Editorial: Research and development for yield of pearl millet

Field Crops Research, 11 ( 1 9 8 5 ) 1 1 3 - - 1 2 1

113

Elsevier Science P u b l i s h e r s B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

EDITORIAL: R E S E A R C H AND DEVELOPMENT F O R YIELD OF P E A R L MILLET

C R A I G J. P E A R S O N

School of Agriculture, University o f Western Australia, Nedlands, W.A. 6009 (Australia)

This issue of Field Crops Research reflects the current state of research on pearl millet. We are fortunate to have an introductory paper b y Dr Glen Burton of Georgia, U.S.A., who has worked for over 40 years towards improving, and drawing attention to the potential of, the crop (Burton, 1985). His paper is followed b y articles reporting independent and uncoordinated work from laboratories in Africa, Australia, India, Great Britain, U.S.A. and U.S.S.R. The lack of coordination is inevitable, b u t no less regrettable, in a world which cherishes "academic freedom". Nonetheless, it i~ good that there are now so many laboratories in temperate areas working on a crop which is grown principally in the tropics. It is also encouraging that these laboratories are concerned with research for the purpose of yield improvement: as pointed o u t elsewhere (Pearson, 1984), the current, high level of research in breeding, plant protection and physiology follows a period in the 1960s when there was little physiological research into pearl millet. Thus, perhaps because of the paucity of background detail, we are at present avoiding the pitfall of, say, research into maize, in which a high proportion of resources was chaneUed into physiological and biochemical studies which have contributed little to yield improvement. Yield improvement is the primary responsibility o f plant breeders. Their current preoccupations, particularly at ICRISAT, the international centre which has responsibility for yield improvement in pearl millet, are illustrated in the paper by Andrews et al. (1985). Plant breeders w h o are appointed n o w will be leading programs into the next century -- it is only 15 years from n o w -- and they will require wide support if they are to produce a sustained improvement in yield over that time. Support comes from the fields of developmental anatomy, physiology, pathology, nutrition, micrometeorology, soil science and agronomy. Recent research in some of these fields has been reviewed by Ferraris (1973) and Pearson (1984). Here I am constrained by space and expertise; I wish to draw attention only to aspects of eco-physiology and development which I think are basic to further research, breeding and yield improvement. With respect to the eco-physiology of the crop, it is significant that the t w o most likely centres for the origin of this crop are in areas which n o w have distinct wet-and-dry tropical climates (Fig. 1). Further, pearl millet has

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U Fig. 1. The four regions of Africa (hatched) which have been proposed as the centre of origin of pearl millet; of these, the Sahara and sub-Sahara (Sahel) appear to be the most likely (after Brunken et al., 1977). been selected by subsistence farmers for its adaptability to drought and low technology, particularly low inputs of fertilizer. T o d a y the vast bulk o f pearl millet is grown where the annual rainfall ranges between 200 and 800 m m (Fig. 2). Crops are grown at low population densities: 30 000--100 000 plants per ha in India and as low as 5000--10 000 plants per ha in the African Sahel, and without inorganic fertilizers. As a consequence, landraces of pearl millet have been selected, probably unwittingly, for conservative development characteristics: profuse tillering, particularly in the axils of upper leaves, following rain (Raymond, 1968), small heads and small individual seeds (e.g. Pearson and Coaldrake, 1983). Furthermore the seed attains viability even if it is incompletely filled (Fussell and Pearson, 1980), as would occur if grain filling were s~opped by drought at the end of the growing season in the wet-and-dry tropics. These ecological characteristics can be screened out and discarded, as was done when high-yielding dwarf sorghums were created for temperate environments (Quinby, 1974). However, I suggest that for the present decade, when there is so much famine in Africa, we should recognize the value of these ecological characteristics and try to improve t h e m to produce pearl

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Fig. 2. Area planted to pearl millet in 1978 in relation to m e a n annual rainfall ( m m ) in Africa and S o u t h Asia. Each d o t equals 20 000 ha (after Bidinger et al., 1981).

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millets which will b e m o r e reliable and higher yielding in t h e i r t r a d i t i o n a l role as the low i n p u t cereal o n the d r y margin o f s o r g h u m or maize c r o p p i n g regions ( N o r m a n et al., 1 9 8 4 ) . I m p r o v e m e n t in yield m a y b e b r o u g h t a b o u t b y judicious a p p l i c a t i o n o f fertilizer and b y b e t t e r m a n a g e m e n t o f pests and diseases. H o w e v e r , it is unr e a s o n a b l e t o believe t h a t yields in the African sahel will be raised in the foreseeable f u t u r e to t h e yield levels c o m m o n f o r t e m p e r a t e cereals in areas having m o r e reliable rainfall, because o f t h e great c o n s t r a i n t which rainfall and available soil w a t e r place o n c r o p d u r a t i o n in West Africa. F o r e x a m p l e , Etasse ( 1 9 7 7 ) , o n t h e basis o f West African e x p e r i e n c e , w a r n e d t h a t fertilizat i o n in excess o f 60 kg N h a -1 w o u l d r e d u c e yields b y p r o m o t i n g vegetative TABLE 1 Response of rainfed pearl millet to different levels of nitrogen and phosphorus over 3 years at Samaru, Nigeria Fertiliser

Rate(kg hg ~)

Grain yield(kg h~ l)

N

0 5O 100 0 66 132

1398 2150 1940 1285 2125 1792

P

Source: Norman et al. (1984), based on Egharevba (1978). 600 I A B ¢oo ..Q cu

"E D

._ L~

200

I

I

I

50 100 150 Planf poputafion (thousands ha-1)

I

200

Fig. 3. Grain yield of four varieties (A: Souna III, B: 47--66, C: IBV8004 and D: IVS 5454) showing low yields and extended yield plateau over a range of plant densities,

in Senegal, West Africa (after L.K. Fussell, pets. commun., 1984).

117 growth but depressing the harvest index of crops which run out of water towards the end of their life. Yield depression in rainfed millet at high levels of nitrogen and phosphorus is shown in Table 1. Likewise, profuse tillering and low harvest indices result in a very broad plateau of grain yield across populations under Sahelian conditions (Fig. 3), so the answer to yield improvement will not be simply to increase plant population. In the longer term, improvement in yield and greater stability of yield will be brought about by selecting genotypes which have developmental patterns optimized for particular environments. Aspects of development which will be important in identifying such "ideotypes" are time of, or size of plant at, floral initiation; tillering; synchrony of initiation and flowering on tillers; stem elongation; and panicle size (number of grain sites) in relation to size of the vegetative parts of the plant. Ong and Monteith (1985) and Pearson (1984) review patterns of development and growth; Pearson and Hall (1984) contrast the development of pearl millet with that of maize. The patterns are both constrained by, and develop around, the progression of the mainstem through juvenility, floral initiation, panicle differentiation and grain filling. Sustained yield improvement will depend on our understanding the control of these developmental events in the mainstem and how the pearl millet plant uniquely regulates its development of basal and aerial tillers. Temperature has an overriding influence on the rate of development of the mainstem (e.g. Pearson, 1975; Fussell et al., 1980; Garcia-Huidobro et al., 1982; Ong, 1983a, 1983b). However, this influence may be seldom exercised in the tropics where the bulk of the crop is grown: at ICRISAT (18°N), temperatures during the main (monsoon or Kharif) season range only from 28 to 35°C during the day and from 22 to 24°C at night, without much seasonal trend (Carberry et al., 1985). It is thus appropriate to consider how the plant might internally control its development. It is plausible that development is controlled by hormone fluxes or balances. However, our knowledge of hormones is generally empirical rather than predictive (Letham et al., 1978). In pearl millet is has advanced to the predictive level only with respect to abscissic acid and drought, due to Henson and coworkers (cf. Henson et al., 1981). A "physical" rather than "hormonal" explanation of control of development relates to the size of the apex of the plant. Photographs of the developing apex have been published by Powers et al. (1980) and Belliard and Pernes (1984). The size of the apical dome increases as each leaf is initiated. Thus, the size of the apex depends on, or at least is highly correlated with, the number of leaves produced on that axis, where the rate of leaf production is sensitive to environment, e.g. temperature (Pearson, 1975; Ong, 1983a) and nitrogen (Coaldrake and Pearson, 1985a). In some varieties the height and diameter of the apical dome and of the apex (the region down to the axil of the youngest leaf which extends beyond the apex; Fig. 4) are the same for meristems which have initiated the same number of leaves, even

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BASE OF APICAL DOME

A

/

BASE OF APEX

Fig. 4. Diagram of a median longitudinal section of a shoot tip showing the apical dome and apex of pearl millet. The line at (1) represents the position where Theodorides (1981) measured the diameter at the base o f the apical dome and at (2) the diameter at the base of the apex; (D) and (A) represent heights of apical dome and apex respectively, used for calculating the volumes o f these regions (Table 2).

though they were grown at temperatures ranging from near-optimum (30/25°C) to near-lethal (18/13°C day/night) (Table 2, from Theodorides, 1981). Consequently, in these cultivars, the volume of the apical dome is the same at any leaf number irrespective of temperature or nitrogen supply. In another cultivar, cv. Ingrid Pearl, Theodorides (1981) found differences in apex size at different temperatures. Floral initiation occurs after a particular number of leaves has been proTABLE 2 Dimensions of the apex of cv. MX001 grown at 30/25 or 18/13°C (16/8h day/night) at an irradiance of 600 uE m -2 s-1. Measurements were made when six leaves were visible, at 18 and 44 days after sowing at 30/25 and 18/13°C respectively.

Diameters (#m) at base of apical dome a at base of apex b Height (~m) of apical dome (D) of apex (A) Volume (ram 3 × 103) of apical dome of apex Volume (mm 3) of shoot tip (above leaf 9) Source: Theodorides (1981). a(1) in Fig. 4. b(2) in Fig. 4.

30/25°C

18/13°C

LSD (P = 0.05)

65 180

58 230

7.8 30.1

31 170

28 175

7.6 16.5

0.089 0.773

0.053 0.892

0.023 0.227

18.54

17.92

4.668

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duced, this number being constant for one variety irrespective of temperature or nitrogen supply (Ong, 1983b; Coaldrake and Pearson, 1985a) although we may presume that this number varies with daylength. It is therefore plausible that floral initiation occurs when the apical d o m e of the mainstem reaches a certain critical size. Further, the timing of differentiation of the panicle b e t w e e n floral initiation and production of the last spikelet can be interpreted as being controlled by, or at least correlated with, the size of the apex (Coaldrake and Pearson, 1985b). Thus the critical sizes of the apex for floral initiation and panicle differentiation may be relatively constant for one variety in a range of environments. On the other hand, variation in critical sizes b e t w e e n cultivars and cultivar X daylength interactions may explain genetic and locational differences in developmental patterns. Such speculations regarding apex size and development may, I hope, lead to research aimed at providing a solid foundation for a theory of flowering and yield determination. Such a theory, coupled with current understanding of germination, leaf appearance and dry weight accumulation (Ong and Monteith, 1985) will allow us to predict mechanistically the developmental patterns, dry weights and yields of crops in various environments. It can be concluded that pearl millet is a crop which is grown widely in the less favourable areas of the wet-and-dry tropics under low inputs (low population, fertilizer and weed and pest control). Its developmental pattern, which we are beginning to understand in some detail, generally results in low grain yield (see Fig. 3) and a low but stable harvest index. The harvest index is often as low as 0.15 (Singh, 1976) although it is sometimes as high as 0.4 (Fig. 10 in Ong and Monteith, 1985). Low yield and low harvest index are offset by a relatively high stability of yield. For example, Pearson and Coaldrake (1983) found yields of one genotype under rainfed conditions (where rainfall during the growing season ranged from 122 to 225 mm at eight sites) were between 0.75 and 1.8 times the yield under irrigation. In the environments and cropping systems in which pearl millet is most widely grown, yield stability may be more important than yield itself. The challenge for field crops research in the next generation is to understand the crop sufficiently that we can retain yield stability while breeding and altering agronomic practices to obtain higher grain yields. ACKNOWLEDGEMENT

I am grateful to Dr Fussell from Niger and Dr Theodorides from Cyprus for permission to use their data in Fig. 3 and Table 2.

REFERENCES Andrews, D.J., King, S.B., Witcombe, J.R., Singh, S.D., Rai, K.N., Thakur, R.P., Talukdar, B.S., Chavan, S.B. and Singh, P., 1985. Breeding for disease resistance and yield in pearl millet. Field Crops Res., 11 : 241--258.

120 Belliard, J. and Pernes, J., 1984. Pennisetum typhoides (Burro.) Stapf et Hubb. In: A.H. Halevy (Editor), Handbook of Flowering, Vol. 4. Commercial Rubber Company Press, Boca Raton, FL. Bidinger, F.R., Mahalakshmi, V., Talukdar, B.S. and Alagarswamy, G., 1981. Improvement of drought resistance in pearl millet. ICRISAT Conf. Pap. 44. Int. Crops Res. Inst. Semi-Arid Trop., Hyderbad. Brunken, J., De Wet, J.M.J. and Harlan, J.R., 1977. Morphology and domestication of pearl millet. Econ. Bot., 31: 163--174. Burton, G.W., 1985. Collection, evaluation and storage of pearl millet germplasm. Field Crops Res., 11: 121--127. Carberry, P.S., Campbell, L.C. and Bidinger, F.R., 1985. The growth and development of pearl millet as affected by plant population. Field Crops Res., 11: 193--205. Coaldrake, P.D. and Pearson, C.J., 1985a. Development and dry weight accumulation of pearl millet as affected by nitrogen supply. Field Crops Res., 11 : 171--184. Coaldrake, P.D. and Pearson, C.J., 1985b. Panicle differentiation and spikelet number related to size of panicle in Pennisetum americanum. J. Exp. Bot., 36: 833--840. Egharevba, P.N., 1978. A review of millet work at the Institute of Agricultural Research, Samaru. Samaru Misc. Pap. 77, 17 pp. Etasse, C., 1977. Sorghum and pearl millet. In: C.L.A. Leakey and J.B. Wills (Editors), F o o d Crops of The Lowland Tropics. Oxford University Press, London, pp. 27--29. Ferraris, R., 1973. Pearl millet (Penniseturn typhoides). Commonwealth Bureau of Pastures and Field Crops Review Series 1/9173. Commonw. Agric. Bur., Farnham Royal, 70 pp. Fussell, L.K. and Pearson, C.J., 1980. Effects of grain development and thermal history on grain maturation and seed vigour of Penniseturn americanum. J. Exp. Bot., 31: 621-633. Fussell, L.K., Pearson, C.J. and Norman, M.J.T., 1980. Effect of temperature during various growth stages on grain development and yield of Pennisetum arnericanum. J. Exp. Bot., 31: 621--633. Garcia-Huidobro, J., Monteith, J.L. and Squire, G.R., 1982. Time, temperature and germination of pearl millet (Pennisetum typhoides S. & H.). J. Exp. Bot., 33: 288--296. Henson, I.E., Mahalakshmi, V., Bidinger, F.R. and Alagarswamy, G., 1981. Genotypic variation in pearl millet (Pennisetum americanum (L.) Leeke) in the ability to accumulate abseissic acid in response to water stress. J. Exp. Bot., 32: 899--910. Letham, D.S., Goodwin, P.B. and Higgins, T.J.V. (Editors), 1978. Phytohormones and Related Compounds: A Comprehensive Treatise. 2 Vols. Elsevier, Amsterdam. Norman, M.J.T., Pearson, C.J. and Searle, P.G.E., 1984. The Ecology of Tropical F o o d Crops. Cambridge University Press, Cambridge. Ong, C.K., 1983a. Response to temperature in a stand of pearl millet (Pennisetum typhoides S. & H.). 1. Vegetative development. J. Exp. Bot., 34: 322--336. Ong, C.K., 1983b. Response to temperature in a stand of pearl millet (Pennisetum typhoides S. & H.). 2. Reproductive development. J. Exp. Bot., 34: 337--348. Ong, C.K. and Monteith, J.L., 1985. Response of pearl millet to light and temperature. Field Crops Res., 11: 139--158. Pearson, C.J., 1975. Thermal adaptation of Pennisetum: seedling development. Aust. J. Plant Physiol., 2: 413--424. Pearson, C.J., 1984. Physiology of Pennisetum millets. In: P.R. Goldsworthy and N.M. Fischer (Editors), Physiology of Tropical Field Crops. John Wiley, London, pp. 281-304. Pearson, C.J. and Coaldrake, P.D., 1983. Pennisetum americanum as a grain crop in Eastern Australia. Field Crops Res., 7 : 265--282. Pearson, C.J. and Hall, A.J., 1984. Maize and pearl millet. In: C.J. Pearson (Editor), Control of Crop Productivity. Academic Press, Sydney, N.S.W., pp. 141--158.

121 Powers, D., Kanemasu, E.T., Singh, P. and Kreitner, G., 1980. Floral development of pearl millet (Pennisetum americanum (L.) K. Schum). Field Crops Res., 3: 245--265. Quinby, J.R., 1974. Sorghum Improvement and the Genetics of Growth. Texas A & M University Press, College Station, TX, 108 pp. Raymond, C., 1968. Pour une meilleure connaissance de la croissance et du developpement des mils Pennisetum. Agron. Trop. (Paris), 23: 844--863. Singh, R.P., 1976. Fertilizer use in dryland crops: effect of levels and methods of nitrogen application on the yield and yield attributes of Kharif cereals. Indian J. Agron., 21 : 27--32. Theodorides, T.N., 1981. Effect of temperature on nitrogen metabolism of pearl millet (Pennisetum americanum (L.) Leeke). PhD Thesis, University of Sydney, Sydney, N.S.W., 202 pp.