1.4.1 Effects of temperature on tomato crop photosynthesis
The effect of temperature on photosynthesis depends on the species, cultivar, the environmental conditions under which the plant was grown and the environmental conditions during measurement. Increases in temperature usually increase photosynthetic rates until enzyme denaturation and photosystem destruction begin (Salisbury and Ross, 1 989). However, respiratory CO2 loss also increases with temperature, and this is especially pronounced for photorespiration, largely because a temperature rise increases the ratio of dissolved O2 to CO2 (Salisbury and Ross, 1989). As a result of O2 competition, net CO2 fIxation in C3 plants is not promoted by increased temperature as much as would be expected.
The promoting effect of a temperature rise is nearly balanced by increased respiration and photorespiration over much of the temperature range at which C3 plants normally grow, so a flat and broad temperature response curve between 15 and 30°C often occurs.
There is also evidence that at high temperatures ATP and NADPH are not produced fast enough in C3 plants to allow increases in CO2 fixation, so formation of ribulose biphosphate becomes limiting (Salisbury and Ross, 1989). Lipton ( 1970), reported that high greenhouse humidity can aggravate the effects of high air temperatures, as the leaves can not transpire efficiently enough to cool the plant. It was found that with air temperatures reaching 30 to 35 °C, the actual temperature of the leaves was close to 35 to 45 °C, temperatures that approach those lethal to young cells of higher plants (Lipton,
1 970).
light and CO2 levels. This is further complicated by the fact that different cultivars exhibit different response curves to light, CO2 and temperature due to different net photosynthetic rates and photochemical and carboxylation efficiencies (Augustine et aI, 1976). Although genotypes vary in the temperature response of leaf net photosynthesis, most show an optimum between 25 and 30 °C.
Many responses to different day/night temperatures have been reported. de Koning ( 1 988) concluded that for young plants a high day/low night temperature regime seems most suitable to achieve a greater increase in light interception and maximum growth. However, great differences between day and night temperature will give elongated weak plants. Later, a low day/high night temperature regime is preferred to produce the maximum yield without loss of fruit quality (de Koning, 1 988).
1.4.2 The effect of solution temperature on tomato plant growth and development
Tomato root system heating has been investigated for at least 30 years and has shown to generally increase vegetative growth with only limited effects on tomato yield at either normal or sub normal air temperature (Hurd and Graves, 1985). An advantage of the NFl' system is the ease at which root temperatures can be maintained or modified with much more flexibility then is possible with soil. A further energy related advantage of Nfl is that the running costs for heating NFl' solutions are low since all the heat subsequently lost from the gullies contributes to heating the air (Hurd and Graves, 1985).
It is recommended that solution temperature be maintained in the range 18 -20 °C, as
high solution temperatures early in the season especially with reduced air temperatures results in poor fruit quality (Drakes et al, 1984). Temperature of nutrient solution, however, should not fall below 1 5 °C, and it has been shown that roots regenerate faster
after root death where the solution is warmed. In situations under warm conditions, solution temperatures in excess of 30 °C have been recorded in exposed return pipes, and this resulted in poor fruit quality (Drakes et al, 1984).
Takano ( 1 988), reported that there was a close relationship between plant growth and root zone temperature in NFf systems, and that heating the solution can supplement greenhouse air heating. It was found that at a solution temperature of 27 °C, dry matter production and mineral uptake were relatively high, even though the air temperature fell to 5 °C at night in winter.. It was concluded that this temperature results in higher photosynthetic and transpiratory efficiency due to the maintenance of lower resistance of water and ion uptake by the root system (Takano, 1 988). It has also been found that in tomato plant roots grown at 20 °C compared to 8 °C, the chloroplasts �ere rounded with clearly visible grana and large osmiophilic granules at 20 DC, whereas the lower temperature resulted in disintegration in the chloroplasts.
The effect of solution temperature on plant growth and nutrient uptake was studied by Moorby and Graves ( 1980) who stated that the rate of water uptake was a positive function of root temperature as was the rate of nutrient· uptake and that the response curve shows a broad optimum in the region of a solution temperature of 25 - 30 °C. In this investigation by Moorby and Graves ( 1 980), it was also found that heating the roots led to initially faster rates of growth of seedlings and larger mature plants with larger leaf areas. Devonald (1987) also found that plant height, fresh weight and dry weight were all affected by solution temperature. The production of larger plants at high root temperatures implies that a greater rate of carbon fixation is occurring, as a result of the increased leaf area in the wanner solution temperatures (25 - 30 °C). Hurd and Graves ( 1985) concluded that the beneficial effects of high solution temperatures on leaf growth and fruiting may be due to increase in water or nutrient uptake by the roots As well as larger mature plants, it was found that there was a larger fruit yield at higher root temperatures resulting from both an increased number of fruit and the production of larger fruit (Moorby and Graves, 1980). This was also reported by Cooper (1973). Although varying the root temperature for a short period did not appear to affect tomato yields, it was found that continuous solution temperatures in the optimum range of 30 -
35 °C increased fruit number, fruit size and fruit yield. However other researchers found the optimum temperature range for increased yield due to a larger fruit size
(Chong and Ito, 1 982; Giacomelli and Janes, 1986; Hurd and Graves, 1 985) or an increased number of fruits (Moss, 1 983) was in the range 20 - 25°C. Maher ( 1980) reported that raising the solution temperature to 25 °C did not affect the development of early fruit or the yield, however, by the end of the experiment, the heated solution produced 3Kg more fruit then the unheated solution.
In another investigation Hurd and Graves ( 1 985) stated that increasing root temperatures of up to 1 2 °C above the ambient of 1 5 °C, resulted in small reductions in yield and quality of early fruit. During the fruiting phase however, root heating increased final yield by about 10% over 20 weeks of harvesting. Hurd and Graves ( 1 985) also reported that high root temperatures (25 °C) gave overall increases in yield and crop value, despite their detrimental effects on early yield and fruit quality. However, by delaying root heating until after fruit set, the detrimental effects on early fruit quality could be avoided.
Maher ( 1980) found that heating the nutrient solution to 25 °C resulted in increased shoot vigour and root size particularly at lower night air temperatures ( 1 3 and 5 °C). This was also reported in a review by Cooper (1973), who demonstrated that shoot dry weight increases with root temperature up to 30 °C. Maher ( 1 980) also found that a solution temperature of 25 °C produced a large increase in root size and raised the levels of minerals in the leaf. It was also noted that total leaf area increases with solution temperature to a maximum of 30 °C then fell off steeply in the 30 - 40 °C range (Cooper, 1 973).
It can therefore be concluded that raising the nutrient solution temperature to 25 - 30 °C has a beneficial effect on several plant processes. These beneficial effects of solution heating include increased transpiration and mineral uptake, increased seedling growth, the development of larger mature plants and increased fmal leaf area, resulting in increases in net photosynthesis and carbon fixation. As a result increases in fruit yield may be expected.
1.4.3 Summary
The growth, development and yield of the tomato crop is influenced by both the genetic potential of the plant and the environment it is exposed to. Process such as nutrient uptake, distribution and photosynthesis provide the basis for plant growth and effect the subsequent growth rate, flowering and fruit development. Environmental factors such as radiation levels, temperature and CO2 all influence crop growth and management of these factors is aimed at maximising assimilate production to obtain the greatest yields.
1.5 THE SOURCE SINK RELATIONSIDP OF THE TOMATO PLANT