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IV. Physiology of the Soybean Plant

IV. Physiology of the Soybean Plant

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86



MARTIN G. WEISS



others failed to flower unless exposed to relatively short days. Relatively

late maturing varieties of soybeans are particularly sensitive to the

photoperiod and have been widely used in studies of “Fjhort day” plants.

Review of all photoperiod studies involving soybeans is not considered

pertinent here, and consequently, hrief consideration will be given only

to those aspects of photoperiodism which relate to soybeans as a crop

and which may serve to explain the response of soybean varieties when

grown in latitudes to which they are not well adapted. A more detailed

summary of the relation of day length and flowering in plants has recently been made by Murneek et al. (1948) and by Borthwick (1947).

a. Varietal Differences. Varieties of soybeans were found by Garner

and Allard to differ strikingly in their response to photoperiod. When

grown under summer daylight conditions in Washington, D.C., four

varieties ranged from 27 to 105 days from germination to blossoming.

When the day length was reduced to 12 hours, however, the varieties all

became early maturing varieties and blossomed 21 to 28 days after

germination, Reduction of the photoperiod altered the blossoming time

of the earliest variety, Mandarin, only slightly, whereas that of the

latest variety, Biloxi, was radically reduced. Biloxi was considered a

“short day” plant and Mandarin was thought to be indeterminate or

day neutral.

Flower primordia were found by Borthwick and Parker (1939) to be

initiated in very early varieties a t all photoperiods, including continuous

illumination, while relatively late varieties differentiated reproductive

primordia only when subjected to periods of illumination not in excess

of 14 hours. Nevertheless, several observations indicated that the fundamental character of the reactions in early and late varieties was similar.

Although flower primordia in early varieties were initiated with exceptionally long light exposures, with photoperiods of greater than 18 hours

the primordia failed to develop into flowers and pods. Similarly, in late

varieties flowers failed to develop a t maximum photoperiods resulting in

primordia initiation. I n later confirmation of these findings, histological

investigations by Neilsen (1942) indicated that the frequency of disintegration of the young sporocytes was associated with the magnitude of

deviation from optimum photo-inductive conditions. Initiation of primordia in early varieties under continuous light was further found by

Borthwick and Parker to occur much less promptly than when illumination was interspersed with dark periods. During late stages of growth,

late varieties flowered with longer photoperiods than during early stages.

It was, therefore, concluded that the fundamental response to photoperiod

is similar in early and late soybean varieties although they may differ

greatly as to relative photoperiod when primordia initiation occurs.



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Several soybean varieties have been noted to respond differentially in

seed and forage yields when grown on soils of different productivity

levels. I n Missouri, the variety Virginia commonly outyields Morse on

soil with low productivity while on soils of high productivity Morse gives

higher yields. Lincoln has been reported to respond relatively more to

fertilizer applications than T 48. T o determine whether the initiation

of flower primordia was associated with the above differential yields, the

varieties, Lincoln and T 48, were grown by Scully et a2. (1945) a t low,

intermediate, and high nitrogen levels with long and short photoperiods.

With long photoperiods total nodes a t which flower primordia were initiated increased progressively with nitrogen concentration in Lincoln and

decreased substantially in T 48. The increase of total nodes per plant

a t higher nitrogen levels was more pronounced in Lincoln. A delay in

flowering was stimulated by higher nitrogen but this delay was more

pronounced in T 48. With short photoperiods neither time of flower

initiation nor total nodes per plant differed with nitrogen levels. I n a

similar experiment with Morse and Virginia, conducted only with moderately short photoperiods, varietal differentiation was not great. Differences as conditioned by nitrogen level seem to occur only a t threshold

durations of the photoperiod when production of flower-producing

substances is relatively low. Although these results are insufficient to

explain differential yield response of varieties a t various soil productivity

levels, the differential response of these varieties at various nitrogen levels

with respect t o time and amount of flower initiation is significant.

b. Duration of Dark and Photoperiods. The effect of duration of the

dark and photoperiods on flower initiation in late soybean varieties has

been the subject of several recent investigations (Allard and Garner, 1941;

Borthwick and Parker, 1938a; Hamner, 1940; Long, 1939; Snyder, 1940).

Certain trends have been established which clarify the nature of photoperiodic induction. I n late maturing varieties, such as Biloxi, alternate periods of darkness and light are required for flower initiation. No

initiation will result from exposure either to continuous light or darkness.

Although exposure to as few as two photo-inductive cycles will result

in flower initiation, an increase in the number of cycles up to 10 will

result in progressively more rapid and more abundant flowering. No

evidence of residual effect is apparent. Exposure to repeated, single,

short photoperiods, when separated by long photoperiods, resulted in no

flowering. Within the range of photoperiods under which Biloxi flower

primordia are stimulated in the 24-hour cycles, photoperiods intermediate

in length (6, 8, 10 and 12 hours) are more conducive to flowering t,han

either extreme (2, 4, or 14 hours). Therefore, periods of darkness 12 to

18 hours in length are most favorable for flower initiation with the 24-



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MARTIN G. WEISS



hour cycles. Use of cycles other than 24 hours alters the length of the

optimum periods somewhat. With long (16-hour) dark periods, flower

primordia are produced more abundantly with 10- to 14-hour photoperiods, and no flowers are initiated with photoperiods in excess of 18

hours. It is apparent that dark periods of sufficient length to stimulate

flowering with the normal 24-hour cycle are ineffective when interspersed

with long photoperiods. With long (16-hour) photoperiods, flowers are

initiated when the dark period is in excess of 10 hours, and most abundant flowering occurs with 12- to 16-hour dark periods. Even with very

short (4-hour) photoperiods, no flower initiation occurs unless the durat4ionof the dark periods exceeds 10 hours. It may be concluded that with

a long photoperiod, which results in no flowering in the 24-hour cycle,

flowers are differentiated provided the dark period is of adequate length.

Furthermore, regardless of length of the photoperiod, the period of darkness must exceed 10 hours for flowering to occur in this variety. The

most critical prerequisite of flower initiation apparently is dark periods

which must exceed minimum durations. Although interspersed periods

of light are necessary, their duration does not seem to be as critical in

fulfilling the requirements for flower differentiation.

c. Light Intensity and Photosynthesis. The intensity of light during

the photoperiod has been found to affect materially flower initiation in

Biloxi soybeans (Borthwick and Parker, 1938c; Hamner, 1940; Parker

and Borthwick, 1940). Intensities of 100 foot-candles for 10 hours or

150 foot-candles for 5 hours are the minimum to stimulate flower initiation. The quantity of flowers produced progressively increases with intensity of light up to full daylight intensities. With 8 hours of natural

daylight, however, the minimum intensity of an 8-hour period of supplementary light necessary to prevent flower induction is 0.6 foot-candles.

The effective length of the photoperiod apparently consists of the period

during which light intensities are in excess of 0.6 foot-candles. However,

high light intensities during a portion of the short photoperiod are a

requisite for flower initiation. Lack of floral initiation a t low light energy

levels is thought to be associated with low photosynthetic activity within

the leaf. Substantiation of the latter theory has been obtained by

retarding photosynthesis by other means. When photosynthesis is materially reduced by exclusion of COz from the air in which the plants are

grown, no flower initiation results. Adequate photosynthesis per unit

area of leaf rather than total photosynthesis per plant is considered the

limiting requirement.

The relative effectiveness of various wave lengths of visible light to

prevent flower initiation also has been studied (Katunsky, 1937; Parker

et al., 1945, 1946). Regions of the spectrum exhibiting maximum effec-



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tiveness were observed to contain the wave bands absorbed by chlorophyll

to the greatest extent. It has been postulated that a flower-forming substance, thought to be normally produced in a long dark period, may be

destroyed as a consequence of some reaction stimulated by energy absorbed by the chlorophyll.

d. Age and Position of Induced Tissue. Sensitivity of various soybean tissues to photoperiodic induction has been determined (Borthwick

and Parker, 1938b, c, 1940; Heinze et al., 1942). Photoperiodic induction

processes have been found to be initiated within the leaves and not in

the stems or growing points of the plant. Based on abundance of flower

primordia produced, the most effective leaf of the plant is that compound

leaf which has attained its full size most recently. I n fact, subjecting

this leaf to short photoperiods results in as great floral stimulation as

when all the leaves of the plant are exposed, and exposure of as little as

one quarter of this leaf results in flower stimulation. The capacity of

leaves to stimulate floral initiation seems associated with their state of

maturity rather than relative position or proximity to growing points.

The flower-forming substance seems to move readily up or down the stem,

into a second branch that has been defoliated, or through grafts of induced leaves on noninduced plants. It is of interest to note that Biloxi

plants will flower with long photoperiods when leaves of an early variety,

which will blossom under continuous illumination, are grafted on to

them. Defoliation of the stock hastens this process.

Differentiation of flower primordia occurs very early in the life of the

soybean plant when subjected to adequately short photoperiods. Limited

floral initiation occurs when Biloxi plants are exposed to short photoperiods two weeks after planting. Flowering is, however, more abundant

when exposure occurs later in the life of the plant. Flower primordia

are not stimulated a t the lower nodes if the plant has attained relatively

large size prior to photo induction.

e. Temperature Effects. Flower initiation has been found to be influenced by temperature differences (Borthwick and Heinze, 1941; Parker

and Borthwick, 1939, 1943). Maximum flower initiation occurs with

day temperatures of 75" and 85" and night temperatures of 65" and 75°F.

Complete inhibition occurs with temperatures as low as 55°F. Exposure

of localized tissues to various temperatures indicates that floral initiation

is influenced much more by differences in temperature throughout the

dark period than by differences throughout the photoperiod. Furthermore, low temperatures in the leaves are more effective in limiting flower

initiation than low temperatures in the petioles or growing points. Inhibition of flower initiation caused by low temperatures, therefore, seems



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atttibutable to the effect on the photoperiodic reactions occurring within

the leaves during the dark period.

b. Nutrition



Nutritional requirements of the soybean plant are high in comparison

with that of grain crops. It has been estimated by Sears (1939) that a

crop of soybeans removes somewhat more phosphorus and magnesium

and appreciably more potassium and calcium from the growing medium

than crops of corn, oats, or wheat with comparable yields. Norman

(1944b) states that per unit area under Iowa conditions the nitrogen

needed by soybeans is somewhat in excess of that needed by comparable

yields of corn. With proper inoculation, of course, a considerable portion

of the required nitrogen is obtained from the air. The appearance in soybeans of numerous deficiencies of minor elements required for plant

growth emphasizes the heavy drain of t+hiscrop upon certain of the micronutrients as well. Although much basic research on plant nutrition was

done prior to the past decade, only recent references are cited herein. I n

many cases the researches cited constitute confirmation of earlier findings

or theories.

a. Nitrogen. Although the soybean plant is capable of utilising soil

nitrogen, as are grain crops, it also has supplementary means of obtaining nitrogen. Like most other legumes it has the ability to enter into a

symbiotic relationship with one of the species of Rhizobium, thereby

making possible the utilization of nitrogen from the air.

The comparative efficiency of free and combined nitrogen for the

nutrition of the soybean plant has been the subject of numerous investigations. The effectiveness of free or combined nitrogen is thought by

Umbreit and Fred (1936) to be a function of the carbohydrate-nitrogen

relation in the plant. When the carbohydrate-nitrogen relation is balanced, the soybean plant primarily utilizes free nitrogen. Under conditions which result in an unbalanced carbohydrate-nitrogen relation,

however, such as low light intensity or unfavorable pH, fixed nitrogen is

required in that it enables plants to survive the unfavorable environment.

Under field conditions these workers believe unfavorable conditions are

the exception and maximum yields should be obtained with nodulated

plants rather than plants which are dependent on fixed forms of nitrogen.

This viewpoint is not entirely supported by other findings.

When Mukden soybeans were grown to maturity by Norman (1944a)

in field experiments on a loess soil low in nitrogen, yield and nitrogen

content of beans from nodulated plots slightly exceeded those from nonnodulated plots receiving 94 Ibs. nitrogen per acre, but were significantly

less than obtained from plots to which 158 lbs, of nitrogen were applied



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per acre. The recovery of nitrogen in plant tops was 24 and 42 per cent,

respectively, for the 94- and 158-lb. rates of application. It was estimated that inoculation resulted in the fixation of approximately 26 lbs.

of nitrogen per acre in the plant tops, which is less than 30 per cent of

the total nitrogen in the tops of nodulated plants. Luxury consumption

was apparent a t the highest rate of application of nitrogen, part of which

was applied at midseason. The oil content- decreased proportionately

with added increments of nitrogen from either combined or atmospheric

nitrogen. With higher bean yields, however, the oil yield per acre increased with additions of nitrogen. It would appear that under these

conditions the inoculated plants did not receive adequate nitrogen through

the fixation process to permit maximum growth and production.

The effect of combined nitrogen on the fixation process has been the

subject of controversial thought by various workers. Early workers generally agreed that addition of nitrogen depressed nodulation, and it was

speculated by some that nitrogen fixation could be entirely suppressed

by the addition of adequate quantities of nitrogen. Considerably more

information has been made available during the past decade. The response of inoculated and uninoculated soybeans grown on acid soils in

the field to lime and various sources of nitrogen was studied by Andrews

(1937, 1938). Based on results obtained when the soybeans were harvested in the small bean stage it was found that addition of ammonium

sulfate in large amounts to nodulated soybeans increased total yields

substantially but had little influence on the nitrogen content. The total

nitrogen increase due to inoculation was as great with a 600-lb. application of ammonium sulfate as with a 75-lb. application, leading the author

to conclude that under field conditions application of ammonium sulfate

at rates as high as 600 Ibs. per acre failed to inhibit nitrogen fixation.

The sources of combined nitrogen, ammonium sulfate, ammonium nitrate,

urea, and cyanamid, varied in effectiveness in stimulating yield. Furthermore, the effectiveness of these forms was influenced differentially by the

addition of lime, and by nodulation of the plants. With heavy applications of combined nitrogen in any form, the nitrogen content of the plants

was not altered by nodulation.

The effect of combined nitrogen on nodulation of soybeans was studied

by Doolas (1938). Inoculated plants were grown in small paraffinedscreen pots placed within much larger pots. As the roots of the plants

penetrated into the outer compartment, a study of the effect of combined

nitrogen on nodulation of the inner and outer zones was made possible.

Addition of nitrogen as calcium nitrate to either the inner or outer zone

reduced the size of nodules materially in that zone but the number of

nodules was reduced only slightly. When nitrogen was added t o the



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MARTIN Q. WEISS



outer zone, the inhibitory effects on nodule size were transferred to the

inner zone, whereas only slight effects were transferred to the outer zone

when the inner zone was enriched with nitrogen. Reduction in size rather

than number of nodules by nitrogen application caused the author to

deduce that the effect of added nitrate occurs after absorption by the

plant root, and that entrance of the organism into the root is not affected.

During the seedling stage soybeans, even though inoculated, frequently undergo a nitrogen hunger period (Fred et al., 1938). This

period of nitrogen deficiency occurs after cotyledonary nitrogen is exhausted and before the nodules supply adequate nitrogen. The nitrogen

hunger period was observed to be prolonged in Manchu soybeans when

grown under bright sunlight. Inhibition of nitrogen fixation was attributed by the authors to an excessive carbohydrate-nitrogen balance

within the plant stimulated by the conditions of bright sunlight. A

reduction in this ratio, accomplished either by shading the plants or by

addition of combined nitrogen, terminated the period of inhibition and

the nitrogen fixation process was initiated.

Studies on effect of combined nitrogen on the fixation process have

been facilitated by the availability of the stable isotope of nitrogen, N15.

Addition of a certain percentage of this fractionally heavier form of

nitrogen to the ordinary nitrogen supplied in the growing medium permits

determination of the proportion of nitrogen in the plant derived from the

Foil. With this isotope of nitrogen, Norman and Krampitz (1946) confirmed previous findings that as combined nitrogen was in greater abundance, the amount of nitrogen fixation decreased even though total

nitrogen per plant increased. The percentage of nitrogen derived from

the atmosphere varied from 100 per cent, when no combined nitrogen was

added, to 30 per cent when large quantities of nitrogen were applied to

the soil. Thornton (1947) similarly using isotopic nitrogen also found

that the amount of nitrogen fixation was inversely proportional to the

amount of nitrogen added. Some degree of nitrogen fixation occurred,

however, even when a near adequate supply of combined nitrogen was

available.

Since nitrogen nutrition is closely associated with other nutrient elements, further aspects of nitrogen nutrition will be discussed in succeeding sections.

b. Phosphorus. During the early stages of development of the

soybean plant phosphorus was found by Hutchings (1936) to be most

efficient in promoting growth on colloidal clay substrate when adequate

levels of calcium are available. With adequate calcium, application of

phosphorus resulted in higher concentrations of the latter element in the

plant tissues. Under similar conditions phosphorus applications also in-



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creased potassium concentrations, although with low calcium concentrations in the substrate the opposite relation existed. Phosphorus was not

found to be a significant factor in controlling nodulation during the early

growth stages of tshe plant. However, applications of phosphorus to the

substrate materially increased the nitrogen concentration of the tissue.

c. Major cations. Calcium has a dominating influence on the nutrition of the soybean plant. Growth responses following application of

lime to acid soils were formerly attributed to the neutralization of the

soil. More recent work shows conclusively that such responses are a t

least partially attributable to the addition of calcium as a nutrient. The

importance of calcium in the nutrition of the soybean plant has been

shown by Horner (1936). Either increase in the calcium level or a

higher degree of calcium saturation of colloidal clay resulted in substantially greater growth, nodulation, nitrogen fixation, and calcium

absorption during the early growth stages of Virginia soybeans. A close

relation between the elements calcium, phosphorus, and nitrogen was

indicated.

Applications of limestone to Norfolk fine sand were found by Beeson

et aE. (1948), in general, to reduce the absorpt,ion of the minor elements,

manganese, iron, cobalt, and copper. Reduction in absorption was pronounced when a supply of minor elements had been added to the soils,

but was small or entirely lacking in soil not treated with these elements.

Toxicity resuking from addition of micronutrients was, therefore, absent

in the presence of high calcium concentrations.

I n a study pertaining to the effects of magnesium on the early growth

stages and nitrogen fixation in soybeans grown on colloidal clay, Graham

(1938) found that adequate magnesium facilitated efficient use of the

calcium offered. At a given level of calcium, nitrogen fixation increased

with higher levels of magnesium and growth was subsequently also increased. No nitrogen fixation occurred in the total absence of magnesium.

Early growth responses of non-nodulated Biloxi soybeans grown in

sand cultures were reported by C. L. Hamner, (1940) when the cations,

calcium, magnesium, and potassium, were varied. A series of nutrient

solutions were employed which could be arranged in a triangle, each

vertex of which represented maximum concentration of a given ion, and

solutions on the adjacent side were entirely deficient in this ion. Of the

three cations varied, variations in potassium concentration resulted in

greatest vegetative growth responses. With high potassium concentrations active vegetative growth resulted even though concentrations of

calcium and magnesium were low. Witsh low potassium, severe chlorosis

of the young plants occurred particularly when calcium or magnesium

concentrations were high.



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MARTIN G. WEISS



Application of potassium to colloidal clay resulted in greater nitrogen

fixation, increased efficiency of phosphorus removed, and less absorption

of magnesium in young nodulated soybean plants (Ferguson and Albrecht, 1941). High carbohydrates in the young plants were found to

be closely associated with increased potassium in the substrate, particularly when the plants were not inoculated.

The function of potassium in the production of carbohydrates was

also noted by Hampton and Albrecht (1944b). Growth of young nodulated Virginia soybean plants was associated with both potassium and

calcium levels. Whereas high carbohydrates were considered attributable

to potassium additions, variations in nitrogen fixation were associated

more closely with calcium concentrations, maximum nitrogen fixation

occurring with low potassium-calcium ratios. However, higher nitrogen

levels in the young plants were closely associated with increased potassium intake. It is of interest that potassium and phosphorus were absorbed or lost to the substrate by the plant roots depending upon relative

concentrations, but in no instance was calcium or magnesium found to

move from the plants to the substrate.

Nutrient studies in sand cultures with the Morse and Virginia varieties by Allen (1943) showed that varieties may differ substantially in

their nutritional requirements. Differences between the two varieties in

forage yields were, in general, not appreciable a t low nutrient concentrations. With high concentrations of potassium and magnesium, however,

yields of the Morse variety substantially exceeded that of Virginia. Differences were attributed to inability of the latter variety to utilize potassium or magnesium a t as high concentrations as the Morse variety, Less

striking differential responses were exhibited by the varieties to levels of

nitrogen, phosphorus and calcium. Ralsoy and Ogden, two soybean

varieties of similar growth habit and maturity, were reported by Nelson

and Hartwig (1948) to interact with fertility level. At high levels Ogden

substantially outyielded Ralsoy whereas a t low levels of fertility the

yield of the two varieties was similar.

Among a number of crop species tested on high lime soils, Bower and

Pierre (1944) found soybeans to be intermediate in response to potassium

fertilization. Crops responding substantially in growth with the application of potassium, such as corn and sorghum, were observed to contain

low concentrations of calcium and magnesium relative to the potassium

concentration in their tissue. Sweetclover and buckwheat which gave no

response were observed to contain high calcium and magnesium concentrations relative t o their potassium content.. Soybeans were intermediate

in this respect. Assuming that absorption of potassium, of which there

was an abundance in this soil, is inhibited by high concentrations of



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calcium and magnesium, t.he authors deduce that plants with high requirements for the latter elements will reduce the concentration of them

in the substrate adjacent to the roots thereby making potassium available, Although soybeans draw heavily on calcium, their potassium requirement also is high, resulting in an intermediate calcium magnesium

: potassium ratio. Response to the application of available potash on

high lime soil is, therefore, intermediate to that of other crops.

d. Micronutrients. Under certain soil types and conditions, chlorosis

attributable to manganese and iron deficiencies readily occurs in soybeans. Manganese deficiency frequently has been observed when soybeans are grown on Maumee loam in Indiana and successful correction

with soil or spray applications of manganese sulphate has been demonstrated by Steckel (1947, 1948). On the high-lime areas of the Webster

series in Iowa, chlorosis due to iron deficiency is common in soybeans.

Spray applications with ferrous sulfate have been found by Nelson (1948)

to be entirely satisfactory for control. Corrective treatment in the above

instances is further discussed in Section VI-2-a.

The ratio of iron to manganese was found by Somers eC al. (1942)

to be of greater importance in promoting normal growth in soybeans than

concentrations of either element. I n greenhouse studies normal plants

were produced only when the ratio of iron t o manganese in the substrate

was between 1.5 and 2.5. With lower iron : manganese ratios the chlorosis

symptoms generally associated with iron deficiency resulted, which the

authors contend are the identical symptoms associated with manganese

toxicity. With higher iron : manganese ratios the chlorosis symptoms

of manganese deficiency occurred, which are thought to be identical wit.h

those of iron toxicity. Even though concentrations of the two elements

were increased 100-fold in the substrate, normal plants resulted when

the above ratio limits were maintained. As iron and manganese are

thought to function as catalysts in cellular respiration, the output of

respiratory COZ from the roots of the plants was measured. The highest

yields of respiratory COz occurred within the range of ratios giving

plants free from symptoms, whereas deviation from this ratio in either

direction resulted in lower respiratory Con.

Field treatment of young chlorotic soybeans on high lime soils by

Nelson (1948) substantiated the above findings. Whereas spray applications of iron compounds increased yields tremendously, and spray applications of manganese compounds showed no effect, spray applications

with both elements resulted in yields materially lower than when iron

alone was applied.

Certain varieties of soybeans were observed by Weiss (1943) t o differ

strikingly in e5ciency of iron utiligation when grown on high lime soils,



+



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MARTIN G. WEISS



By growing advanced hybrid progenies of efficient and inefficient varieties

on a substrate with a differentiating level of iron, it was determined that

the difference in efficiency was conditioned by a single gene. The expressed cell sap of efficient types was of lower p H than that of inefficient

plants. The composition of aerial tissues indicated that absorbed iron

was precipitated within the inefficient plants when grown on high lime

soils and was thereby rendered unavailable to the plant.

The concentration of boron necessary for normal soybean growth has

been determined by Rogers (1947) to be somewhat less than required

by alfalfa or Crimson clover, whereas 0.4 to 0.3 p.p.m. in the substrate

was found by Hodgkiss et al (1942) to give visible toxicity symptoms.

Optimum concentrations of boron for production of fresh tissue by Harbinsoy soybeans were found by Minarik and Shive (1939) to lie within

the range of 0.025 to 1.0 p.p.m. in the substrate. Maximum calcium

absorption also occurred within this range. A marked tendency was

evident for the moisture content of tissues to decrease with increasing

concentrations of boron in the substrate. This occurrence was interpreted

as support for the theory that the function of boron may be concerned

with the regulation of water absorption by plasma colloids.

Boron deficiency in soybeans was found by MacVicar and Struckmeyer (1946) and Struckmeyer and MacVicar (1948) to be accompanied

by a marked increase in vascular tissue resulting from abnormal cambium

activity. Boron deficiency symptoms and the accompanying cambial

activity were particularly noticeable when Biloxi soybeans were grown

under long-day conditions. When the Biloxi variety was grown under

short days, or Pagoda, a day-neutral variety, was grown with either

long or short days, boron requiremen& were considerably lower. The

decrease in cambial activity associated with photoperiodic induction of

flowering in the latter cases was thought to cause a reduction in severity

of boron deficiency symptoms.

Varieties of soybeans were found by Earley (1943) to exhibit differential tolerance to zinc concentrations in excess of nutrient requirements

in crushed quartz substrate flushed with slightly acid nutrient solutions.

Tolerant varieties, such as Hudson Manchu, showed no toxicity symptoms a t concentrations of zinc 8 to 12 times as great as those tolerated

by susceptible varieties.

Illini soybeans were found by Martin and Trelease (1938) to have

greater tolerance for excesses of selenium in the growth medium than

tobacco. However, 2 p.p.m. of selenium caused a marked stunting of

soybean plants. Selenium absorption was closely associated with its

concentration in the substrate. The presence of sulfur, with sublet,hal



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IV. Physiology of the Soybean Plant

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