500–3000 mm per
∗Corresponding author. Fax:+94-8-388041.
E-mail address: [email protected] (W.A.J.M. De Costa)
year is considered optimum (Carr, 1972; Squire and Callander, 1981; Watson, 1986). All tea-growing ar-eas in Sri Lanka receive rainfalls over 2000 mm per year. However, the distribution of this rainfall within the year is distinctly bi-modal because of the season-ality of monsoons which cause rainfall. Consequently, most tea-growing areas in Sri Lanka experience a continuous dry period of about 2–3 months during which green leaf yields decrease significantly.
High transpiration rates from extensive tea canopies cause significant soil water deficits which are respon-sible for decreased leaf expansion rates (Monteith and Elston, 1983; Squire, 1990; Stephens and Carr, 1993). Even when the soil is wet, the excessive transpiration rates resulting from higher levels of irradiance and sat-uration deficits around mid-day could cause transient water deficits within the plant (Kramer, 1988; Smith et al., 1994). Transpiration is closely linked to photo-synthesis which is the primary physiological process responsible for growth of young leaves which form the economic yield of tea. De Wit (1958) showed that plant biomass production is directly proportional to transpi-ration. The proportionality constant has been termed transpiration efficiency (Tanner and Sinclair, 1983) or dry matter:water ratio (Monteith, 1986; Squire, 1990). The saturation vapour pressure deficit (D) of the air exerts a significant influence on transpiration effi-ciency (TE) of a crop by controlling the water vapour
pressure gradient between the leaf sub-stomatal cham-ber and the outside air (Jones, 1992). For a given level of leaf conductance, a greater leaf-air vapour pressure deficit causes a higher transpiration rate and consequently decreases TE. It has been shown that the
product between transpiration efficiency (TE) and D
is a constant for a given crop species (Bierhuizen and Slatyer, 1965; Monteith, 1986). On the other hand, D could exert a direct effect on stomata of many plant species by decreasing stomatal conductance with in-creasing D (Jarvis and Morrison, 1981). Because of the varying degree of transpiration control by stom-atal conductance depending on the degree of coupling between canopy and air (Jarvis and McNaughton, 1986), the overall effect of D on transpiration and TE
may vary with canopy characteristics and atmospheric conditions.
Despite the above complications, for most practical purposes, an estimation of TEand transpiration would
enable the prediction of tea yields in a given environ-ment. The main objectives of this experiment were to measure transpiration from a mature tea canopy and to determine its controlling factors and the relationship with yield as estimated by TE.
Tea is considered a shade-loving plant (Squire and Callander, 1981) and is normally grown as a mixture with trees which provide a natural shade. As solar irradiance is the primary source of energy for tran-spiration, shading could reduce the transpiration rate
of tea growing under shade. Hence, the effect of ar-tificial shade, which simulated the different levels of natural shade, on transpiration of tea was investigated in this study.
There have been several previous studies on water use of tea (Dagg, 1970; Willat, 1971, 1973; Carr, 1974, 1985; Cooper, 1979; Callander and Woodhead, 1981; Stephens and Carr, 1991). However, in all these stud-ies, evapotranspiration which includes both transpira-tion and soil evaporatranspira-tion, had been measured. These measurements of water use include a variation com-ponent which does not directly contribute to the in-ternal functioning of the plant as soil evaporation can vary depending on the degree of ground cover and top (0–5 cm depth) soil water availability (Ritchie, 1972). Therefore, a measurement of transpiration from tea canopies is needed to predict tea yields accurately. In the present experiment, the heat pulse technique was used to measure the transpiration from tea plants.
2. Materials and methods
The experiment was carried out at Talawakelle, St. Coomb’s estate of the Tea Research Institute of Sri Lanka (latitude, 6◦4′N; longitude, 80◦40′; altitude,
1380 m a.s.l.). The soil is a fine, mixed thermic Trop-udult (Panabokke, 1996). The field capacity was 44% (v/v) and the permanent wilting point of 26% (v/v). These were determined using the pressure plate ap-paratus. Field capacity and permanent wilting point were defined as the soil water contents (S) at matric potentials of−0.01 and−1.5 MPa, respectively. The soil available water content (Sa) was defined as the
difference between S at field capacity and permanent wilting point. The soil profile contains about 200 mm of available water in its top 1 m.
canopy at a height of around 12–15 m. Excavation studies have shown that clonal tea has a maximum rooting depth of around 0.9–1 m, but more than 90% of the roots are located within the top 0.6 m of the soil profile (Anandacoomaraswamy, unpublished). In contrast, Grevillea has a deeper root system with the maximum rooting depth exceeding 3 m. The lateral spread of the tea root system is over an area of 1.2 m in diameter. In each plot, 10 plants with round stem bases and a stem diameter of approximately 55 mm were selected for transpiration measurement.
2.1. Measurement of transpiration
Transpiration was measured by the heat pulse tech-nique as described by Cohen et al. (1981). It is based on measuring the time required for a heat pulse applied at a given point of the stem to be transferred to a given point downstream. This gives a measurement of the rate of sap flow in the xylem. The flux density of sap is then calculated using flow geometry, cross-sectional area and flow velocity (Swanson, 1994). The measur-ing instrumentation consisted of a line heatmeasur-ing needle (resistance, 38), a thin rod (30 mm in length) con-taining three thermocouples along its length at 5, 15 and 25 mm from the tip (Thermalogic, USA) and a data logger for recording the thermocouple output sig-nals. The heating needles and the thermocouples were inserted in to the stem base of each of the selected plants. Before insertion, the bark was trimmed away and holes were drilled using a drill guide template. The thermocouple rod was inserted so that the three temperature sensors were placed at radial distances of 5, 15 and 25 mm beneath the outer surface of the stem. Diameter of the stem bases after trimming away the bark was 50 mm. The inserted thermocouples were at a distance of 5 mm downstream of the heating nee-dle. After installation of the heater and the thermocou-ples, the stems were covered with insulating material to prevent radial heat loss to the outside. The heater was powered by a 12 V battery. The heater and ther-mocouples were connected to a 21X Campbell data logger (Campbell Scientific Inc.). The heat pulse was switched on and off every second by a control port on the data logger and the duration of the heat pulse was controlled by the data logger through a programmed counter as described in Ishida et al. (1991). The data logger was programmed to record the rise in
temper-ature of the thermocouples every second and the data were averaged and stored at 30 min time intervals. The single-sensor heat pulse method used in the present study is appropriate for relatively high sap speeds whereas the up- and down-stream dual-sensor method is appropriate for slower sap speeds (Swanson, 1994).
2.2. Calculation of volumetric water flux
The volumetric water flux, J (m3h−1), was the sum of fluxes in different radial rings of the stem as mea-sured from thermocouples inserted at different radial depths:
J =J1A1+J2A2+J3A3 (1)
where A1, A2and A3are the respective stem ring areas
from which flux densities J1, J2and J3are measured.
The respective stem ring areas were calculated as
A1=π[R2x−(Rx−0.5)2],
A2=π[(Rx−0.5)2−(Rx−1.5)2],
A3=π[(Rx−1.5)2−(Rx−2.5)2] (2)
where Rxis the xylem radius as obtained by
measur-ing the circumference (C) of the stem after removmeasur-ing the bark (Rx=C/2π). The flux density Ji through a given area Ai was calculated following the procedure described by Cohen et al. (1981). This is based on the equation of Marshall (1958) to describe the temper-ature rise detected at a distance r from a line heater after a time t following a heat pulse
T = H
where H is the heat output per unit length of the heater,
ρ, c andκ the density, specific heat and thermal dif-fusivity of wet wood, respectively, x and y distances related to the distance r between the line heater and the thermocouples as r=(x2+y2)1/2. V is the
convec-tive velocity of the heat pulse.
The temperature rise T reaches a maximum at time tm after the heat pulse. At this point when dT/dt=0,
Eq. (3) gives an expression for the convective velocity of the heat pulse as
V =(r 2−4κt
m)1/2 tm
When tm0 is the time required to maximize T at
By combining Eqs. (4) and (5), V can be computed as
V = r(1
−tm/tm0)1/2 tm
(6)
Here, tm0was determined by temperature rise data
collected at night during which the sap flow rates were assumed to be zero. Ji can be given as:
whereρc is the volumetric specific heat of tea wood (3.2×106J m−3K−1 as determined previously by Anandacoomaraswamy, unpublished) and ρlcl is the
volumetric specific heat of water (4.18 J m−3K−1). Total flux of water was then calculated from Eq. (1). Transpiration rates of three trees of G. robusta grow-ing in the same field were measured simultaneously with tea using the same method.
The transpiration measurements obtained from the heat pulse method, were of the same order of magni-tude as the evapotranspiration values estimated by the soil water balance method during the corresponding period of the previous year (Anandacoomaraswamy, unpublished). As the tea canopy in the measured fields, covered the soil completely, soil evaporation could be assumed to be negligible. Hence, the above evapotran-spiration measurements could be safely assumed to reflect the transpiration levels.
2.3. Experimental treatments
To examine the factors influencing the transpiration rate of tea, three treatments were imposed on the ex-perimental plots at different times. To determine the influence of irradiance, four different levels of shad-ing (25, 60, 80 and 85%) were imposed on the plots by covering them with varying layers of shade net-ting. The whole plot area of 8 m×6 m, which included about 66 tea plants, was covered. A control plot was kept unshaded. One week after the imposition of shading treatments, transpiration was measured
simultaneously over a period of 3 days at different shade levels and the control. During these measure-ments, all plots were irrigated to eliminate water stress.
To examine the influence of soil water availabil-ity, some of the plots were irrigated to field capacity on the day prior to the commencement of the tran-spiration measurements. Soil water content at a depth of 15 cm was measured gravimetrically on a weekly basis.
To examine the effect of a change in canopy re-flectance on transpiration, a 5% solution of Kaolin was sprayed on the canopy of some of the plots. Canopy temperature in the Kaolin-sprayed and control plots was measured using an infra-red thermometer (HORIBA, IT-330) simultaneously with the transpi-ration measurements.
Continuous measurements of incident solar radia-tion, air temperature, relative humidity and wind speed at 1.2 m above the soil surface were made by Kipp so-larimeter, thermometer, hygrometer and anemometer, respectively, in an automatic weather station (Camp-bell Scientific Inc.) located in the experimental field. Measurements were recorded at 5 min time intervals, integrated or averaged over 30 min intervals and stored in a data logger. Saturation vapour pressure deficit of the air (D) was computed from relative humidity measurements as the difference between the satura-tion vapour pressure at air temperature and the actual vapour pressure.
2.4. Tea leaf yield
Leaf yield of tea was measured by plucking all the new shoots emerging above the canopy surface at 7-day intervals. Dry weight of leaf yield was measured after oven-drying at 105◦C to a constant weight
usu-ally over a period of 12 h. Transpiration efficiency (TE)
for leaf yield (in terms of made tea) was estimated by a linear regression between harvested leaf dry weight and transpiration. Value of the product between TE
Table 1
Meteorological conditions during the experimental period (from 1 January to 13 February 1999)a
Day of Mean solar irradiance Mean maximum Mean minimum Mean vapour pressure the year (MJ m−2per day) temperature (◦
C) temperature (◦
C) deficit (kPa)
0–7 22.26 23.8 9.9 0.52
8–14 20.51 24.7 12.0 0.62
15–21 23.63 24.4 9.7 0.81
22–28 24.11 24.5 7.7 0.88
29–35 22.91 22.9 10.3 0.82
36–42 21.37 21.4 8.6 0.85
aMeans of temperatures and vapour pressure deficits were measured at 1.2 m above soil surface and are 24 h mean values. No rainfall occurred during the experimental period.
3. Results
3.1. Meteorological conditions during the experimental period
The experiment was done during a rain-free pe-riod. There were clear skies and high irradiance levels (Table 1). Maximum and minimum temperatures were typical for this agroclimatic zone. The 24 h mean sat-uration deficit did not exceed 1 kPa throughout the experimental period.
3.2. Variation of transpiration with soil water content
The simultaneous variation of transpiration rate and top soil (0–15 cm depth) water content with day of the year during the period between 1 January and 14 February 1997 is shown in Fig. 1. The soil water content (S) decreased from the field capacity water content of 44% at Day 1 to 15% at Day 23. The transpiration rate also showed a similar pattern with 1.59 l per plant per day near field capacity to 0.7 l per plant per day at 15% which was below the permanent wilting point. After Day 23, S increased up to 24% due to irrigation. Transpiration rate also increased up to 1.1 l per plant per day (Fig. 1).
Fig. 2 shows the variation of transpiration rate with S. The transpiration rate declined only slightly when S decreased from field capacity (44%) to 33%. However, the transpiration rate showed a rapid decline between S of 33 and 15%.
The diurnal variation of transpiration for two plants on Day 1 (S near field capacity) and on Day 28 (S near permanent wilting point) is shown in Fig. 3. The transpiration rates were much less on Day 28 as
compared to Day 1. When S was near field capac-ity, maximum transpiration rates of 0.53–0.93 l per plant per day occurred between 1000 and 1500 h. The corresponding maxima when the S was near perma-nent wilting point were 0.27–0.53 l per plant per day. Although irradiance and saturation vapour pressure deficit (D) were higher on Day 28, the transpiration rates were lower than on Day 1. This indicates the important role of S in controlling transpiration rate in rainfed conditions. Fig. 3 also shows that transpira-tion rates vary appreciably from plant to plant even within the same plot. This could be due to differences in both canopy area and rooting patterns. Although plants with approximately similar stem bases were
Fig. 2. Relationship between soil water content and transpiration rate of mature clonal tea. The curve is drawn by hand.
selected for measurement, the number of branches, the canopy foliage area and the depth and spread of the root system could be different for different plants. A plant with a greater canopy area could intercept a greater amount of solar irradiance and consequently could have a greater transpiration rate than a plant with a similar stem base, but a smaller canopy size. Likewise, a plant with a more extensive root system may have a greater transpiration rate because of its greater rate of water extraction from the soil.
3.3. Variation of transpiration with shading
Fig. 4 shows the relationship between transpiration rate and irradiance as a percentage of the unshaded control. Transpiration decreased linearly with decreas-ing irradiance throughout the range of radiation lev-els tested at a rate of 0.031 l per plant per day per % reduction in solar irradiance.
The diurnal variation of transpiration for the un-shaded control and 85% shade treatment (i.e. 15% irra-diance) is shown in Fig. 5. It shows that the reduction of transpiration due to shading occurred during the period around mid-day between 1000 and 1500 h. The sudden drop in transpiration rate and its subsequent increase around 1500 h in the unshaded plant could probably be due to the shadow of a nearby Grevillea tree. Because of the diurnal variation of solar angle,
even tea plants in the open (i.e. not under the canopy of Grevillea trees) could experience transient shading. Fig. 5 shows similar transpiration rates in both the shaded and unshaded treatments from 1500 h on-wards. This could probably be due to sunlight coming under the shade at lower sun angles and increasing the transpiration rate of the shaded treatment. It is also possible that D was lower under the shade than in the open. Although, this may have increased stomatal conductance in the afternoon, the lower canopy-air vapour pressure gradient could negate any significant increase in transpiration rate due to greater stomatal conductance.
The effect of natural shade provided by a 12-year-old G. robusta on transpiration of tea is shown in Fig. 6 as a comparison with the effects of artificial shade. The daily transpiration rates of the two tea plants (1.07 and 0.42 l per plant per day) under the Grevillea tree were considerably lower than the value of tea plants in the open, 3.511 l per plant per day (Fig. 4). The variation between the two plants in their transpiration rates was probably due to the varying levels of shade provided by the Grevillea canopy as determined by the respective location of each plant in relation to the canopy overhead. The transpiration rates of the two plants corresponded to shading levels of around 75 and 85% in Fig. 4 (i.e. 25–15% irradi-ance). Grevillea had a daily transpiration of 2.2 l per plant per day. Except during early morning and late evening, the diurnal variation pattern of transpiration rates were similar for tea and Grevillea.
3.4. Effects of change in canopy reflectance on transpiration
Fig. 7 shows the effects of spraying Kaolin on the diurnal variation of canopy temperature and transpi-ration rate. Kaolin decreased the canopy temperature by 2–4◦C especially around mid-day (Fig. 7a). This
is probably because of the increased reflectance of Kaolin-sprayed canopies. Kaolin also decreased tran-spiration slightly during the period between 1000 and 1500 h (Fig. 7b).
3.5. Transpiration efficiency (TE)
Fig. 5. Diurnal variation of transpiration rate of mature clonal tea under open conditions (unbroken line) and at 85% artificial shade (broken line).
Fig. 4. Response of transpiration of mature clonal tea to shad-ing over a period of 1 day. Estimated linear regression line is: transpiration=0.238+(0.031×% irradiance). Standard error of regression slope=0.004 and adjusted R2=0.95.
TEas given by the slope of this relationship was 9.637
kg (made tea) ha−1mm−1of water transpired. The re-lationship between total dry matter yield and the ratio between transpiration and mean D also was linear. The proportionality constant was 6.9 g kg−1kPa. Standard-ization of transpiration values with D did not increase the precision of the linear relationship significantly.
4. Discussion
Fig. 6. Diurnal variation of transpiration rate of Grevillea robusta (unbroken line) and two tea plants (broken and dotted lines) under the shade of Gravillea.
experiment, Ritchie (1973) found that in maize, about 80% of available water had to be depleted before transpiration fell significantly. However, this limiting value of available water also depends on the atmo-spheric demand for water vapour (Denmead and Shaw, 1962) as determined by irradiance, vapour pressure deficit and boundary layer resistance (Penman, 1948). In the present experiment, S was measured only in the first 15 cm of the soil profile. In the clonal tea used here, a high proportion of roots were present in this layer. However, most probably, the roots were ab-sorbing water from deeper layers in the soil profile as well to maintain transpiration rates at maximum lev-els. Therefore, the limiting depletion value of 65% available water was most probably determined by the soil water availability in the deeper layers of the soil profile and the depth and extent of the root system as well. Transpiration rate could be expected to decline at a higher level of soil water availability when the soil water availability and rooting depths were lower and the atmospheric demand was higher. Neverthe-less, the limiting value of 65% depletion of available soil water has a practical significance for the specific conditions prevailing in the tea-growing regions at
higher altitudes in Sri Lanka. Based on the results of Carr (1969, 1974); Willat (1971); Stephens and Carr (1989); Stephens and Carr (1991), it was concluded that for tea growing in Tanzania and Malawi, the ac-tual evapotranspiration begins to decline significantly below the potential evapotranspiration when approxi-mately 30–40% of the soil available water is depleted. This was a much lower limiting depletion level than the value of 65% found in the present experiment. This was probably because of the lower atmospheric demand as indicated by the lower D (<1 kPa, Table 1) of the present experiment as compared to 2 kPa in Stephens and Carr (1991).
The decrease of transpiration rate with the decrease of available soil water can be due to a combination of several phenomena such as increased canopy re-sistance (Monteith et al., 1965), increased hydraulic resistance within the xylem and increased resistance at the soil–root interphase (Passioura, 1988a). Further experimentation is needed to separate the relative con-tribution of each of the above factors.
Fig. 8. Relationship between transpiration and yield of made tea for weekly periods during the experiment. Estimated linear regression line is: yield=9.637×transpiration. Standard error of regression coefficient=0.953 and adjusted R2=0.64.
in the canopy. The sensitivity of transpiration to irra-diance is dependent on the degree of coupling of the canopy to the environment (McNaughton and Jarvis, 1986) with the sensitivity increasing with decreased coupling. Tea has a short (about 1 m high) and smooth canopy which could be expected to have a low degree of coupling to the surrounding environment (Jones, 1992). This agrees with the observed sensitivity of transpiration rates of tea to irradiance in the present experiment.
On the other hand, the environment in which the present experiment was done experiences strong winds which increases the coupling between the canopy and the surrounding environment to a certain extent. Therefore, the decoupling coefficient (or the McNaughton and Jarvis factor) of tea canopies of the present experiment could have been around 0.5 (on the basis of the values given in Jones, 1992) which makes the transpiration of tea sensitive to both available energy supply (i.e. irradiance) and stomatal factors as determined by the soil water availability (Passioura, 1988b).
The value of transpiration efficiency (TE) for leaf
yield of tea in the present experiment (9.637 kg ha−1 mm−1) is higher than the range of water use efficien-cies of 1.5–5.2 kg ha−1mm−1 observed by Stephens
and Carr (1991). This may be partly because Stephens and Carr (1991) values are based on water use which includes both transpiration and soil evaporation. The value of 6.9 g kg−1kPa for the product between TE
and D observed in the present experiment is slightly higher than the maximum of 5.0 g kg−1kPa observed for groundnut (Ong et al., 1987), a C3 crop like
tea. In agreement with theoretical analyses of Bier-huizen and Slatyer (1965); Monteith (1986) and Jones (1992), the value for tea is lower than the range of 8.4–10.6 g kg−1kPa observed for a C4 crop, pearl
millet (Squire, 1990).
5. Conclusions
Transpiration rates of field-grown mature tea canopies under rainfed conditions are determined by both soil water contents and irradiance levels. In the atmospheric and soil conditions of the tea-growing areas of the higher altitudes of Sri Lanka, around 65% of available soil water has to be depleted before a significant decrease of transpiration occurs. Tran-spiration decreased with a decrease in irradiance at a rate of 0.031 l per plant per day per % reduction in so-lar irradiance. The estimated transpiration efficiency of tea for the present environment is 9.637 kg (made tea) ha−1mm−1of water transpired. The product be-tween TE and D for tea in the present experiment is
6.9 g kg−1kPa.
Acknowledgements
The authors wish to acknowledge the help of Dr. J. Mohotti, Mrs. S. Nawaratne, Mrs. Sithakaran and staff of the Physiology Division of the Tea Research Institute, Talawakelle, Sri Lanka.
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