African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 9, Num. 2, 2001, pp. 369-375

African Crop Science Journal, Vol. 9. No. 2, pp. 369-375

Inheritance of Shortday Induced Dwarfing in Photosensitive cowpeas

M. F. Ishiyaku and B.B. Singh¹
Institute for Agricultural Research (IAR), Ahmadu Bello University, Zaria, P.M.B. 1044, Nigeria
¹International Institute of Tropical Agriculture (IITA), Kano Station, PMB 3112, Kano, Nigeria

(Received 10 April, 2000; accepted 5 December, 2000)

Code Number: CS01018

INTRODUCTION

Cowpea [Vigna unguiculata (L.) Walp.] is an important source of dietary protein and nutritious fodder in the semi-arid tropics, particularly in West and Central Africa. It is normally grown in intercropping with cereals in complex cropping systems and contributes to soil fertility and sustainability of the systems (Mortimore et al., 1997; Singh et al., 1997; Tarawali et al., 1997). The traditional varieties are photosensitive such that when planted in long days (June - July) they grow profusely and remain vegetative until the onset of short days (September) when they begin flowering. They become ready for harvest in October-November taking 120 to 130 days from sowing to maturity. Farmers have consciously selected such varieties because i) they flower and mature after millet has been harvested and therefore no shading during reproductive stage, ii) they mature after the rains ensuring good seed quality, iii) they mature when competition for labour is minimum, and iv) they provide grain as well as fodder (Mortimore et al., 1997). Therefore, current cowpea breeding efforts in the region aim to improve these local photosensitive varieties by incorporating resistance to diseases, insect pests and parasitic weeds such as Striga gesnerioides and Alectra vogelii in them (Singh, 1994; Singh et al., 1997).

The effects of photoperiod and temperature on reproductive development of cowpea have been studied by several workers (Njoku, 1958; Hadley et al., 1983; Craufurd et al., 1996a, 1996b; Ishiyaku, 1997). However, none of the reports have mentioned the effect of photoperiod on the vegetative growth of photosensitive and photoinsensitive cowpea varieties. In our routine breeding programme we have noticed that when cowpea varieties are grown in shortday conditions (off season) for generation advance or for seed multiplication, the photosensitive varieties flower early as expected, but they also become extremely dwarf in contrast to the photoinsensitive varieties which show near normal growth and maturity. This suggests that photosensitivity affects both vegetative and reproductive stages. This experiment was conducted to confirm the dwarfing phenomenon observed in short days and its inheritance in photosensitive cowpea varieties.

MATERIALS AND METHODS

This study was conducted at the International Institute of Tropical Agriculture (IITA) Kano Station, Kano, Nigeria which is located at 12° : 03’ N latitude and 8° : 32’ E longitude. The first part of the study was to confirm the dwarfing effect of short daylength on photosensitive cowpeas and the second part dealt with the inheritance of this phenomenon.

Dwarfing due to short days. Two photosensitive cowpea varieties (Kanannado and IAR 1696) and two photoinsensitive cowpea varieties (IT87D-941-1 and IAR-48) were evaluated in pot culture at four planting dates, 12 February 1993, 16 May 1993, 16 July 1993 and 16 October 1993. These dates were chosen so that the vegetative phase of the plants will coincide with short or long days as well as possible commercial planting dates by the farmers in West Africa, representing both rainy season and dry season crops. At each date, three seeds of each variety were sown in 25 cm plastic pots filled with a 90:10 mixture of soil and farmyard manure. Each variety was planted in six pots constituting six replications. The pots were arranged in a completely randomised design on table tops in the screenhouse. After germination, thinning was done to maintain one plant per pot. Pots were regularly watered and kept weed free. Plants were sprayed with Sherpa plus at 1 lt ha^-1 to protect them against insects. Data recorded were days from sowing to first flower, days from sowing to first pod maturity, and plant height at first pod maturity. Plant height was measured from soil level to the growing tip on the main stem. Data were subjected to Analysis of Variance using MSTATC programme.

Inheritance study. The photosensitive variety, Kanannado, was crossed with the photoinsensitive early maturing variety, IT87D-941-1 and sufficient F₁, F₂ and backcross seeds were obtained between July 1993 and October 1994. Fifty seeds of each of the two parents, 25 F₁ seeds, 20 seeds of each of the two backcrosses and 150 F₂ seeds were sown on 18 November 1994 in 19 cm plastic pots in the screenhouse when the daylength became less than 12.5 h. One seed was sown in each pot. However, not all the seeds germinated. The pots were arranged in a completely randomised design. The plants were regularly protected against pre-flowering and post-flowering insects using Sherpa plus at 1 lt ha^-1. Data were recorded on days from sowing to first flower together with plant height at first flower, for each plant. Plant height was measured from soil level to the tallest growing tip on the main stem. Differences between tall and dwarf plants were quite pronounced and therefore, classification based on plant height was easy. However, genetic classification based on days to flower was difficult because differences with respect to days to first flower were not clear among the two parents and the overall range in F₂ was also not great. This was due to the fact that photosensitive plants flowered earlier due to short days and photoinsensitive plants were not affected by daylength and flowered early. Therefore, genetic classification was restricted to growth habit only. The numbers of tall and dwarf plants in F₁, backcross and F₂ populations were recorded and tested against expected genetic ratios using Chi-square. All the dwarf segregates were progeny tested in the field under long day condition in 1995 to check whether they all represent photosensitive class as expected.

RESULTS AND DISCUSSION

Significant varietal differences with respect to growth and reproductive development were observed at different dates of planting. The effect of dates and genotype x date interactions were also significant.

Days taken to first flowering. The number of days taken to first flower of different varieties at different dates of planting is presented in Table 1. The mean number of days to flowering was 46.2 for February, 80.0 for May, 52.1 for July and 50.0 for October plantings. However, there was significant variety x planting date interaction which affected the means for different dates of planting. Thus, even though the mean number of days to flowering for February, July and October are similar, the values for the photosensitive and photoinsensitive varieties differed markedly within and between dates (Table 1). In general, photosensitive varieties flowered later in longer days (May and July plantings, 117.5 d and 65.1 d, respectively) compared to shorter days (February and October plantings i.e., 45.7 d and 51.6 d, respectively). The corresponding values for the photoinsensitive varieties were 42.6 and 39.1 d for May and July plantings, and 46.6 and 49.0 days for February and October plantings, respectively.

Days to first pod maturity. The mean time to first pod maturity differed significantly between sowing dates and between varieties. The variety x sowing date interaction was also significant (Table 2) and therefore, the mean days to first pod maturity for different dates of planting were not a true reflection of the effect of dates. For example, even though the mean maturity period for May planting was maximum, this was due more to delayed maturity of photosensitive varieties and not so much due to photoinsensitive varieties. In general, photosensitive varieties matured earlier than photoinsensitive ones in short day sowings and later in long day sowings (Table 2). The mean days to pod maturity for the photosensitive varieties in the short day plantings of February and October were 63.5 days and 79.7 days, respectively and the corresponding values for the same period in the photoinsensitive varieties were 72.5 days and 74.0 days, respectively. The photosensitive varieties matured in 131.0 and 78 days in the long day sowings of May and July, respectively. The photoinsensitive varieties on the other hand took 60.5 and 58.5 days for the same sowing dates.

Plant height. Planting dates had significant effect on plant height and May planting gave the tallest plants. However, the effect of sowing date on plant height varied with varieties, the photosensitive varieties being more affected than photoinsensitive varieties (Table 3). The average plant height of photosensitive varieties for May and July plantings were 142.0 and 89.0 cm, respectively, compared to 31.5 and 26.0 cm for the photoinsensitive varieties (Fig. 1A, 1B, 1C). In short day sowings, the photosensitive varieties became extremely dwarf whereas the photosensitive varieties grew normally (Fig. 1D, 1E, 1F, 1G). The mean heights in the February and October sowings were 20.5 and 15.0 cm for photosensitive and 39.0 and 24.5 cm for photoinsensitive varieties, respectively (Table 3). The decrease in plant height due to change of sowing date from May (long day) to February (short day ) is more than 400% (23 cm versus 141 cm in Kanannado). Generally plant height in photoinsensitive varieties was less affected by sowing date (Fig. 1D, 1G). Extreme dwarfing of the photosensitive varieties occurred in the February and October sowings when daylength was shorter than 12.5 h. The effect of temperature was ruled out by the results of an experiment conducted concurrently by growing selected photosensitive and photoinsensitive varieties under short day condition (February 4 planting) with ambient temperatures (33.8/17.5 °C) in the screenhouse and under heated conditions (37.4/22.1 °C), in a glasshouse. The dwarfing effect of the shortday sowing date (4 February) on the photosensitive varieties was not significantly different in the heated glasshouse from that of the relatively cooler ambient temperatures in the screenhouse. This indicates that the phenomenon of dwarfing exhibited in the photosensitive varieties is due to the effect of the short days. Also, since this response is common to all photosensitive varieties, it must be related to differential expression of photosensitive and photoinsensitive gene(s) during short and long days.

Inheritance of shortday induced dwarfing. The segregation patterns for dwarfing in different populations derived from the cross between a photoinsensitive variety, IT87D-941-1 and photosensitive variety Kanannado grown under shortday condition are presented in Table 4. All the 12 plants of Kanannado were dwarf while all the 14 plants of IT87D-941-1 were tall. The mean plant height of Kanannado was 11.9 cm while the mean plant height of the variety IT87D-941-1 was 28.6 cm. The F₁ plants were as tall as the tall parent, with a mean plant height of 26.6 cm indicating complete dominance of photo-insensitivity in respect of growth in short days. The F₂ population segregated into 95 tall to 33 dwarf plants giving a close fit to a 3 tall : 1 dwarf ratio. The backcross F₁ plants involving Kanannado segregated into 2 tall to 4 dwarf which fits closely to 1:1 ratio, even though population size is small. All the 6 plants of the backcross involving IT87D-941-1 were tall with mean plant height of 26.9 cm. Thus, even though the backcross population size is small the results show that the dwarfing of photosensitive genotypes grown under short day is controlled by a single recessive gene pair.

In order to test whether the same gene governing photoperiod sensitivity for flowering is responsible for the control of dwarfing under short day conditions, seeds from all the dwarf F₂ plants were planted in the field in 1995 in long day conditions. All the progenies had profuse growth like the photosensitive parent Kanannado and flowered when the days became shorter. This suggests that the photosensitivity and dwarfing due to short days may be a pleiotropic effect of the same gene, which is being designated ‘psps’ (photosensitive).

The earlier studies on inheritance of photosensitivity in cowpea in cowpea were made by growing the segregating populations under long day conditions (Sene, 1967) and the results indicated a dominant gene inheritance. This may be due to late flowering of photosensitive segregates and difficulty in separating them from the medium/late photosensitive segregates, giving an appearance of dominant inheritance. The evaluation of segregating populations in the short day conditions makes the classification more discreet due to extreme reduction in height of photosensitive plants giving a clear monogenic recessive inheritance. It is earlier believed that in cowpea, daylength affect only the rate of reproductive development. However, this study show that short daylength not only causes early flowering in photosensitive cowpea varieties but it also induces dwarfing.

The simple inheritance and extreme dwarfing of the photosensitive plants under shortday conditions makes it a useful marker for selection of photosensitive plants in the dry season (short days) because during the main rainy season (long days), photosensitive genotypes grow profusely (Fig. 1A) and must be planted in very wide spacing to permit genetic studies or individual plant selection.

The seeds of parental material used in this study are being maintained at IITA Kano Station and are available to researchers on request.

REFERENCES

1. Craufurd, P.Q., Aiming, Q., Ellis, R.H., Summerfield, R.J. and Roberts, E.H. 1996a. Development in cowpea (Vigna unguiculata). II. Effect of temperature and saturation deficit on time to flowering in photoperiod-insensitive varieties. Experimental Agriculture 32:13-28.
2. Craufurd, P.Q., Aiming, Q., Summerfield, R.J., Ellis, R.H. and Roberts, E.H. 1996b. Development in cowpea (Vigna unguiculata). III. Effects of temperature and photoperiod on time to flowering in photoperiod-sensitive genotypes and screening for photothermal responses. Experimental Agriculture 32:29-40.
3. Hadley, P., Roberts, E.H., Summerfield, R.J. and Minchin, F.R. 1983. A quantitative model of reproductive development in cowpea [Vigna unguiculata (L.) Walp] in relation to photoperiod and temperature and implications for screening germplasm. Annals of Botany 51: 531-543.
4. Ishiyaku, M.F. 1997. Inheritance of time to flowering in cowpea [Vigna unguiculata (L.) Walp.]. Ph D Thesis, University of Reading, U.K.
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7. Sene, O. 1967. Determinisme genetique de la precocite’ chez Vigna unguiculata (L.) Walp. Agronomie Tropicale 22:309-318.
8. Singh, B.B. 1994. Breeding suitable cowpea varieties for West and Central African Savannah. In: Progress in Food Grain Research and Production in Semi-Arid Africa. Menyonga, J. M., Bezuneh, T.B.,Yayock, J. Y. and Soumana, I. (Eds.), pp. 77-85. OAU/STRC-SAFGRAD, Ouagadougou, Burkina Faso.
9. Singh, B.B., Chambliss, O. L. and Sharma, B. 1997. Advances in cowpea breeding. In: Advances in Cowpea Research. Singh, B.B., Mohan Raj, D.R., Dashiell, K.E. and Jackai, L.E.N. (Eds.), pp. 30-49. International Institute of Tropical Agriculture and Japan International Research Center for Agricultural Sciences. IITA, Ibadan, Nigeria.
10. Tarawali, S.A., Singh, B.B., Peters, M. and Blade, S.F. 1997. Cowpea haulms as fodder. In: Advances in Cowpea Research. Singh, B.B., Mohan Raj, D.R., Dashiell, K.E. and Jackai, L.E.N. (Eds.), pp. 313-325. International Institute of Tropical Agriculture and Japan International Research Center for Agricultural Sciences. IITA, Ibadan, Nigeria.

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