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II. Genetic Variation in Response to Environment

II. Genetic Variation in Response to Environment

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environments can be quite misleading. Zaleski (1954) classified as late-flowering

several varieties which are regarded in the countries to which they are adapted as

early-flowering, It is almost certainly because they were winter-active, and

sustained such injury during more inclement conditions than those to which

they were accustomed, that growth during spring was delayed because of their

weakened condition. Song and Walton (1974) have suggested that breeding for

late autumn growth, without the accompanying development of physiological

adaptation to winter survival, can result in reduced plant vigor in the following


Furthermore, it is often impossible to tell how or even whether a particular

phenotypic character contributes to survival. In the extreme climate of eastern

Anatolia, for instance, wild forms of Medicago sativa with spreading, prostrate

stems are found in the same area as forms which are perfectly erect (ChristiansenWeniger and Tarman, 1939). Citing the example of M. asiatica, which is highly resistant to leaf loss in Afghanistan, but undergoes severe leaf shedding when grown

in Europe, Bennett (1 970) warns that “Except when employed for characteristics

with a very high heritability (which can only be determined by prior genetic

studies), or when conducted in an environment closely resembling that in which

the collected material is to be utilized without further genetic manipulation,

phenotypic selection is an unreliable basis for sampling.”

Genetic+nvironment interactions represent possible sources of variation of

uncertain magnitude to be kept in mind when discrepancies between reports are

encountered. It is therefore remarkable that winter-dormant and winter-active

types have been found to be not different in photosynthetic rates at different

temperatures (Pearson and Hunt, 1972a) or in water-use efficiency (McElgunn

and Heinrichs, 1975).

Ill. Shoot Growth


The following brief formal description indicates how one may proceed from

measurements of differential growth to the quantitative assessment of total

yield. The shoot is regarded as consisting of internodes, i in number, each of

which contains a segment of stem of length 1 and cross-sectional area A , and two

oppositely placed trifoliate leaves with thickness t and surface area S (one side

only). The total aerial volume V of a plant with n shoots is then given by



(IA t rS)

If V is made up of a proportion D of plant dry matter with density ~ ~ ( 1 . 8



glml), according to Hundtoft and Wu (1970), W of water with density p w and

void space A , the fresh weight Y is specified in appropriate units by


The formulas indicate the minimum requirements for the full interpretation of

data, while the true picture will often be considerably more complex. No

allowance is made for the incidence of branching, for example; nevertheless, a

high correlation is likely to exist between leaf and internode numbers per stem

(Liang and Riedl, 1964). Another problem is the high mortality of shoots during

growth (Christian er al., 1970), representing considerable wastage of dry matter

accumulation. Petioles, stipules, stem tips, flower buds, and other reproductive

structures are reported to comprise a fairly constant 11% of total herbage dry

matter from the bud stage onward (Fick and Holthausen, 1975).


While the number of internodes which a plant produces may be characteristic

in a given environment, the position at which flowering first occurs differs

strikingly among phenotypes (Jones, 1950). Medler et al. (1955) concluded

from a study of nine winter-hardy clones that flower positions at the 14th or

15th node were associated with long day length, while at short day lengths,

flowers first appeared at about the 10th node. Using creeping-rooted clones

under controlled conditions, Carlson (1965) found a similar relationship and

noted that under long photoperiods plants continued to produce nodes even

after starting to flower.

In contrast, Dobrenz et al. (1965) found a high inverse correlation between

minimum temperature and the number of nodes to the first raceme in a

nonhardy variety “Moapa,” and to the time from cutting to floral initiation.

However, Field and Hunt (1974) reported that the number of leaves and the

node of first flower were unaffected by temperature at a photoperiod of 16.5


It is difficult to reconcile this evidence except by postulating different responses of winter-dormant and winter-active varieties to day length and temperature. The contrasting experimental conditions may also have had some influence.

Internode number is reduced by water restriction (Perry and Larson, 1974), and

early transition from the vegetative to the reproductive stage may well be a

response to stress, whatever its nature. The possible consequences on production

of high temperatures close to dry and almost bare soil following cutting in

summer warrant closer investigation.




Lengths of internodes, and the ratios between them, are varietal characteristics

in a given environment (Sheridan and McKee, 1968). In general, the length

increases for the first few internodes, then steadily decreases toward the plant

apex, while similar trends are apparent in leaf size. Leaf weight increases almost

linearly with shoot height, with fairly constant chemical composition and

uniformly high digestibility (Smith, 1970a; Christian et al., 1970). Because the

stem has the function of a support, however, its cross-sectional area increases at

the base, and the weight of stem dry matter increases almost as the square of the

height (Christian er al., 1970). The top segments of shoots are of similar

composition and of high digestibility, irrespective of shoot height, but make up

only a small proportion of the total dry weight. The major weight gains during

maturation are in the heavily lignified tissues of the lower stems, and hence

nutritive quality falls off at an increasing rate as growth continues. The best time

to harvest material of high digestibility is before severe leaf drop occurs and

while the stems can still be readily cut at the base with a pair of scissors. Protein

content is highly related directly to digestibility and inversely to lignin and

cellulose contents.

Weight increase of shoots is approximately linear over most of the period of

vegetative growth (Pearson and Hunt, 1972d), although there appears to be an

increase in the rate of stem thickening and elongation around the bud stage

(Dent, 19.55; Nishikawa, 1966), with little increase in height or yield after the

full bloom stage (Raguse and Smith, 1966).

Large leaves are often associated with thick stems (Zaleski, 1954), and therefore probably with long internodes. Plants of Medicago fulcata type are usually

regarded as leafier than M. sutiva types, but this may be merely due to an

initially slower rate of stem development after cutting. Taken at the same stage

of growth, different varieties have been found to have similar leaf stem ratios

(Davies, 1960) and similar leaf and stem protein levels (Dobrenz er al., 1969). As

maturity approaches, growth rates of M. falcata types become more rapid than

those of M. sativa types, so that comparative yields become dependent on

harvest date (Sprague and Fuelleman, 1941; Tysdal and Kiesselbach, 1939).


Cell division is most active during the early stages of leaf expansion, and is

inversely related to the rate of cell enlargement (Koehler, 1973). Eventually, at a

particular leaf length, cell division ceases, and mean cell size then increases in

proportion to leaf length. Leaf growth may therefore be modified in different

ways at different stages of development, depending on whichever process is



predominant. Area, weight, and photosynthetic activity all increase at comparable rates in developing leaves when expressed as proportions of the values at

full expansion (Wolf and Blaser, 1971b). The rate of development, or number of

days to full expansion, is related, naturally enough, to the rate of leaf and

internode appearance (Wolf and Blaser, 1971b; Field and Hunt, 1974).

Turrell (1942) classified alfalfa leaves as primary, secondary, and so on,

according to the origin of the shoots that bore them (main stem, axillary bud

from primary leaf, etc.). Leaves of higher order formed increasingly at the lower

nodes as the plant aged; they tended to become successively less in area and

thickness, with thinner epidermal, palisade, and spongy mesophyll layers. The

earlier and larger leaves had high interna1:external surface ratios and high

intercellular volume; although stomate density was lower than in smaller leaves,

pore size was greater. These factors were hghly correlated with susceptibility to

damage from SOz, indicating that gas exchange rates were proportionately

higher in larger leaves. Evidently the smaller and later leaves are restricted at the

cell enlargement stage, and are unlikely to become as photosynthetically effective.

Considerable interest has been shown over the last decade in the measurement

of specific leaf weight (SLW) as a possible index of photosynthetic activity.

Designating leaf dry weight as YD, SLW may be expressed in terms of the

formula given earlier as

SL W = Y D / S= t p D D

Clearly, SLW may vary according to leaf thickness, dry matter content, or void

space, or any combination of these. In Lolium varieties, SLW may be higher in

thin leaves with small mesophyll cells than in thick leaves with large cells (Wilson

and Cooper, 1969). Under and conditions, the desert shrub Enceliu furinosu has

leaves of very high SLW, with compact mesophyll cells and very little intercellular space (Cunningham and Strain, 1969).

In alfalfa, Delaney and Dobrenz (1974) found that SLW in different genotypes

was related directly to the thickness of the leaf and of the palisade tissue, and

inversely to leaf area. However, Barnes et al. (1969) concluded that SLW and

leaf area were under separate genetic control, with all possible combinations

being encountered.

IV. Root Growth

In the young seedling, root growth starts more slowly than shoot growth, and

is mainly confined to the tap root, with little lateral development. Within 2-4

months, however, the shoot:root ratio declines from about 2.0 to 1.O (Gist and

Mott, 1958; Matches et al., 1962). Thereafter, the ratio is likely to be governed



to an increasing extent by environmental factors. After shoot maturation, tap

root diameter and total weight continue to increase steadily (Crowder et al..,

1960; Nishikawa, 1965).

Where the effective depth is 6 feet or less, maximum penetration depth may be

attained within 1 year, with subsequent growth devoted to increasing the

number and thickness of taproots and laterals (Upchurch and Loworn, 195 1).

Thick lateral roots develop sporadically and secondary thickening extends down

the whole length of the tap root (Tanaka, 1971a,b). Severing the tap root some

inches below the soil surface results in increased lateral root development, with

little effect on other growth (Klebesadel, 1964). Large varietal differences in the

number of branch roots and in the proportion of primary roots showing

branching have been observed (Smith, 1951).

Little is known of the seasonal pattern of root growth. Jones (1943) distinguished the permanent or cambial roots, consisting almost entirely of secondary growth and providing transport and storage, from the transient roots, which

are primary in structure and which undergo growth in spring and autumn and

decay during summer.

The typical pattern of root growth for many species was described by Loomis

and Ewan (1936) as geotropic movement down to dry soil, followed by lateral

branching in the moist layer above, without hydrotropic response. The process

may be briefly described as an osmoregulated force extended by the plant

(Creacen and Oh, 1972) against the mechanical resistance of the soil, which is

primarily a function of bulk density and water content (Taylor and Gardner,


A similar behavioral response apparently ensues when a sudden transition

occurs to any inhospitable region, such as one of mineral deficiency or toxicity,

or a hardpan layer. Where the restriction is less severe, growth is restricted to the

tap root, and there is little or no lateral formation. The tap root always seems to

develop first, even in compact soils, where branch roots eventually become more

important (Carlson, 1925). On eroded slopes, the tap root may disintegrate, its

functions being taken over by lateral roots (Lapinskiene, 1966).

In deep sandy soils, there is little mechanical impediment, but the water-holding capacity is often low, encouraging tap root extension (Lamba et al., 1949;

McCleUand, 1969). On fine textured soils, growth is often restricted by compaction, which may hamper growth by preventing the entry of root hairs or by lack

of aeration (Scott and Erickson, 1964), and may be aggravated by animal traffic

(Tanner and Mamaril, 1959). Increase in bulk density leads to lower plant yields

(Cifford and Jensen, 1967), even when root growth is not visibly restricted

(Peterson, 1971). Hence plants grown on clay soils are usually small (McClelland,

1969), with slower growth rates than on lighter soils (Dent, 1955), although the

variety “Hairy Peruvian” is reported to be an exception to this rule (Rogers,

1963). Compaction generally increases with depth, which contributes to the



frequent observation that most of the root system is contained within the first

foot or so of soil (Lamba et aL, 1949; Upchurch, 1951; Bennett and Doss,

1960). Root penetration and water storage may be improved by mixing high

density subsoil with topsoil (Cary e f al., 1967).

In heavily compacted subsoils, tap roots follow cracks and cleavage planes

(Fehrenbacher e t a l , 1965), often becoming thinner and crooked, but resuming

normal growth further down (Scott and Erickson, 1964; Safta and Balan, 1971).

In moist swelling subsoils, roots may follow earthworm burrows and gaps left by

former decayed roots and evidently combine removal of water with root extension down the crevices left by the cracking of the soil mass (Fredricksen, 1938).

The intervening blocks of soil are often bypassed, and the few lateral roots are

confined to the more friable regions (Paltridge, 1955). Root growth is not

limited to the vertical plane. Paltridge (1955) found that when roots were unable

to penetrate a L'self-sealing'' layer, they spread laterally, up to 16 feet or until

the roots of neighboring plants were reached. He affirmed that alfalfa is not

necessarily or genetically a deep-rooting plant, but that its roots will grow in any

plane where water is available.


Environmental Factors and Vegetative Growth


I . DayLength

Low growth rate under short day lengths is one of the most distinctive features

of winter-dormant varieties. In particular, internode elongation is greatly reduced (Carlson, 1965). Much branching takes place, presumably as a result of

loss of apical dominance, since at long day lengths, adventitious stem production

is inhibited (Carlson, 1965). Since even normally developed stems in these

varieties are often thin, the combination of reduced supporting ability and the

change in load distribution probably contributes to the typically decumbent

growth habit.

The effects of day length and temperature overlap, and the interaction is likely

to produce different responses in different varieties, but because of the great

variability between individual plants, few detailed comparisons have been carried

out. At temperatures of 15"/10°C (the figures written in this form will be used

to indicate day and night temperatures, respectively) and 8-hour photoperiod,

Sato (1971) found that leaves of Du Puits (an intermediate type) were thicker,

but much shorter and narrower, than at higher temperatures or longer day

lengths. As the temperature is raised, the growth inhibition by day length

disappears. The data of Schonhorst et al. (1957) show that at 15.5"C the shoot



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


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

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II. Genetic Variation in Response to Environment

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