Drought tolerance in the common bean

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Drought tolerance in the common bean possible regulatory mechanisms and breeding strategy
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Common bean -- Drought tolerance -- Testing   ( lcsh )
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Drought Tolerance in the Common Bean: Possible Regulatory

Mechanisms and Breeding Strategy.





Fluctuations in amount and frequency of natural rainfall in

bean production areas in Latin America, North America, and Eastern

Africa are sufficiently great that both short-term and season-long

periods of drought are experienced by some bean producers almost

every season. The growing of beans on coarse-textured soils and/or

with an associated crop further enhance the likelihood of a water

shortage sometime during the growing season.

The climate and soils of major bean production zones in Mexico

(Zacatecas, Durango, Aguascalientes, Chihuahua) render bean

production in these regions particularly vulnerable to drought

stress.

The common bean (Phaseolus vulgaris) and its immediate

ancestors are indigenous to Mexico, and, for this reason, it might

be expected that present day varieties would display some tolerance

to the adverse growing conditions represented by the low and

variable rainfall of the semi-arid and sub-humid zones of Mexico.

Nevertheless, the common bean is not accounted as having resistance

to drought, certainly not in the same degree as the Tepary bean (Ph.

acutifolius) or the Cowpea (Vigna unguiculata). We are generally

familiar with the plant symptoms of drought stress in beans -













s tunted plants small leaves short ened int ernodes delayed

development, dark green foliage color, leaf flagging, wilting, and

leaf drop, weak root development, flower and pod abortion, premature

senescence, shriveled pods, and smaller seeds.

Physiological responses of beans to drought stress are less

obvious, but include changes in relative water content, elevated

leaf temperature, reduced rate of transpiration, diffusive

resistance, stomatal behavior and leaf water potential changes, all

of which are measurable by instrumentation. Leaf, internode, or pod

growth rates, which basically depend upon water for cell wall

extension, are good indicators of water status, but are more

time-con~suming for reliable measurements. Leaf thickness, under

stress, may help to maintain a higher photosynthesis rate than if

leaves were thin, and thus be of positive value to a variety

subjected to water deficit.

What are the bean plant characteristics which, singly or

collectively, tend to render the plant more tolerant to drought

stress? Is the genetic regulation of an individual trait

sufficiently strong that it can be selected efficiently? Table 1

lists the plant characteristics which various authors (see

Literature Review) have suggested as contributory to drought

tolerance.

We must note, however, that some of these traits are

dependently related to others in the list. Water use efficiency is













dependent upon stomatal closure and cuticular vax as well as upon a

deep and ramifying root system, and maintenance of a high leaf water

potential. Leaflet orientation is strongly associated with leaflet

size.

Furthermore, some traits are not inherently related to high

yield. Early maturity is one example. Small leaves, while

favorable to leaflet orientation, can not easily be combined with

large seed size. Self-pruning, while a tactic that conserves

moisture, reduces photosynthically-active leaf surface which is

deleterious to yield.



TABLE 1. Plant Characteristics Contributory to Drought Tolerance in

the Common Bean.



1. Early maturity (drought avoidance)

2. Deep and/or ramifying root system

3. Small leaves

4. Leaflet orientation

5. Leaf Thickness (under certain circumstances)

6. Self-pruning of leaves

7. Cuticular wax on leaves

8. Stomatal closure to conserve leaf water

9. Water use efficiency (WUE)

10. Maintenance of high leaf water potential












11. Remobilizationiof previously stored carbohydrate

12. Developmental plasticity affecting timing and frequency oP

flowering.

13. Unidentified intrinsic physiological properties that

affect yield and survival under drought stress.



Behavior of the pinto bean variety 'San Juan Select,' from the

high dry plains of southwestern Colorado, illustrates a special

adaptive response to drought stress. Once established on residual

winter moisture, the plant responds to each rare summer rain by

producing a flush of blossoms some of which become seed-bearing

pods, following which the plant suspends reproductive growth until

the next rain at which time it repeats the flowering and

seed-forming cycle. This is an example of developmental plasticity.

We believe that each of the first 12 characteristics listed in

Table 1 is subject to both environmental and genetic influences, but

we have no information at this time on the relative proportions or

importance of each type of regulation. Some traits may be more

easily scored and selected for than others, for example, early

maturity or small leaves or leaf orientation, as compared with a

deep or ramifying root system. This is true not only because of the

relative convenience of scoring, but also because of the relative

heritabilities of each trait. Genetic linkages and/or developmental

inter-dependencies among certain characteristics may complicate the












task of assembling the most favorable combinations of genes into a

common superior genotype. 1

In the remainder of this paper we wish to consider just one of

the 12 traits -- the characteristic of remobilization of

previously-stored assimilate-as contributory to seed-filling under

drought stress. The basic idea is simple if a variety can produce

an excess of carbohydrate (or nitrogen-containing materials) during

pre-anthesis or even early post-anthesis, store it temporarily in

leaves, stems, or root, and subsequently remobilize a substantial

portion to seeds during the critical filling stage, when the plant

.is_ drought-stressed, it should help to maintain a near normal

seed-filling rate or to minimize the yield reduction attendant upon

lowered photosynthesis induced by the stress.

There is some evidence, in wheat, rice, and soya that this may

occur. There is also qualitative evidence in beans that different

varieties store and remobilize different amounts of starch.

Measurements of dry weight losses in leaves, stems and roots and dry

weight gains in pods and seeds during particular short periods of

reproductive development in beans support this hypothesis. Such

experiments are not critical, however, because concurrent

photosynthesis and respiration rates have not been estimated.

In particular, in this paper, we want to focus upon data from a

recent greenhouse experiment in which we grew two bean cultivars in

quartz sand medium in polyvinylchloride cylinders nourished by












complete nutrient solution daily over the complete life cycle. We

used radio-active CO2 gas, applied at three stages, to label th'e

carbohydrate, in order to estimate the proportions of assimilate

found in seeds at physiological maturity (PM) which came from

pre-anthesis storage, from storage during flowering to

mid-pod-filling (F-MPF), or from MPF to PM. A severe water

(drought) stress was imposed at MPF. The labelling apparatus used

is shown in Figure 1. It consists essentially of both 1CO2 and
14
CO2 gas regulated by a flowmeter and introduced into a gas-tight
saran-plastic chamber where the gaseous air was circulated by a

.small fan around 16 bean plants in their PVC cylinders (8 stressed

and 8 non-stressed, of a given variety at a given labelling time,

one of 3 such times). The chamber was illuminated overhead by 2

high intensity sodium vapor lamps during the exposure period of

about I hour 45 minutes, including the gas flushing time. The gas,

after passing through the chamber, is moved by pump through a drying

medium and/or by-passed into the infra-red gas analyzer (IRGA) for

monitoring purposes, and .directly through the ROH trap, a cooling

ice-bath, and returned to the chamber.

The bean genotypes of the study were N81017, a navy bean shown

in field tests in both Michigan and Iguala, Mexico, to have some

drought tolerance, and B790131, a tropical black seeded type,

thought to be drought sensitive. In some figures, these lines are

denoted as CV.1 and CV.2,. respectively.












Experimental Results



Average changes in dry weight (in gas) of seeds, stems, leaves,

roots and total (per cylinder-2 plants) at flowering, mid-pod

filling (15 days after flowering), late-pod filling, and

physiological maturity, for N81017 and B790131 under "plus water"

and "minus water" treatments are shown in Figures 2-5. For N81017,

the most pronounced change, aside from the lower total and seed

weight, was the stage at which a marked reduction in leaf weight was

measured. This took place in the "plus water" treatment gradually

from MPF to PM, whereas in "minus water" the leaf weight loss

occurred precipitously from the stage at which drought was imposed,

that is, MPF. Roots and stems incurred smaller weight losses than

leaves, more in "minus water".

In B790131, leaves were also the target of dry weight losses,

but in this line the loss came in the period of late pod-fill (LPF)

to PM.

Distribution of dry weight by variety and treatment is shown in

Table 2 and the associated A.0.V. in Table 3.. These show values of

Biological Yield, Economic Yield, and Harvest Index. B790131 shows

a low HI because, under the "plus-water" regime, this line produced

and retained leaves, stems, and roots so that total weight

(Bioyield) was still increasing at PM, thus lowering HI for the












"plus-water" treatment. This shows up as a significant cultivar by

treatment interaction in Table 3.

Values for yield components and their AOV are given in Tables 4

and 5, respectively. Inasmuch as the stress was imposed at MPF, one

would not expect number of pods to be significantly altered by

treatment. In fact, however, there was found a highly significant

cultivar x treatment interaction for pod number, for which we have

no explanation. N81017 incurred a small, and B790131 a large,

reduction in single seed weight due to stress. Clearly, the two

cultivars responded quite differently to drought, as reflected in

their yield component changes.

The distribution of radio-activity, by variety and stage of

labelling, is shown in Tables 6, 7, and 8. When labelled at F, the

percentage recovery of radio-activity at PM on the whole plant basis

was similar for both cultivars, and slightly higher for the stressed

plants. When labelling was done at MPF and at LPF, percentage

recovery at PM was, as expected, higher than for the earlier

labelling, but the two cultivars displayed strikingly different

values under the "minus-water" treatment. Percent recovery was only

b to 5 in 790131 as compared to 81017, for which percent recovery

was 3/4 and 9/10 at MPF and LPF, respectively. These differences

are explainable on the grounds that stress induced earlier

senescence and lower respiration in N81017 and delayed senescence












vith no change in respiration in B790131, as compared with the

non-stress treatment.

Tables 7 and 8 show the distribution in the plants of

radio-activity, expressed as a percentage of total whole-plant-

radio-activity recovered at PM.. Table 7 is particularly

interesting. It shows that seeds of both cultivars possessed a

greater percentage of label when their plants were'stressed than

when non-stressed. From what other plant parts was label supplied,

stems, pod walls, leaves, or roots? For cultivar 1 (N81017), it

appeared that stems, pod walls, and especially leaves, may have been

.the source of 1C-assimilate, but clearly not the roots. For CV.2

(B790131), leaves and roots were the source of 1C-assimilates to

the seeds and clearly neither stems nor pod walls. This is the

situation for assimilate fixed at F and counted at PM.

When labelling was done at the later stages, MPF and LPF, a

greater fraction of the 1C-assimilate, and therefore of all carbon

photo-assimilated at those times, was found in the seeds, regardless

of cultivar or treatment, except for CV.1 where an appreciable

portion was found in pod valls. Clearly, most carbon fixed at MPF

and later was directed mainly to seeds in both stressed and

non-stressed plants. There is no evidence whatsoever for storage of

assinflate in vegetat ive structures in N81017 after MPF, and

evidence for only moderate storage in leaves and stems for B790131

at the MPF or later stages.












If we plot, as in Figures 6 and 7, increments of growth in

grain weight against increments of total net dry weight increase folr

the periods F to MPF (Fig. 6) and MPF to PM (Fig. 7) for cultivars 1

and 2 under the two conditions of moisture, an informative picture

emerges. For the data of Fig. 6, since the period F to MPF does not

include the stress (stress was imposed at MPF), only two points are

shown, representing non-stress. The position of the diagonal line

shows where a cultivar would lie if all the assimilate produced

during F to MPF went into the grain. CV.2 displays that behavior.

CV.1 lies further from and below the line and clearly is

assimilating carbon at a greater rate than it is using for grain

growth.

We turn next to Fig. 7, which covers the period MPF to PM, the

period following imposition of drought stress. Stressed plants of

both cultivars, but particularly CV.1, take a position above the

diagonal line, indicating grain growth exceeds total growth for that

period, implying the grains had to be receiving assimilate produced

and stored in an earlier period, there being no other source of dry

weight to draw upon.

For the "plus-water" treatment, CV.1 (N81017) lies close to the

diagonal line, indicating essentially complete utilization of

photosynthate produced in the MPF to PM period for grain growth.

For CV.2, however, the photosynthate produced in MPF to PM was

clearly under-utilized in grain development. This is shown












graphically also in Figure 8. The display in Figure 7 argues

strongly if not compellingly for the view that, under drought

stress, grain growth will require and utilize previously stored

assimilate, and that cultivars may differ in the extent to which-

such storage materials are used.

Figure 8 presents the situation in somewhat more graphic terms.

For CV.1 (+H20) the histogram shows that to form 43.4 grams of seed

in the MPF to PM period, 2.81 grams of assimilate produced in the F

to MPF period was required to supplement assimilate production

(current net photosynthesis) in the MPF to PM period. In the case

of CV.2 (+H20), as noted previously, seed growth of 21.12 grams

consumed only about 54% of the net photo-assimilate produced in the

MPF to PM period and none from any stored previously.

For CV.1 (-H20), the histogram indicates that 19.74 grams of

seed growth made in the MPF to PM period utilized 4.75 grams from

concurrent photosynthesis or 24%, 10.5 grams from assimilate

produced in the F to MPF period or 53%, and 4.5 grams or 23%

remobilized from pre-anthesis assimilate production.

For CV.2 (-H20), grain growth of 15.09 grams depended upon both

concurrent assimilate production (76%) and assimilate remobilized

from production in the F to MPF period (24%).

The results support the hypothesis that under a drought stress,

remobilization to the seeds of previously stored assimilates is

enhanced. Under conditions of this experiment, however, the rate












and exteat of storage and remobilization were insufficient to

maintain seed growth at the normal non-stressed rate. I

We can offer the opinion that under conditions of relatively

luxuriant assimilate storage followed by a short-term stress upon-

photosynthesis, remobilization in some genotypes would be sufficient

to maintain a near normal rate of seed growth. Under more severe

conditions seed filling will surely be adversely affected and yields

reduced. Nevertheless, we conclude that, in breeding for

resistance/tolerance to drought stress in beans, it would be

advantageous to breed~ for genotypes that possessed the ability to

store and remobilize carbohydrate when exposed to drought stress

during the seed-filling period.



CONCLUDING COMMENTS



Among the 12 characteristics listed in Table 1 as contributing

to drought tolerance in beans, we have singled out assimilate

storage and remobilization for particular experimentation and

discussion here. This is only one aspect, however, of the plants'

morphological-phenological-

physiological response to drought stress. Drought tolerance in the

bean is surely as complex as yield itself, and yet, as with yield,

genetic advances would appear to be possible. Indeed,












experimentalists in CIAT, Mexico, and California are already finding_

that this is so.

From the standpoint of breeding strategy, we anticipate the

emergence of two schools of thought: One, the group that accepts

drought tolerance as a complex genetic-environmental response that

can best be selected for in advanced hybrid populations screened

rigorously in the field, with the major selection criterion being

yield itself under stress conditions, or geometric mean yield under

stress and non-stress environments; two, the group that decides that

certain identifiable plant characteristics account for or lead to

.some degree of drought resistance and that selection can be

practiced for these traits in particular populations, and that,

given time, favorable multiple-component combinations can be

constructed which will confer higher levels of drought tolerance

upon genotypes possessing them.

For the present, the strategy of the first school is likely to

be the choice of most breeding programs. It can be simply and

objectively employed in arid environments with a line ~source

irrigation facility and does not require knowledge of mechanisms of

resistance. For the longer run, and probably for the higher levels

of resistance,'a knowledge of mechanisms at the physiological and

genetic level is required. As such information is gained, the

strategy of the second school becomes more and more realistic and

appropriate.

















Cultivar Treatmaent Biological*1 Economic*1 Harvest
Yield Yield Index


81017 Irrigated 85.30 47.15 0.55

Stress 52.59 28.17 0.54

790131 Irrigated 75.14 29.73 0.40

Stress 47.64 24.20 0.52


7-~c~ ,2~


Distribution of Dry Weight


*1 grs./ pot















Analysis of Variance for Distribution of Dry Weight*1



Biological Economic Barvest
Yield Yield Index



Water treatment ** **

Cultivar ** **

Cultivar x **
Treatment


*1 t = significant at 5% level
** = significant at 1% level






















Cultivar Treatment Number seeds/Pod Seed vt.*1 Number
of Pods (grs.) of Seeds




81017 Irrigated 38 5.28 23.73 199
Stress 28 4.63 21.83 130

790131 Irrigated 28 3.96 26.66 112
Stress 32 3.87 19.62 125


YIELD COMPONENTS


*1Weight of 100 seeds.




















____ __ _U__I____________________ ______~_____


Water treatment ** *

Cultivar ** **

Cultivar x ** ** **
Treatment


Analysis of Variance for Yield Components*1


Seeds/Pod Seed rpt.*2 Num~ber
(grs.) of Seeds


Number
of Pods


*1 = significant at 5% level
** = significant at 18 level


*2Weight of 100 ~seeds.














Labelling % of Radioactivity
Time Cultivar Recovered at Phys. Maturity
+H20 -H20

Flowering I 32.5 36.2

2 36.6 38.9


Mid-Pod Filling 1 65.2 94.6

2 57.0 52.2


Late-Pod Filling 1 88.8 75.6

2 88.3 25.1


RADIO-ACTIVE LABELLING EXPERIMENT ON BEANS, GH-1984
















Labelling Cultivar Seeds Stems Pod Walls Leaves Roots
Time +H20 -H20 +H20 -H20 +H20 -H20 +H20 -H20 +H20 -H20




Flowering 1 19.1 25.4 39.4 34.6 9.9 5.3 17.5 5.9 13.9 28.7


2 21.4 37.3 23.7 27.1 5.7 7.1 11.7 1.3 37.3 27.2


RADIO-ACTIVE LABELLING EXPERIMENT ON BEANS, GH-1984
Distribution of Radioactivity Recovered at Physiological Maturity
(expressed as % of total radioactivity recovered at P.M.)
















Labelling Cultivar Seeds Stems Pod Walls Leaves Roots
Time +H20 -H20 '+H20 -H20 +H20 -H20 +H20 -H20 +H20 -H20

Flowering 1 19.1 25.4 39.4 34.6 9.9 5.3 17.5 5.9 13.9 28.7

2 21.4 37.3 23.7 27.1 5.7 7.1 11.7 1.3 37.3 27.2


Mid-Pod Filling I 56.3 56.9 6.4 8.0 32.8 29.0 3.0 3.3 1.4 2.8

2 84.0 91.6 1.5 1 .0 11.0 6.3 2.7 1.1 0.7 0.4


Late-Pod Filling 1 90.1 81.8 2.3 5.2 4.3 5.8 2.1 3.3 1.1 3.9

2 64.6 53.2 9.6 15.8 3.8 5.1 15.8 14.5 5.9 11.3


7~~~


RADIO-ACTIVE LABELLING EXPERIMENT ON BEANS, GH-1984
Distribution of Radioactivity Recovered at Physiological Maturity
(expressed as % of total radioactivity recovered at P.M.)


















.CLAMP
*VALVE:
O FLOWMETEI









PLUS WATER


581017


9


ToTAL-



SEEDS




ST EMS
LEAVES


A80
M
S 70


S40

203


PM


PHYSIOLOGICAL STAGE


BEANI LINIE


FELOW MPli LPF









100
90
- 80
70
G60

40
30
L..

O0


T OT AL


SEEDS


ST EMS
RO15OTS5


STAGE


PHYSIOLOGICAL


BEANI LINIE N88101~7 M I NUS WATER


FLOW MPF LPF PM







B790131


100
90
8 8
78
60
50
-40
30
20
iG


M
S
I)
R
Y
0
T
S


;; T oTL



95EEDS


Is r EMS


FLOW MPT LPF PM
PHYSIOLOGICAL STAGE


PLUS WATER~


BEANI LINE




;Z~- "-


B790131


-TOTAlL



SE EDS

.ROQ OT S
ST En5


PM


P H Y SI O LOGCI C AL STAGEI


BEAN LINE


MINJUS WATER


D
R 4
Y


~FLOW MPF LPF
















S20

O> CV-2 OCV- I




O 20 40
NET PHOTOSYNTHATES (grs.), F -MPF








C V. I
+H9O


40





CV. 1 +H20
rr ~-He 0
20 0ck2







O 20 40

NET PHOTOSYNTHATES (grs.), MPF-PM














C V. I
+ HgO


C V. I
-Hg0


C V. 2
+Hg0


C V. 2
-H20


43.40






10.5
4.75


2.8 I



40.59


39.29


21.12


19.74


15.09


3.67
I I.42


C I Total growth (grs.) MPF -PM
M Seed growth (grs.) MPF- PM
I Proportion of seed growth accounted for F MPF assimilates
SProportion of seed growth accounted for pre nthesis assimilates




Full Text








complete nutrient solution daily over the complete life cycle. We

used radio-active CO2 gas, applied at three stages, to label the

carbohydrate, in order to estimate the proportions of assimilate

found in seeds at physiological maturity (PM) which came from

pre-anthesis storage, from storage during flowering to

mid-pod-filling (F-MPF), or from MPF to PM. A severe water

(drought) stress was imposed at MPF. The labelling apparatus used

12
is shown in Figure 1. It consists essentially of both 1CO2 and
14
CO2 gas regulated by a flowmeter and introduced into a gas-tight

saran-plastic chamber where the gaseous air was circulated by a

small fan around 16 bean plants in their PVC cylinders (8 stressed

and 8 non-stressed, of a given variety at a given labelling time,

one of 3 such times). The chamber was illuminated overhead by 2

high intensity sodium vapor lamps during the exposure period of

about 1 hour 45 minutes, including the gas flushing time. The gas,

after passing through the chamber, is moved by pump through a drying

medium and/or by-passed into the infra-red gas analyzer (IRGA) for

monitoring purposes, and directly through the KOH trap, a cooling

ice-bath, and returned to the chamber.

The bean genotypes of the study were N81017, a navy bean shown

in field tests in both Michigan and Iguala, Mexico, to have some

drought tolerance, and B790131, a tropical black seeded type,

thought to be drought sensitive. In some figures, these lines are

denoted as CV.1 and CV.2, respectively.












stunted plants, small leaves, shortened internodes, delayed

development, dark green foliage color, leaf flagging, wilting, and

leaf drop, weak root development, flower and pod abortion, premature

senescence, shriveled pods, and smaller seeds.

Physiological responses of beans to drought stress are less

obvious, but include changes in relative water content, elevated

leaf temperature, reduced rate of transpiration, diffusive

resistance, stomatal behavior and leaf water potential changes, all

of which are measurable by instrumentation. Leaf, internode, or pod

growth rates, which basically depend upon water for cell wall

extension, are good indicators of water status, but are more

time-consuming for reliable measurements. Leaf thickness, under

stress, may help to maintain a higher photosynthesis rate than if

leaves were thin, and thus be of positive value to a variety

subjected to water deficit.

What are the bean plant characteristics which, singly or

collectively, tend to render the plant more tolerant to drought

stress? Is the genetic regulation of an individual trait

sufficiently strong that it can be selected efficiently? Table 1

lists the plant characteristics which various authors (see

Literature Review) have suggested as contributory to drought

tolerance.

We must note, however, that some of these traits are

dependently related to others in the list. Water use efficiency is




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"plus-water" treatment. This shows up as a significant cultivar by

treatment interaction in Table 3. 4

Values for yield components and their AOV are given in Tables 4

and 5, respectively. Inasmuch as the stress was imposed at MPF, one

would not expect number of pods to be significantly altered by

treatment. In fact, however, there was found a highly significant

cultivar x treatment interaction for pod number, for which we have

no explanation. N81017 incurred a small, and B790131 a large,

reduction in single seed weight due to stress. Clearly, the two

cultivars responded quite differently to drought, as reflected in

their yield component changes.

The distribution of radio-activity, by variety and stage of

labelling, is shown in Tables 6, 7, and 8. When labelled at F, the

percentage recovery of radio-activity at PM on the whole plant basis

was similar for both cultivars, and slightly higher for the stressed

plants. When labelling was done at MPF and at LPF, percentage

recovery at PM was, as expected, higher than for the earlier

labelling, but the two cultivars displayed strikingly different

values under the "minus-water" treatment. Percent recovery was only

- to in 790131 as compared to 81017, for which percent recovery

was 3/4 and 9/10 at MPF and LPF, respectively. These differences

are explainable on the grounds that stress induced earlier

senescence and lower respiration in N81017 and delayed senescence












11. Remobilization of previously stored carbohydrate

12. Developmental plasticity affecting timing and frequency ot

flowering.

13. Unidentified intrinsic physiological properties that

affect yield and survival under drought stress.



Behavior of the pinto bean variety 'San Juan Select,' from the

high dry plains of southwestern Colorado, illustrates a special

adaptive response to drought stress. Once established on residual

winter moisture, the plant responds to each rare summer rain by

producing a flush of blossoms some of which become seed-bearing

pods, following which the plant suspends reproductive growth until

the next rain at which time it repeats the flowering and

seed-forming cycle. This is an example of developmental plasticity.

We believe that each of the first 12 characteristics listed in

Table 1 is subject to both environmental and genetic influences, but

we have no information at this time on the relative proportions or

importance of each type of regulation. Some traits may be more

easily scored and selected for than others, for example, early

maturity or small leaves or leaf orientation, as compared with a

deep or ramifying root system. This is true not only because of the

relative convenience of scoring, but also because of the relative

heritabilities of each trait. Genetic linkages and/or developmental

inter-dependencies among certain characteristics may complicate the