Studies with roots are particularly difficult, because soil temperatures at dffferent dep�hs are correlated, and the development of different organs and dffferent root numbers is also correlated. These multiple correlations make definite statements about which environmental factor controls individual processes very difficult. The rapid development of main and lateral roots could result from either, a direct effect of local temperature or a direct effect of shoot meristem temperature or, an indirect effect of the shoot meristem, or temperature affecting shoot growth and therefore the supply of assimilates and honnones to the roots (Gregory 1 983). In addition, the work of Watts (1 972b)
has drawn attention to the significance of the shoot meristem, but the soil temperature was also recognized as an important determinant of early shoot I growth, because the temperature of the root system can affect plant growth by
altering the rate of absorption and movement of water and nutrients, and the production and translocation of plant regulating hormones. Air temperature is also important as high leaf temperature can lower rates of net photosynthesis
root growth than shoot growth and hence high shoot : root ratios.
Lahav and Trochoulias (1 982) demonstrated that at low temperature avocado
plants had better root growth and more dry matter accumulation compared with high temperature, as at high temperature the pattern of distribution of assimilates had less going to the roots and no change in the movement of carbohydrates out of the leaves. Furthermore, more roots appeared to have died during the high temperature treatment. The optimal temperature for root growth-was D21/N1 4°C for cv. 'Hass' (heat sensitive) and D25/N1 8°C for cv. 'Fuerte' (heat insensitive). Thus high temperature was more advantageous in all parameters measured except for root dry matter production, and the heat insensitive genotype had a higher optimal temperature compared to the heat sensitive one. However, the shoot/root balance was changed in favour of shoots by high and supra-optimal temperatures. A similar situation may exist in asparagus, where growth in tropical climates may favour the shoot, but heat insensitive genotypes may maintain a lower shoot/root ratio at high tern peratures.
Hurewitz and Janes (1 983) showed that tomato plants grown with a cool root
zone temperature (1 8°C and below) had thicker darker green leaves with increasing purplish coloration on the underside of their leaves. Their roots were short and thick as opposed to plants grown at warmer root zone temperatures (26°C and above) whose root were long, thin and profusely branched. lt appears, therefore that an environment which provides for satisfactory root growth would also benefit the shoot growth of the plant. Cool root zone temperatures are capable of inhibiting and warm temperature are capable of
stimulating plant growth when air temperatures are not limiting growth, I
because vital temperature dependent processes (e.g. sink activity of roots) which occur in the root exert control over shoot as well as root activities.
The examination of rape by Cumbus and Nye (1 982) also indicated similar results, in that root dry weight (and root length) were clearly temperature
dependent; increasing from 1 0 to 30°C, and decreasing at 35°C. Root
branching increased with higher temperatures as did the intensity of brown pigmentation. In apple trees, supra-optimal root temperatures showed no serious damage to roots up to 35°C, except for their reduced size. At 40°C,
different degrees of dieback appeared, and the quantity of dead roots increased. Leaf senescence was noticeable at 40°C. At 35°C similar symptoms
occurred m uch later and less severely. It was suggested that the reduction of leaf chlorophyll level at supra-optimal root temperature was due to a reduction in the supply of cytokinins from less active roots· ( Gur et al. 1976).
Hence it is likely that the control of the root growth of asparagus in tropical climates will be complex, because root growth is associated with assimilation and assimilate partitioning, which is strongly influenced by temperature. A thermotolerant line may have a greater ability to maintain normal growth. High rates of root growth at high temperature would be a favourable characteristic in asparagus, because the carbohydrate pool for spear growth is the roots.
1 .6. Effects of Abscisic acid on Growth at High Temperatures
1 .6.1 . Plant Hormones and High Temperatures
Environmental stress is known to cause hormonal changes. Such changes are evident when plants are exposed to heat, water, salinity or flooding, and may be part of the mechanism enabling plants to survive environmental stress. When the stress is relieved, the reverse process begins in which metabolic processes tend to normal conditions ( ltai et a/, 1 978). Yakushkina and Tarasov (1 982) reported that the high growth rate of maize seedling at 25°C
I
was accompanied by high activity of auxins, GA, and cytokinins and low activity of ABA; the reverse occurred at 46°C. Application of GA at 1 00 mg/1
increased growth and removed the inhibitory effect of high temperature. This result was contrary to that of Onwueme and Laude (1 972), who found that 0.5
the GA concentration was too low. Matsui et a/ (1 986) also found that when
grapevines were exposed to temperatures exceeding 40°C during the growing
season, the content of GA in the berries declined thus indicating that fruit cells appeared to lose their ability to synthesize or import the growth promoting h ormone and assimilate from leaves and buds. The growth inhibition resulting from decreased GA level was partially reversible by application of GA3 or with promalin.
1 .6.2. Changes in Endogenous ABA at High Temperatures
Plants exposed to stress, such as high temperature or low moisture, react by producing ABA, which functions to slow/stop growth and induce resistance,
which in turn may hasten the senescence process (Hellali and Kester 1 979).
For example the ABA concentrations was significantly higher in young leaves
of N-deficient (stressed) plants of tomato (Daie et al. 1 979). Durley et al.
( 1 983) examined the ABA and IAA concentrations in leaves of sorghum under
drought stress, and found that there was considerable genotypic variation, and that the mean leaf ABA concentration in drought-stressed plants was positively related to a reduction in grain yield. They concluded that it is possible to evaluate genotype drought resistance to a given stress treatment in sorghum by examination of ABA levels.
lt is suggested that ABA is a stress hormone, which regulates stress
responses common to many environmental stresses (Boussiba et a/ 1 975). ltai
et al. (1 978) reported that the ABA content was not consistent with this
assumption. Rikin et al. (1 975) showed that ABA and GA are involved in an
interrelationship between morphogenesis and cold resistance in seedlings of '
two alfalfa cultivars (Ranger and Hairy Peruvian), while Waldman et al. (1 975)
found that ABA-Iike activity was hardly affected by thermophotoperiod
(alterating day I night temperature regime). Thus they suggested that the
m odification of the ABNGA balance through the decrease of the GA level, m onitors the capacity of the two alfalfa cultivars to become cold acclimated
when exposed to low temperature. Atkin et al. (1 973) measured the inhibitor
content in xylem exudate of maize at various temperatures, and found that there was a lower inhibitor activity of xylem exudate at 33°C than at both 28°C
and 23°C, and that at 33°C the temperature was optimal for shoot extension
growth. ltai et al. (1 973) treated the root system of tobacco and bean for 2
m in utes at 46-47°C (extremely high tempera1ure), and found that heat
treatment reduced cytokinin levels and increased ABA levels in the xylem
exudate of maize.
Daie and Campbe/1 (1 981 ) proposed that ABA may be a common mediator for
all stressed plants, and may also be involved in a stress syndrome response, because stressful temperatures, whether constant or diurnal, caused an increase in ABA levels of tomato plants (Daie and Campbe/1 1 981 ). However
when Eze et al. (1 983) examined the effects of temperature and moisture
stress on the accumulation of ABA in bean, they suggested that the rapid accumulation of ABA during temperature stress is a function of induced m oisture stress deficits and does not result from high or low temperature per se, in contrast with those responses reported for tomato by Daie and Campbe/1 ( 1 981 ).
Radin and Hendrix (1 986) found that the accumulation of ABA in leaf discs of corn incubated in an osmotic stress solution (polyethylene glycol 8000) from 20 - 35°C depended strongly upon temperature, being maximum at 20°C, while
at 35°C the amount of ABA accumulated was 45-80o/o less that at 20°C. In fact,
the production of ABA in cotton leaf at 35°C is higher than at 20°C, but due to
the higher turnover rate at 35°C, the ABA accumulation was less at 35°C than
at 20°C. Daie et al.(1 981 ) exposed the seedlings of cool season crops (beet, I
lettuce, radish, pea) and warm season crops (bean, corn, eggplant, okra) to supra- and sub-optimal temperatures (40, 25, 1 0°C), and the results supported
the hypothesis that ABA is produced under temperature stress. For cool
season crops 40°C was a marginally stressful temperature, and at
contrast, all warm-season crops showed as much as a 5-fold increase in free ABA at 1 0°C compared to the concentration at 25°C. Thus the ABA levels or turnover rate appears to reflect the extent of stress, the effect depending on species.
1 .6.3. Exogenous ABA on Growth at high temperatures
The retardation of plant growth under high temperature has been described earlier, but the way in which high temperature retards growth is not clearly understood although, as discussed earlier, one cause may be the elevated
ABA content in both roots and shoots at high temperature (Biddington and
Dearman 1 982), since elevated ABA depressed cell elongation and cell
division in roots ( Torre et al. 1 972). However, Gaither et al. (1975) reported
that although ABA probably retains its physiological role as a growth inhibitor in roots of many species, it may also play a promotive role in the hormonal regulation of pea root growth. For example, the application of 1 JlM ABA increased root growth about 25 - 30%, while 0.01 JlM ABA enhanced and 1
JlM - 1 00 JlM ABA inhibited the elongation of maize root segments (Pi/et and
Rebeaud 1 983). Consequently it is suggested that low concentrations of ABA
m ay stimulate the growth and development of roots and shoots (Yamaguchi
and street 1 977; Abou-Mandour and Hartung 1 980). Thus ABA can act as a
growth promoter and this action probably involves an interaction with other
endogenous growth regulators ( Yamaguchi and Street 1 977).
The work of Biddington and Dearmen (1 982) showed that the addition of ABA
to the nutrient solution increased the root to shoot ratio of hydroponically gro�n cauliflower by reducing the dry weight of the shoot and increasing that
of the root. Similarly, Watts et al. (1 981 ) reported that elevating endogenous
ABA by water stress or by the application of exogenous ABA had similar effects on the growth of roots and shoots and ultimately the root to shoot dry weight ratio. Generally, low concentration ABA applications and mild water stress stimulated root growth, but higher levels of ABA application and water
stress reduced root growth. Overall the effect of stress was to increase the
ratio of roots to shoots. However, Creelman et al. (1 990} reported that
exogenous ABA did not induce the changes in sugar accumulation, polysome status and mRNA populations that were observed after soybean seedlings were water stressed.
1 .6.4. Possible R ole of ABA In Plant Growth at High Temperatures
The possibility that ABA is involved in a common stress regulation system had been proposed by Boussida et al. (1 975), who showed that tobacco plants
pre-exposed to drought, mineral deprivation or salinity, exhibited elevated levels of endogenous ABA and demonstrated resistance to a secondary stress of sub-zero temperature. Furthermore, it has been reported that application of ABA to cultured tobacco cells accelerated the rate of adaptation of cells to NaCI stress and cold (Keith and MeKersie 1 986; Orr et al. 1 986; Chen and
Gusta 1 983; Singh et al. 1 987). The work of Bonham-Smith et al. (1 987} also
showed that maize seedlings can acquire thermotolerance by pretreating seedlings through the roots, with either a heavy metal stress or a water stress.
Bonham-Smith et a/ (1 988a,b) also reported that maize seedlings pretreated
with an exogenous application of ABA or Triadimefon, (a fungicide capable of blocking GA synthesis), enhanced the plant's ability to withstand the effects of a 3 hour sub-lethal {40°C) or lethal {45°C) heat shock. The level of protection provided by these agents was dependent upon the length of time that the plant was exposed, as prolonged exposure reduced tolerance to subsequent stress.
1 .6.5. Endogenous Hormone Levels and Application of Plant Hormones on Growth at High Temperatures
Roots are known to produce hormonal factors that are required for sustained shoot growth and metabolism. Atkin et al. {1 973) found that the export of GA
from corn root declined markedly due to root heating. The heat stress not only affected root growth, but also affected the tops; thus temperature had a
diverse and complicated impact on plant growth. Menzel ( 1 980; 1 981 ; 1 983a,b;
1 985) found that a mildly high root temperature increased the activity of GA, increased stem growth and inhibited tuberization of potato. Moreover when
Hiller et al. (1 979) measured the relationship between seedstalk height and GA
content of the vernalized seedstalk of carrots, they found that supra-optimal temperatures severely reduced ultimate seedstalk height and endogenous GA
activity. In addition Matsui et al. (1 986) found that Napa Gamay grape berries
from heat stressed vines were smaller than from non-stressed vines. The effect of heat ,Stress was overcome by application of GA, but not BA. Thus h igh temperature stress may cause a decline in the GA level, but in contrast,
slightly lower temperatures may increase GA levels. Furthermore, Rikin (1 975)
showed that ABA and GA are involved in the interrelationship between morphogenesis and cold-resistance in the seedlings of alfalfa.
Chen et al. (1 986) found that GA applied to mungbean during the heat-shock
period not only increased the subsequent thermotolerance of seedlings treated at 45°C but also allowed survival at a higher temperature (48°C) . . Some of this effect appeared to be due to the inhibition of RNA and protein synthesis at high temperature. They suggested that GA treatment may enhance the thermotolerance of mungbean seedlings through stimulation of induction and
accumulation of heat shock proteins in the seedling dur_ing the heat shock
period. Lin et al. (1 985) reported that heat shock proteins function to prevent
leakage of various substance from soybean cells. This suggests that GA may promote the heat shock process by protecting membrane integrity. In addition,
Lafuente et al. (1 991 ) showed that although heat shock elevated ABA levels,
endogenous ABA levels were not directly related to the rate of electrolyte
1 .6.6. Other Hormones in Relation to High Temperatures
Cytokinins are synthesized in the roots and appear to play an important role in the processes of plant development. Atkin et al. (1 973) found that a high
temperature (33°C) caused a declined in cytokinin activity. This result is the reverse of that of Skene and Kerridge (1 967), who showed that a mildly high ·
temperature increased cytokinin activity, but Atkin et al. (1 973) explained his
high temperature result as being due to a reduction in exudate flow from the roots.
ltai et al. (1 973) exposed the root system of tobacco and bean to a
temperature of 46-4rC for two m inutes, and found a lower cytokinin activity and a higher ABA content. Mauk and Langille (1 978) also showed that zeatin
riboside levels in potato plants grown at 028/N 13°C with 1 0 hour photoperiod were significantly higher than those in plants grown at D30/N28°C with 1 8 hours photoperiod. Skogovist (1 974) speculated that if a certain content of
cytokinins in roots is required for keeping the cells alive, heat treatment could have suppressed or inhibited the cytokinin synthesis to such an extent that the root meristem died. Application of kinetin was found to maintain endogenous cytokinin above critical levels, possibly by preventing the formation of hydrolytic enzymes such as proteinase, peptidase, RNase and B-1 ,3-glycan hydrolase.
Kuroyanagi and Pausen (1 988) also showed that high root temperature
increased the activities of protease and RNase, and caused a loss of chlorophyll, protein and RNA in shoots, whereas low root temperature had the opposite effect. High root temperature directly induced shoot senescence, probably by the disruption of root processes (i.e. the export of cytokinins).
Optimal or mildly high temperatures may maintain or increase the level of auxins. In contrast, at supra-optimal temperatures, the levels of auxins are
significantly decreased (Lee et al. 1 979). Hadid et al. (1 986) showed that high
and low temperatures caused a depression of the concentration of auxins, but
(1 984) also reported that high temperature decreased the level of auxin, and caused a problem with fruit set of tomato.
A number of environmental factors such as temperature, drought, salinity, flooding, light, C02 and 02 have been shown to influence ethylene production in plant tissue. High temperature may cause impairment of ethylene production by perturbing cellular membranes, resulting in the inhibition of membrane associated oxidation of ACC to ethylene. Moreover the conversion of ACC to ethylene in auxin treated mungbean hypocotyl was inhibited only aJ__ temperatures at and above 42.5°C. High 'temperature inhibited auxin induction
of ACC synthase ( Yang and Hoffman 1 984; Biggs et al. 1 988). Yu et al. (1 980)
also concluded that the conversion of ACC to C2H4 is highly vulnerable to high
temperature inhibition. Also Chan (1 986a,b) demonstrated that heat can
decrease the activity of EFE (ethylene forming enzyme system). Horiuchi and
lmaseki (1 986) also reported that the conversion of ACC to ethylene was
greatly inhibited at high temperature.