Agricultural and Forest Meteorology 103 2000 375–386
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
a
Tea Research Institute, St. Coomb’s Estate, Talawakelle, Sri Lanka
b
Department of Crop Science, Faculty of Agriculture, University of Peradeniya, Peradeniya 20400, Sri Lanka
c
Decagon, 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
− 1
h
− 1
occurred 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
− 1
h
− 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 T
E
was 9.637 kg ha
− 1
made tea mm
− 1
of 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
− 1
kPa. © 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: costacropsci.pdn.ac.lk 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.
0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 1 3 4 - 9
376 A. Anandacoomaraswamy et al. Agricultural and Forest Meteorology 103 2000 375–386
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 T
E
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 T
E
. It has been shown that the product between transpiration efficiency T
E
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 T
E
may vary with canopy characteristics and atmospheric conditions.
Despite the above complications, for most practical purposes, an estimation of T
E
and 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 T
E
. 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 evaporation, 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
