The increasing worldwide shortages of water and cost of irrigation are leading
to an emphasis on developing methods of irrigation that minimize water use (maximize
the water use efficiency). This has lead to irrigation scheduling which is conventionally
aimed to achieve an optimum water supply for productivity, with soil water content
being maintained close to field capacity. In many ways irrigation scheduling
can be regarded as a mature research field which has moved from innovative science
into the realms of use, or at most the refinement, of existing practical application.
Nevertheless, in recent years there has been a wide range of proposed novel
approaches to irrigation scheduling which has not yet been widely adopted; many
of these are based on sensing the soil moisture status directly (Jones,
1990). Regardless of irrigation scheduling plant still regulates their diurnal
water status at a favourable level by the control of stomatal aperture. Stomatal
closure helps to maintain a high leaf water potential, which leads to a reduction
in photosynthetic activity. Stomatal closure reduces CO2 entry into
leaves which reduces the intercellular CO2 concentration and lowers
C fixation. This causes an imbalance between photochemical activity at photosystem
II (PS II) and electron requirement for photosynthesis and leads to increased
susceptibility to photo-damage (Jones, 2004).
Plant use the sunlight in any growth stage and after using the light energy
for living activities, unused energy in that stage is re-emitted as fluorescence
(Saito et al., 2003).
Chlorophyll fluorescence is a very sensitive probe of the physiological status
of leaves; it provides very rapid assessment of plant performance in a wide
range of situations. Chlorophyll fluorescence parameters such as PS II photochemical
efficiency (FV/FM), chlorophyll Fluorescence decrease
ratio (Rfd) and size of the acceptor pools available to PSII (Sm)
indicates seasonal changes in the activity of photosynthetic apparatus (Krause
and Wies, 1991).
However under stress, the photosynthetic quantum conversion declines and correspondingly heat emission and chlorophyll fluorescence increases considerably.
Krause and Wies (1991) reported that chlorophyll fluorescence
indirectly measures photosynthesis efficiency. If a leave is placed in a dark
for a couple of minutes and then is returned to the light, fluorescence quickly
rises to an initial level (FO). Fluorescence increases from FO
to its maximum (FM) due to the rapid decrease of electron accepting
QA (quinone-type acceptor) molecules. The variable fluorescence (FV)
is the difference between FM and FO and is extremely sensitive
to changes in the ultrastructure of membranes and rates of electron transfer.
Hence FV/FM can be presented as the yield of photochemical
reaction (Krause and Wies, 1991).
The study was undertaken to identify ways in which chlorophyll fluorescence
may be used effectively to select the best irrigation interval for tomato plants
MATERIALS AND METHODS
The study area: The study was carried out at the University of Cape
Coast (UCC) Teaching and Research Farm from 2006-2007. It falls within the coastal
savanna zone of Ghana between latitude 05° 03N and 05° 15N,
longitude 01° 13W and 01° 13W. The area is characterized by a
mean annual rainfall, which varies from about 750 to 1200 mm. The area has two
seasons that is dry season and wet season. The wet season can also be divided
into two, the minor one and the major one. The major season is from May to July
with a peak in June and the minor season is from September to November with
a peak in October. The main dry season is from December to February (Ayittah,
Temperatures are uniformly high throughout the year with an annual average minimum of 30°C. Diurnal variations in temperature are greatest in February and March.
Determination of Etc: The mean daily Eto for the study area was computed
using Modified Penman method (Sam-Amoah, 1996). The
Etc was then derived from the formulae below:
||Reference Evapotranspiration (Modified Penman Method)
Total volume of water for the growing period was 31.48 m3. The same
amount was used for each of the treatments.
Chlorophyll fluorescence determination: In order to determine the effect
of irrigation interval on the chlorophyll fluorescence parameters of tomato
plant, only the leaves that had most recently matured. i.e., third leaves from
the apex were used for measuring the leaf photosynthetic performance using Compact
Continuous Violet Laser-Induced Fluorosensor (CVLIF) (Anderson
et al., 2004). The photosynthetic apparatus was used to register
and determine the continuous chlorophyll fluorescence spectra and induction
kinetics in the 685 and 740 nm as well as the ratio F685/F740. Radiations from
a continuous-wave violet laser diode emitting at 396 nm through a fibre was
closely incident on the detached leaves of tomato to excite chlorophyll pigments,
which was detected by an integrated spectrometer with CCD readout. The chlorophyll
fluorescence spectra with peaks at 685 and 740 nm were monitored for 180 s for
each of the tomato leaves giving fluorescence kinetics curve or Kautsky curve
(Anderson et al., 2004). The Kautskys curves
were monitored from a maximum intensity level (Fmax) followed by a fluorescence
decay until a steady state (Fs) was achieved. From the slow fluorescence decrease
(Fd = Fmax-Fs), the fluorescence decrease ratio (Rfd = Fd/Fs) of the leaf was
calculated. Stress adaptation index (Ap) was then computed from the formulae
||Stress adaptation index
||Fluorescence decrease ratio for far red band
||Fluorescence decrease ratio for red band
Experimental design for irrigation interval on tomato growth: A randomized
complete block design (RCBD) was used. There were 16 plots and each plot size
was 8x8 m. There were eight rows with plant spacing
of 1xl m and plant population per plot was 64.
Seed: The Wosowoso variety of tomato was used. It was obtained from a certified seed company in Cape Coast.
Nursing and planting: The seeds were nursed and planting was done a month after nursing. Growing period was 90 days.
Irrigation: There were four treatments with four replications. The treatments were: daily application of water (T1), every third day (T2), every fifth day (T3) and every seventh day (T4). The treatments were imposed two weeks after transplanting.
Moisture determination: Soil water contents were measured before and after irrigation in the laboratory by the gravimetric method.
Crop measurement: Data collected on plant growth included: Chlorophyll fluorescence and adaptability. There were five sampling times and measurements were taken on five plants on each.
Cultural practices: The plants were staked and fertilizer application to all treatment was done. Plants were kept free of weed by repeated hand weeding and insects, pest and diseases were controlled with fungicide and insecticide.
RESULTS AND DISCUSSION
Soil moisture content: From Table 1, T1 (15.1) and T2 (15.1) had the highest mean soil moisture content after irrigation. T3 (5.4) had the highest mean soil moisture content before irrigation but T4 (4.2 and 11.7) had the lowest mean soil moisture content before and after irrigation, respectively.
From Table 2, the soil water depth increased as the amount applied increased. During the growing stage, more water was required at the development stage and mid-season and then it declined during the late-season.
Fluorescence decrease ratio (Rfd at F 690) and Irrigation interval: From Fig. 1, T1 had the highest fluorescence decrease ratio (Rfd) at F690 of 2.465 but T2 recorded the lowest fluorescence decrease ratio (Rfd) at F 690 of 1.690.
From Fig. 2, T1 had the highest fluorescence decrease ratio (Rfd) at F740 of 1.929 but T4 recorded the lowest fluorescence decrease ratio at F740 of 1.101.
|| Soil moisture content before and after irrigation for the
||Soil water depth at different growth stages
||Fluorescence decrease ratio (Rfd) and irrigation interval
fluorescence decrease ratio (Rfd at F 740) and irrigation interval
|| Fluorescence decrease ratio (Rfd) and irrigation interval
Adaptability and irrigation interval: T1 had the lowest adaptability of 0.155. The highest adaptability value of 0.253 was recorded by T3.
There were significant differences among the treatments for adaptability.
Effect of soil moisture content on chlorophyll fluorescence: In this
experiment after irrigation, one day irrigation interval had the highest moisture
content which resulted in higher plant growth which was in agreement with Wang
et al. (2003). They indicated that drought stress is one of the major
causes for crop loss worldwide, reducing average yields with 50% and over. Maxwell
and Johnson (2000) also indicated that plant growth depends on cell division,
enlargement and differentiation and all these are influenced by other physiological
processes and it was in agreement with the present study.
From the present study, it was observed that increase in chlorophyll fluorescence
and adaptability had effect on the growth of the tomato plant which was in agreement
with Krause and Wei (1991). They reported that chlorophyll
fluorescence indirectly measures photosynthesis efficiency.
Hetherington et al. (1998) also assessed the
photosynthetic activities of different chlorophyll containing part of tomato
plants. They concluded that the non-leaf green tissues of tomato are quite active
photosynthetically and therefore potentially contribute significantly to plant
growth which was in agreement with the present study.
Fluorescence decrease ratio (Rfd), adaptability and irrigation interval:
Chlorophyll fluorescence parameters such as adaptability and fluorescence decrease
ratio were affected by change in irrigation interval.
|| Adaptability and irrigation interval
The decrease of fluorescence decrease ratio (Rfd) in tomatoes under the sprinkler
irrigation could be explained as photoinhibitory effect on the Calvin cycle
by changes in the environmental conditions such as temperature, oxygen, light
etc and physiological status of plants (Krupa et al.,
1993). T4 had the lowest fluorescence decrease ratio of 1.702 and this could
be due to decrease in the rate of utilization of Adenosine Triphosphate (ATP)
and Nicotinamide Adenine Dinucleotide Phosphate (NADP) in photosynthetic metabolism
which could not be compensated for the next time of application of water. Increase
in irrigation interval could cause inhibition of photosynthetic metabolism and
can result in the decline in photosynthetic potential which when sufficiently
large to overcome stomatal limitation will result in a further decrease in CO2
assimilation rate (Lawlor and Cornic, 2002).
From Fig. 2, T2, T3 and T4 had
Rfd values less than 2.0 but T1 had Rfd values greater than 2.3.
This can be speculated that the lower fluorescence is due to increase in the
nonradioactive energy dissipation. It is also known that drought may lead to
an increase in nonphotochemical quenching (Sheuermann et
From Fig. 3, it can be seen that as irrigation interval increases
then adaptability also increases and the higher the Rfd value the higher the
adaptability. T3 recorded the highest adaptability (Vitality) Value
of 0.253 and was well adapted with other environmental stress conditions present.
Pukacki (1991) indicated that for well-stressed adapted
plants the Rfd value is less than 2.3 and plant of lower vitality indicate Rfd
Value less than 2.0 and it was in conformity with the present study.
Irrigation interval of five days, using the same amount of irrigation water was physiologically (in terms of adaptability) better than 1, 3 and 5 and adaptability could be used as an effective parameter to estimate the water status of tomatoes under various environmental stress conditions. Tomato plant under T1 resulted in higher fluorescence decrease ratio (Rfd) than those under the other treatments.
T3 (0.22) should be practiced so that a higher fluorescence decrease
ratio and adaptability can be achieved for tomato production during the dry
Author wish to express his sincere thanks to the Department of Agricultural Engineering, University of Cape Coast, Ghana for supporting this experimental research. Author thanks are also extended to his family especially his twin brother Mr. Paul Osei Boamah for his encouragement and financial support.