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V. Environmental Factors and Vegetative Growth

V. Environmental Factors and Vegetative Growth

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K. R. CHRISTIAN



height of winter-dormant strains does not increase when day length is extended

from 8 to 12 hours, whereas in winter-active varieties at low temperatures and in

all varieties at 26.7"C, shoot height is almost proportional to photoperiod.

However, the results of Iversen and Meijer (1967) suggest first that weight

responses may be much greater than responses in height, and second that the

complete picture may be quite a complex one.

Leaf development may be more rapid at long day lengths (Wolf and Blaser,

1971b) but leaf size is apparently not affected, and since stem diameter as well

as length is increased (Sato, 1974), 1eaf:stem ratios are accordingly reduced

(Coffmdaffer and Burger, 1958; Sato, 1971).

Growth rates are reported to be highest under continuous light (Guy et uZ.,

1971). Light interruption of the dark period was found to increase plant height

during cooler, shorter days, with winter-active and intermediate types producing

more erect and fewer semierect stems (Massengale et UZ., 1971). However, yields

were in general higher with natural daylight only, particularly during the summer

months.

Reports on the effect of day length on root growth differ, suggesting that stage

of growth or other interactions may be involved. Coffindaffer and Burger (1958)

and Carlson (1965) observed no appreciable effect on root weights, and Seth and

Dexter (1958) showed that there was little effect on tap root lengths of either

hardy or nonhardy varieties. Hanson (1967) recorded increases in dry matter

yields of all plant parts at long day lengths, although the proportion of roots was

less. Sat0 (1971) found that root:top ratios tended to be highest at a 12-hour

photoperiod. On the other hand, long day lengths have been observed to

accelerate thickening of tap roots and growth of the root system as a whole

while short days retarded root growth (Ueno and Tsuchiya, 1968).

2. Light Composition



Winter-active varieties increase in stem length much more rapidly than winterdormant varieties under mixtures of blue and red lights or green and red lights,

particularly at low temperature (Nittler and Gibbs, 1959). Inhibition of hypocotyl elongation was also greatest in winter-dormant varieties. The diversity of

reactions to selected colors found by these workers may indicate one reason for

conflicting results between different laboratories and between the laboratory

and the field. A complete, or at least a balanced, spectrum is required for

maximum growth, particularly root growth (Heinrichs, 1973).

The proportion of far red (740-mm) light transmitted by the alfalfa canopy is

much higher than that of red (640-mm) light (Robertson, 1966); this would

favor photomorphogenic reversal at the plant bases. It may also be significant

that the proportion of far red radiation in natural sky light increases at twilight,

particularly when the sky is clear; the effect will assume greater importance at



ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH



193



high latitudes (Robertson, 1966). In Phaseolus, internode elongation is a function of phytochrome in the far red absorbing form at the start of night, and is

proportional to the number of hours of darkness. The process differs from that

due to long days and becomes less important at higher internode number

(Vince-Prue, 1975).

It seems possible that the initiation of new growth from the plant base could

be inhibited by the canopy above through selective radiation effects as well as

through apical dominance. Light intensity alone is insufficient to explain the

effect, since bud elongation takes place in defoliated plants even in darkness and

in plants where the crowns become exposed to direct sunlight because of

lodging.

There seems to be no evidence to suggest that changes in spectral composition

during cloudy or overcast days are large enough to modify plant growth, but the

possibility should perhaps not be overlooked.

3. Light Intensity

Reduced light intensity removes growth inhibition by light, and the immediate

result is rapid internode elongation (Pritchett and Nelson, 1951). The formation

of the layer of woody tissue inside the cambium is reduced, so that the stems are

much thinner than in normal plants and have the appearance of very young,

succulent internodes. The effect is transient, coming to a halt more quickly at

low light intensities, so that greatest stem height is likely to be found at

intermediate levels (Pritchett and Nelson, 1951).

Relative growth rates are increasingly reduced at light intensities below

50-75% of full daylight, and follow the general trend of the hyperbolic lightphotosynthesis curve. SLW increases with light intensity (Cooper and Qualls,

1967). Mean leaf area is diminished to a much lesser extent (McKee, 1962), so

that the net effect may be that the leaf weight:total plant weight ratio remains

fairly constant (Cooper, 1966, 1967). Stem growth is proportionately increased

at the expense of root growth, particularly in the early stages of seedling growth

(Cooper, 1966, 1967; Pritchett and Nelson, 1951). Nodulation is even more

severely affected, presumably because of inadequate carbohydrate supply

(Pritchett and Nelson, 1951; McKee, 1962).

As shoot length increases, shading of lower leaves becomes an increasing

limitation to production. Thomas and Hill (1949) found that net assimilation in

plants 48 inches tall was only twice as great as in plants 6 to 8 inches tall, even

though the leaf weight was 3.6 times as great. King and Evans (1967) observed

that net photosynthesis in single plants approximately doubled as the leaf area

index increased from 2 to 10. It should be noted, however, that the light

response curves show that shaded leaves are photosynthetically just as efficient

as younger, sunlit leaves at the light levels at which they are operating and,



194



K. R. CHRISTIAN



because of their low respiration rates (King and Evans, 1967), they are evidently

not parasitic (Wolf and Blaser, 1971a).

Experiments have shown that the longevity of leaves can be increased by

thinning the stand (Wolf and Blaser, 1972; Pearce et al., 1968). However,

removal of a large proportion of the photosynthetic area will almost certainly

divert nutrients and growth factors to the remainder. Lower leaves continue to

senesce even in full sunlight, but rejuvenation occurs within a few days if the

upper part of the shoot is removed (Hodgkinson et al., 1972). The importance of

apical dominance was further demonstrated by the observation that high rates of

photosynthesis were maintained for a longer time when new shoot buds were

removed as they appeared.

Net photosynthesis per unit leaf dry weight was found by Pearce and Lee

(1969) to be reasonably constant, whether differences in SLW were due to

differences in environment or in genotype. The experiments of Delaney and

Dobrenz (1974) also show that photosynthesis per unit leaf weight is not related

to SLW. In the field, however, the decline in photosynthesis with the degree of

shading experienced in dense canopies was much greater than that in SLW (Wolf

and Blaser, 1971a). Pearce and Lee (1969) also observed that although photosynthesis was more constant on a leaf dry weight basis than on an area basis,

other factors involving senescence resulted in different relationships in the field

from those in the growth chamber.

Changes in light intensity have been shown to result in very substantial

adaptation within 2 weeks, both in photosynthesis and in SLW, with the result

that photosynthesis per unit weight was much the same, irrespective of previous

light treatment (Pearce and Lee, 1969). Similar trends were observed in canopies

thinned to one stem per plant (Wolf and Blaser, 1972) although the response in

photosynthetic rate was less at high light and greater at low light than the

response in SLW.



4. m e Diurnal Cycle

Increases in starch and sugar fractions of alfalfa herbage during daylight have

been reported by a number of workers. The variability in the results may be

partly attributed to analytical methods as well as to stage of plant growth and to

environmental effects. Lechtenberg et al. (1971) found that the starch content

of leaves increased by 10%during the day while that of stems showed almost no

change, accounting for most of the concurrent increase in 1eaf:stem ratio from

1.1 to 1.5. Chatterton et al. (1972) observed that the rise in total nonstructural

carbohydrates in daylight hours would account for 70% of the overall change in

SLW. Concentrations fell during the first few hours of sunlight and increasei

thereafter, with rapid responses to changes in light intensity.



ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH



195



Chatterton (1973) also found that SLW and net carbon exchange showed

closely related inverse trends during the day, and suggested that the buildup of

assimilates could have been inhibiting photosynthesis. However, the data could

equally be used to suggest that fluctuations in photosynthesis due to other

causes would bring about corresponding changes in SLW. Pearson and Hunt

(1972a) reported a slight decline in net carbon exchange in alfalfa after several

hours of illumination under controlled conditions, but it occurred earlier at

higher temperatures, when starch would be less likely to accumulate. Although

feedback mechanisms have been suggested as possible limitations to photosynthesis on a number of occasions, the changes in concentrations have not been

large enough to be particularly convincing.

The reverse trend-the loss of carbohydrate and dry matter from the shoots

during the hours of darkness-has been the subject of controversy for some time.

Knapp et al. (1973) measured a gain of about 300 kg per ha during the day,

while about the same amount disappeared during the night at the late bud stage,

and less than half as much at about early bloom. Starch and sucrose amounted

to 70% of the weight changes. Dry matter losses tended to be greater on warm

nights, but the relative proportions lost through respiration and transpiration

were not determined,

Tracer studies by Hodgkinson and Veale (1966) showed that assimilated 14C is

rapidly translocated in the form of soluble carbohydrate to the stems and, to a

greater extent, to the roots, where it is steadily converted into insoluble forms.

In the light, however, the formation of insoluble carbohydrate in stems is greater

than in the dark, and there is considerable accumulation of insoluble carbohydrate in the leaves, while transfer to the roots is slowed down. Evidently,

assimilate which has recently been formed is not used for respiration during the

light (Ludwig and Canvin, 1971).

The magnitude of the variations in carbohydrate concentrations creates uncertainty concerning the trends which have been observed in other constituents.

Changes in nitrogen fractions, although significant, have been reported to be

small in relation to pool sizes, and intermediates do not normally accumulate

from rate-limiting steps (Youngberg et al., 1972). No day-to-day weather effects

were found.



B. TEMPERATURE



Many reports have shown the importance of air temperature on shoot development. The most consistent effect is that of rate of maturation or time to first

flower, ranging from more than 40 days at day temperatures of 20°C or below

to 20 days or less at day temperatures of 30°C or above (Jensen et al., 1967;



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K. R. CHRISTIAN



Nelson and Smith, 1969; Marten, 1970; Smith, 1970b; Pearson and Hunt,

1972c; Smith and Struckmeyer, 1974). The differences appear to be greater up

to the bud stage than between the bud stage and first flower (Greenfield and

Smith, 1973), and in general are less at high temperatures. At 35"C, inhibition of

floral initiation may cause delay in flowering (Pearson and Hunt, 1972~).

Maturation is linked with the rate of node formation, and also possibly with

node number, as mentioned in Section III,B. Field and Hunt (1974) reported

that the Qlo for node formation was 2.03, with nodes being formed every 3.38

days at 15"/1O"C and every 1.19 days at 30"/25"C. Pearson and Hunt (1972~)

observed a similar trend, with a slightly slower rate of 35"/30", attributed to

possible water stress. They also found that the rate was faster for regrowth of

seedlings than for the primary growth. The effect is paralleled by the more rapid

rate of leaf expansion and maturation as the temperature is increased (Wolf and

Blaser, 1971b).

Dry matter yields at first flower are higher as a rule at lower temperatures

because of the extended growing period (Nelson and Smith, 1969; Marten, 1970;

Smith, 1970b; Lee and Smith, 1972b; Smith and Struckmeyer, 1974). Growth

rates appear to be highest when daylight temperatures are in the region of

20"-25"C (Smith, 1970b; Ueno and Smith, 1970a; Guy ef al., 1971; Bula,

1972).

The preceding experiments were all carried out under controlled environment

conditions. In the field, Evenson and Rumbaugh (1972) reported that when

wheat straw mulch was applied, soil temperatures were lowered by as much as

9"C, plant maturity was delayed, and yields increased by more than lo%, under

conditions where it was considered that reradiation and soil moisture differences

were of no importance.

Plant height, and therefore internode length, do not appear to be greatly

affected by temperature (Cowett and Sprague, 1962; Nelson and Smith, 1969),

although there are indications that plants in cool regimes tend to be slightly

taller than those in warm to hot conditions (Smith, 1970b; Ueno and Smith,

1970a; Vough and Marten, 1971).

Although stem and leaf weight and leaf area are all increased at low temperatures, stem growth is proportionately greater than at higher temperatures.

Nelson and Smith (1969) found that the total yield at 18"/1O"C was three times

that at 32"/24"C, while the leaf weight was only twice as much. Marten (1970)

reported that the 1eaf:stem ratio at first bloom was lower at 16"/10"C than at

27"/2 1°C. Stem diameter is greater under cooler conditions (Vough and Marten,

1971), and the weight of stem per unit length of internode declines at an

increasing rate as the temperature rises (Field and Hunt, 1974).

Leaf area at full expansion is greatly influenced by temperature, reaching a

maximum in the vicinity of 20". It decreases gradually as the temperature is

lowered (Wolf and Blaser, 1971b; Sato, 1974), and more rapidly as the tempera-



ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH



197



ture is raised (Nelson and Smith, 1969; Wolf and Blaser, 1971b; Bula, 1972;

Pearson and Hunt, 1972c; Sato, 1974). The rate of increase in leaf area is

equivalent to the area at full expansion divided by the rate of leaf appearance,

and there is some evidence to suggest that this is highest at intermediate

temperatures (Nelson and Smith, 1969; Wolf and Blaser, 1971b; Sato, 1974).

SLW was reported by Smith and Struckmeyer (1974) to be almost twice as

great at first flower under a 21"/12"C regime as compared with 32"/24"C, and

this was associated with starch concentrations of 40% and 6%, respectively.

Microscopic examination showed that under cooler conditions, chloroplasts

accumulated so much starch that cell lumina were not discernible. Leaflets had

highly thickened sclerenchyma and phloem cell walls, and were 30% thicker than

leaflets at the high temperature regime, with more compact palisade and spongy

parenchyma cells. When expressed on a starch-free basis, SLW showed much less

variation with treatment.

Sato (1974) also found that SLW, leaf thickness, and mesophyll thickness were

all greater at 15°/100C than at higher temperatures, while palisade cell diameter

decreased steadily with temperature. Stomata1 and epidermal cell densities were

lowest and intercellular volume greatest at 20'11 5°C.

In contrast, Bula (1972) observed a tendency in three different varieties for

SLW to increase from 25" to 35°C. Leaflets grown at 20" and 25°C had larger

cells, particularly in the xylem and bundle sheath parenchyma, and more

intercellular spaces. There were no obvious trends in leaf thickness. Pearson and

Hunt (1972d) found that SLW was lower at 20°/15"C than at 30"/25"C, while

Field and Hunt (1974) reported that the decline in SLW was rapid between

15"/lO"C and 20"/15"C, but became more gradual as the temperature increased,

with little difference between 25"/20"C and 30"/25"C.

These results indicate that starch accumulation is mainly responsible for

changes in SLW below about 20°C but that at higher temperatures different

rates of cell division and development modify the leaf anatomy, altering cell size,

the proportion of void space, and also possibly the dry matter content. It also

appears from measurement of carbohydrate levels that leaflets are just as able to

adapt themselves to changes in temperature (Greenfield and Smith, 1973) as

they are to changes in light intensity.

The temperature coefficient for photosynthesis is close to unity over the

normal range (Thomas and Hill, 1949; Stanhill, 1962; Pearson and Hunt, 1972a),

with a rapid decline below 5°C and above 30°C (Murata et al., 1965). However,

when Pearson and Hunt (1972b) raised the temperature in steps over the day

from 10" to 40"C, they found that net carbon intake decreased from 20 to

about 5 mg per dm2 per hour, suggesting that treatment interactions of some

kind may have been involved.

Pearson and Hunt ( 1 9 7 2 ~ )found that the root:shoot ratio of seedlings increased more rapidly with time as the temperature was raised, but that the



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K. R. CHRISTIAN



asymptotic value finally reached was lower at the time of 50% flowering. The

nature of the curves indicated a possible reason for inconsistencies among other

results obtained at different stages of growth. However, Smith (1970b) reported

that, as temperature increased, plants harvested at first flower had lower total

yields but higher root:shoot ratios.

It is usually assumed that in a controlled environment chamber, soil temperature follows air temperature, with a lag period depending on the size of the

container. However, such factors as radiation level, air movement, and soil

moisture content can produce large temperature gradients in the region of

greatest root density close to the surface.

Using controlled soil temperatures, with air temperatures fluctuating between

15°C and 32"C, Heinrichs and Nielsen (1966) found for a wide range of cultivars

that herbage growth was higher at a root zone temperature of 27°C than at

lower temperatures, whereas root growth was greater at 12°C than at higher

temperatures, and much greater than at 5°C. Despite considerable temperature X

variety interactions, the fmdings are consistent with other indications that while

fairly low temperatures are suitable for root growth because of reduced respiration requirements, top growth i s restricted as a result of inadequate supplies of

nutrients and possibly of water. It seems that root temperature has no appreciable effect on shoot maturation (Nielsen et al., 1960; Heinrichs and Nielsen,

1966; Jensen et al., 1967). Dermine et al. (1967) found that whereas a change

from 15.6'/4.4"C to 26.7"/15.6"C resulted in an immediate shoot growth

response, the reverse change took 2-3 weeks for growth to slow down, with no

comparable reduction in root growth. This could be interpreted as the result of

reduced mineral uptake at the lower temperature regime, with a depletion of the

reverse already present.

The effect of variation between day and night temperatures has received little

attention. Steinke (1968) found no significant differences in yield between

plants grown at 18"/1OoC and 18"/4"C. Robison and Massengale (1969) concluded that high night temperatures might have been partly responsible for a

decline in vegetative growth, carbohydrate reserves, and plant vigor, but other

environmental differences between plants grown in the greenhouse and in the

field may well have had greater effects. Smith and Struckmeyer (1974) found

that plants grown at 3Oo/3O0C not only had yields similar to those at 32"/24"C

but were higher in leaf carbohydrate content. Pearson and Hunt (1972a) inferred

from their results that translocation from shoots to roots during darkness may

have been less at higher temperatures, but more direct evidence would be

valuable,

Dark respiration increases almost linearly with temperature, corresponding

roughly to a Q l o of 2.0 over the lower part of the range (Thomas and Hill,

1949; Murata et al., 1965; Pearson and Hunt, 1972b). There is evidence of

possible acclimation (West and Prine, 1960; Pearson and Hunt, 1972b), al-



ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH



199



though its significance is not clear. Comparing successive harvests in the field

over an entire season, Delaney et al. (1974) found an inverse relationship between

temperature and leaf respiration, which they suggested might be due to lack of

substrate during the summer months. However, the results may be associated

with the rate of growth and degree of maturation of the leaves at the time of

harvesting. Although respiration is often stilI thought of as wasteful dissipation

of assimilate, it is in fact concerned with two vital processes: the repair and

maintenance of existing tissues and the provision of energy for synthesis and

growth.

The deleterious effects of exposure to high atmospheric temperature have been

demonstrated by Pulgar and Laude (1974). Plants subjected to treatments of

52°C for 2.5 hours or 46°C for 6.5 hours showed reductions in shoot number

and height within 7 days. The effect persisted during the next regrowth period,

and shoot lengths were 20% lower than in the controls even at 70 days after

treatment. Although roots were not visibly damaged, root dry weights were

slightly reduced.



C . WATER



I . Root Growth

Plant roots will not grow for any appreciable distance into dry soil, and the

extent to which they develop in moist soils depends on the supply of assimilate.

Janson (1975b) showed that, in a climate subject to drought, herbage yield, root

weight, and root depth during the establishment year were linearly related to the

amount of irrigation water, irrespective of time, frequency, or rate of application. The results indicate first that in the coarse free-draining shingle used in this

trial, root growth is stimulated by moist, not dry, conditions, and second that

the plant is able to take full advantage of water supplies as they become

available. With established plants grown in a fine sandy loam of bulk density

1.61, Bennett and Doss (1960) obtained no consistent relation between root

weight and soil moisture, but found that effective rooting depth was greater at

low soil moisture levels. It is evident that no simple generalization is possible,

and that such factors as soil penetrability and water-holding capacity must be

taken into consideration.



2. Water Uptake

In considering water extraction by the plant, it is useful to start from the

premise that uptake from the soil is directly proportional to the root density and

to the difference in water potential between root xylem and soil in any given



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K. R. CHRISTIAN



region (Bahrani and Taylor, 1961;Kohl and Kolar, 1976). In addition, soil water

conductivity may be expected to become limiting at low water potentials. There

remains considerable doubt concerning the measured values of plant water

potential, and it has been recently reported that levels below -20 bar may

commonly occur in alfalfa (Cary and Wright, 1971; Kohl and Kolar, 1976).

In fine textured soils, the great majority of the roots are in the top 6-12

inches, in the best position to encounter incoming water from rainfall and

irrigation. It is therefore usually observed that soil water in this region is

removed much more rapidly than at greater depths (Bennett and DOSS,1963;

Lucey and Tesar, 1955). Alfalfa is able to deplete the surface layers just as

rapidly as other species, but because of its deeper rooting system it extracts

water from lower levels during dry periods (Chamblee, 1958b; Van Riper, 1964).

In sandy soils, water is likely to be more evenly extracted at all depths. Under

moderate to high saline conditions, alfalfa removes water at depth at tensions

well below -15 bar (Brun and Worcester, 1975).

At low temperatures, water uptake may become limiting. Ehrler (1 963) found

that, compared with rates above 20"C, water absorption was reduced by 20% at

10" and by 70% at 5°C. There were no interactions between contrasting varieties

and temperatures.

3. Plant Growth and Water Use



If water uptake were the only consideration, plant growth would be greatest

when the soil was at field capacity (100% water availability), but would show

little reduction until the soil water potential reached a value of about -2 bar

(Kemper and Amemiya, 1957). Since this figure may correspond to less than

25% water availability in sand and more than 75% in clay, it is hardly surprising

that trials in which stands have been irrigated at various levels of soil water

availability have yielded such different results (Bourget and Carson, 1962; Hobbs

et al., 1963; Bezeau and Sonmor, 1964; Peyremorte et al., 1971). The additional

complication of excess water supply will be discussed later.

Alfalfa has acquired the reputation of being an extravagant consumer of water,

despite considerable evidence that this is an unbalanced verdict. Trials in a

number of regions have shown that daily consumptive use is similar to that of

other crops in which full ground cover is established (Fredricksen, 1938; Halkias

et al., 1955; Krogman and Lutwick, 1961; Peck et aL, 1958; Bennett and Doss,

1963; Szeicz et aL, 1969). Various workers have concluded that annual water

use depends not so much on species as on length of growing season, proportion

of ground cover, rooting depth, and crop yield (Chamblee, 1958b;Krogman and

Lutwick, 1961; Sonmor, 1963; Tadmor et al., 1966). During the dry summer

months, water consumption by actively growing alfalfa may be similar to that of

dormant or semidormant species (Snaydon, 1972~).



ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH



20 1



During the spring, water demands are comparatively low, and it is usually only

during periods when potential evaporation rates are high and growth rates low

that water losses become excessive. Yields often decline with successive harvests

during the season to a much greater extent than evapotranspiration, with water

requirement (weight of water used per weight of dry matter produced) being

almost doubled (Cohen and Strickling, 1968; Vorhees and Holt, 1969; Daigger et

al., 1970). Hanson (1967) reported that both yields and water-use efficiencies

were higher on frequently irrigated plots, and that consumptive use was greatest

when irrigation was delayed until essentially all moisture had been depleted to a

depth of 6-12 inches. Gifford and Jensen (1967) found that water-use efficiency

declined at lower soil water availability, but Lucey and Tesar (1965) reported

that it was independent of irrigation regime. High evaporative demand alone

does not seem to be responsible for wastage, since Tadmor et al. (1966) showed

that under arid conditions, water requirement was 820 in one year and 740 in

the next, compared with the mean of 859 established by Schantz and Piemeisel

(1927).

The general impression is therefore that alfalfa uses water wastefully because it

continues to function during periods when water stress restricts its growth and

when other plants remain dormant, neither growing nor using water. This places

the crop at a further disadvantage. In summer, the deep green mass of an alfalfa

field often presents the appearance of an oasis among the surrounding areas, and

for this reason becomes subject to the “oasis effect.” Advective winds brought

across dry, bare ground supply latent heat flux energy to the crop, increasing

evapotranspiration rates by up to 40% (Blad and Rosenberg, 1974), or even

more (Hudson, 1965; de La Sayette, 1967).

Where moisture supply is adequate and advection is absent, evapotranspiration

from a crop which has established full ground cover is close to potential

evaporation as governed by meterological factors, particularly incoming radiation (Jackson, 1960; Nicholaichuk, 1964; Krogman and Hobbs, 1965; Hobbs

and Krogman, 1966). As the soil dries out, evaporation rates decline according

to the gap between the transpirational demand arid the water resources available

to the root system. Van Bavel (1967) found that stomata1 conductance started

to decrease when the soil water potential reached -4 bar. At -11 bar, evapotranspiration was less than 0.2 of potential evaporation and was controlled by

the plant such that it did not exceed 20 mm per hour, regardless of demand; at

this stage, the crop was emitting heat to the atmosphere.

Numerous trials have shown that evapotranspiration is least in the week

following defoliation, despite the higher daytime temperatures above bare

ground, and is usually one-quarter to one-half of the rate at full bloom. Where

plants remain dormant following harvest during summer drought, water use may

be as low as 0.1-0.3 mm per day (Tadmor e l al., 1966). Transpiration increases

not only as full ground cover is attained (Krogman and Hobbs, 1965) but also as



202



K. R. CHRISTIAN



crop height increases, exposing greater leaf surface area and creating greater air

turbulence by increasing the aerodynamic roughness of the canopy. The evapotranspiration rate from a tall crop can be twice that from a short one (Hafeez

and Hudson, 1965; Grassi and Chambouleyron, 1965). A stand cut weekly is

likely to produce somewhat less dry matter than one allowed to reach maturity,

but may use only half as much water (Sprague and Graber, 1938). Water use

becomes greatest at full flower and during seed formation and ripening (Ktidrev

et al., 1970), and it is during this time that differences between cultivars in water

requirement become most apparent (Cole e t a l , 1970). Genotypes within

cultivars show considerable differences in water-use efficiency (Cole and

Dobrenz, 1970), which is associated with htgh forage production, particularly of

stems, but is not related to palisade cell density or leaf thickness (Dobrenz et al.,

1971).

4. Water Deficit



Bauman (1957) distinguished five phases in moisture stress effects on growth,

related to the osmotic potential of the plant during a drying cycle: above -10

bar, optimal growth; -10 to -12 bar, little effect on growth; -12 to -17 bar,

growth very slow; -17 to -32 bar, no growth; and below -32 bar, dry weight

loss.

Stem growth is most affected by water deficit; stems per plant, internodes per

stem, internode length, and branching are all reduced (Perry and Larson, 1974),

and eventually elongation ceases (Lucey and Tesar, 1965). Under mild deficits,

the lower yields may be of higher quality than in plants with adequate soil

moisture (Jensen et a l , 1967), but further desiccation leads to general plant

deterioration. Shoot growth appears to be slightly more affected than root

growth (Bourget and Carson, 1962; Janson, 1975b), presumably because of the

water potential gradient within the plant, and growth appears to be diverted to

carbohydrate storage in the root (Willard, 1951 ; Cohen et al., 1972).

Striking changes in leaf morphology under arid conditions have been described

by Gindel (1968). Mean leaflet size was only 1 cm2 during the dry season,

compared with 4 cmz at the end of the wet season. Stomate and epidermal cell

densities were somewhat greater in the smaller leaves, but it appeared that cell

expansion was reduced more than cell division. Small cells, having proportionately less volume reduction when desiccated, and high negative osmotic

values are characteristic of drought-hardy plants (Russell, 1959).

Alfalfa has no more ability than many other plants to remove water from soil

before permanent wilting occurs (Briggs and Shantz, 1912). Murata et al. (1966)

found that respiration was reduced when the soil water content was reduced to

45%, photosynthesis at 35%, and leaf water content at 25%. All three values



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