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IX. Physiology of Tropical Legumes

IX. Physiology of Tropical Legumes

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was highly correlated with mean seed weight of each species, siratro

having the greatest seed and seedling weights and greenleaf desmodium

the smallest. At the second harvest, the real differences between the legumes in growth rate were expressed. Growth was abnormal and reduced

markedly at the lowest temperatures 15110°C and 18/ 13°C (daylnight).

Optimum temperature for growth of all the legumes was 30/25 2 3°C

which is lower than for tropical grasses and higher than for temperate

legumes and grasses. Above 33/28"C growth rate declined, particularly

in the Desmodium species, but not so markedly in siratro.



In experiments with Townsville stylo, 't Mannetje (1965) extended the

photoperiod of 8 hours sun with incandescent light. He found that it was

a short-day plant in temperatures of 30°C (day) and 25°C (night) and that

dry matter yields in 12- and 14-hour photoperiods were greater than those

in 8- and 10-hour ones. Sweeney's results, quoted by Humphreys (1967),

showed optimum dry matter production at 33/28"C. In a study of the flowering behavior of seven selections (early to late) of Townsville stylo, D.

F. Cameron ( 1967a) showed that day length was the main factor controlling flowering and that they all had a strong short-day response. At normal temperatures, maximum day lengths (critical day lengths) at which

all plants flowered were 13 hours for the early selections, 12 hours for

the midseason and late midseason, and 1 1.5 hours for the late. Both high

night temperature and low day temperature delayed or inhibited flower

initiation in the early and midseason selections, and these effects were

greater at the critical day length.

D. F. Cameron's field and shadehouse experiments (1967b) with different sowing dates and locations gave similar results to his controlled

environment studies. In the early December sowing, the range in flowering time between maturity groups was 56 days because the longer day

lengths promoted flowering in early types and prevented floral initiation in

late types. With the late March sowing, day length was short enough to

promote flowering in all maturity types so the range in flowering time was

only 8 days. At the southerly locations, most selections flowered later

because the longer day lengths delayed flowering time.

C. Glycine wightii

Edye and Kiers observed ( 1966) variation in maturity, stolon development and frost resistance in 50 accessions ofglycine at Lawes, southeastern

Queensland. The discontinuous variation in flowering enabled definition



of very early, early, midseason, and late maturity types. Generally, collections from below 15" latitude are early, midseason, or late types and

are more strongly stoloniferous and more frost susceptible whereas collections from above this latitude are predominantly early types, which

are less stoloniferous and more frost resistant. Tow ( 1967) compared the

growth of Tinaroo glycine and green panic in controlled conditions. He

found that green panic produced much more dry matter than glycine per

unit of intercepted light and per unit of water transpired and that green

panic had higher shoot-root ratios than Tinaroo glycine. He investigated

five other varieties of glycine and all had higher shoot-root ratios than

Tinaroo, some with similar ratios to green panic. Wutoh et al. (1968a)

showed that flowering in five glycine introductions (very early to late) is

affected by both photoperiod and temperature. Temperatures of 27/2216°C (daylnight) appeared to be best for growth and seed production.

All accessions studied were short-day types and in the sensitive ones

temperature had little effect on flowering. In others, lowering day or night

temperature or both induced flowering in long (16-hour) days and could

prevent it in short days. Seed formation did not occur in day temperatures above 27°C.

Response to salinity in glycine was investigated by Gates el al. ( 1 966a,

b) as soils with a high salt content occur in the brigalow lands of northeastern Australia, where this legume has potential. With the highest salt

level (240 meq of NaCl per liter) yield of Tinaroo glycine dropped to 25%

of the control but nitrogen content was unaffected. Phosphorus content

rose by up to 100% in the roots but did not change in the tops. Percentage

of soluble nitrogen increased by more than 50% as salinity rose, indicating

impairment of protein synthesis. Glycine can adapt to high levels of sodium chloride unaided by divalent ions provided the increase in salinity is

gradual. In their experiments with 22 glycine accessions, Gates et al.

(1966b) used four salinity levels up to 140 meq of NaCl per liter. The

highest salinity level had a relatively greater impact on growth than the

others. Differences in dry weight of the glycines at all salinity levels were

of similar proportions to those at the control level. The normal capacity

for growth of an accession seemed to be an important feature in determining its response to salinity.


In a controlled environment experiment (Hutton, 1964), growth of

siratro was very poor at 18/ 13°C and 2 1 / 16°C in short (8-hours) and long

(8-hours sun 8-hours incandescent light) days. Over the temperature

range 24/ 19"C, 27/22"C, 30/25"C, and 33/28"C, dry matter production




of siratro in a short day averaged 30% of that in a long day. Maximum dry

matter yield in the long day was at 27/22"C and 30/25"C and was reduced by 30% at 33/28"C and by 7% at 24/19"C. Flowering occurred in

short and long days at all temperatures except 18/ 13°C and was best at

24/19"C, 27/22"C, and 30/25"C. At a constant temperature of 28"C,

Whiteman (1969) found that siratro flowered in daylengths of 8, 10, and

12 hours but not at 16 and 24 hours, indicating that siratro is a short-day


In a comparison between siratro and hamilgrass ( P . maximum) in a

controlled environment, Ludlow and Wilson ( 1968) found that relative

growth rate of the grass was almost twice that of siratro. This was due to

a much higher net photosynthetic rate in the grass which also had a higher

respiration rate than siratro.



Although not strictly tropical, the African trifoliums have some potential in the subtropics. 't Mannetje and Pritchard (1968) studied the reactions of 13 species and varieties of these trifoliums in controlled environment and glasshouse experiments. T. baccarinii, T. pseudostrictum,

and T . usambarense behaved as sensitive short-day plants, and T. burchellianum var. burchellianum and T. africanum behaved as sensitive

long-day plants. The other species were either day-neutral or could not

be clearly classified because of strong effects of night temperature on

flowering. T. semipilosum flowered in all treatments except the 10-hour

photoperiod at 25/20"C and better in a low than high night temperature.

Conditions favorable for flowering were usually the best for growth.

X. Breeding and Genetics of the Main legumes

This was reviewed by Hutton ( 1965), and only the more recent findings

will be discussed here. The complement of characters present in the current range of legume cultivars in Australia, with the exception of siratro

(Hutton, 1962), have resulted from natural selection in various native

habitats overseas. Thus a number of the legumes are not fully adapted to

conditions in northern Australia and need further breeding or selection to

adapt them more closely to the soil-pasture-animal system.


Breeding systems vary (Hutton, 1960) from close pollination as in

Townsville stylo, siratro, Indigofera, glycine, and Miles lotononis (Byth

1964) through a combination of self- and cross-pollination in the desmodi-



ums (Rotar et al., 1967) to cross-pollination in lucerne, white clover, and

some of the African trifoliums including T. semipilosum (Pritchard and

't Mannetje, 1967). In the self-pollinating tropicals a high relative humidity is needed for pollen-tube growth on the stigmas, and seeds are set only

if the humidity is adjusted to the optimum level around the emasculated

and hand-pollinated flowers. In leucaena the round heads of small flowers

are self-pollinated but can be emasculated after anthesis by washing in a

very weak solution of a nontoxic wetting agent and then hand-pollinated.



D. F. Cameron (1965) found that significant variation occurred in

flowering time, growth habit, plant growth, and seed yields among a large

number of collections made from the naturalized populations in Queensland and the Northern Territory. There was a continuous range in flowering time from early to late, and plant growth was related to time of flowering. Late types gave higher dry matter yields than midseason types, which

gave higher yields than early types. The midseason types gave the highest

seed yields.

Time of flowering and seed setting are very important in Townsville

stylo to ensure production of large amounts of seed for reestablishment

in the following wet season. D. F. Cameron (1967~)related flowering

time of 58 ecotypes with their collection site characteristics. In general

the late-flowering types were collected from areas with an annual rainfall

over 45 inches, and early types came from drier areas with a rainfall of

23-35 inches. Distribution was also correlated with latitude. The five

collections obtained south from Rockhampton were early or midseason

and all collections from the northern parts of Cape York Peninsula, the

Northern Territory, and Western Australia were late flowering. Late

types usually give higher yields than early types in areas with a long growing season, because they continue vegetative growth after early types

have set seed and stopped growing. With a short growing season, late

types are unable to flower and seed and fail to produce a dense sward in

the following season whereas early types regenerate well because they

flower and set seed before soil moisture is exhausted. Erect types usually

give higher yields than prostrate, particularly when associate grasses are

present in the pasture.

Inheritance of flowering time has been studied (D. F. Cameron, 1968)

in a diallel cross between nine lines ranging from early to late. In all

crosses involving two late-flowering types, late flowering was strongly

dominant and the 3 : 1 segregation indicated a major single gene difference

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IX. Physiology of Tropical Legumes

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