Restructuring maize plant type for higher productivity
--J. K. S. Sachan

Ever since the advent of the modern era in maize cultivation, there always existed a stereotypic image of a maize plant type marked by the presence of one or two cobs in the middle of the plant. Our analyses of prehistoric wild corn vis-a-vis Sikkim Primitive maize (SP) and other primitive and advanced races of maize, including inbreds and local varieties of the Northeastern Himalayan region and the plains of India, effectively emphasize the values of landraces in developing maize cultivars. Identity of landraces or the prehistoric corn is progressively lost during 10,000 years of domestication, population growth and deforestation. Breeders are responsible, to some extent, for the extinction of maize diversity. Inbreds and hybrids with one or two ears in the middle of the plant were preferred to better suit mechanized farming and ease of harvesting by combine harvester. This had a devastating effect on potentialities of this taxon, fondly called corn. With a growing realization of the stagnation in the yield potential of maize, one begins to ponder the importance of wild relatives and landraces. A significant question arises--do we really have such a germplasm that can contribute to a quantum jump in maize yield? The answer is positively 'yes'. The SP maize, a germplasm par excellence, offers advantageous opportunities to the breeder for a sizable leap in yield and productivity. It is worthwhile in this context to mention the salient features of SP maize.

Information obtained from our studies on botanical, C- and Q-banding; pachytene analysis, ethnobotany, and interpretation of the archaeological findings have provided tangible evidence that pre-historic wild corn, which evolved in the extreme desert environment in the Tehuacan valley of Mexico, is well preserved in the form of Sikkim Primitive (SP) in remote and isolated pockets of the Northeastern Himalayan Region (NEH) (Sachan and Sarkar, Indian J. Genet. 46:153-161, 1986). SP maize, which has 10,000 years of history, was evolved under extreme desert conditions. With its unique plant architecture, it offers a golden opportunity to breeders to utilize a reservoir of favourable genes, both for biotic and abiotic stresses as well as several other agronomic traits. There is strong evidence that SP maize plant type conforms to the most competitive and advantageous attributes for survival in wilderness. Reproductive efficiency and the defense mechanisms of SP maize are the fittest attributes to survive against the vicissitudes and vagaries of weather, potential enemies, biotic and abiotic stresses. Studies have revealed that SP maize harbours a gene for drought tolerance (Mani et al., MNL 61:2, 1987). The most important physiological attributes of SP maize are a complete lack of apical dominance, prolificacy (5-9 ears) with uniformity in ear size (Fig. 1); erect leaves for developing maize varieties for high population density, top bearing habit and drooping tassel to ensure effective fertilization (Sachan and Sarkar, MNL 56:121-124, 1982). It stays green after maturity; thus it is also good for fodder purposes. It is resistant to stalk rot and has tremendous stem strength which prevents lodging.

Some of the factors influencing photosynthetic efficiency are the availability of water, carbon dioxide, light, nutrients, temperature, plant age, leaf age and the genetic makeup of the plant. The internal control of photosynthesis in maize is the rate at which photosynthetic products are translocated from subtending leaf and leaves in the upper canopy to the sink (cob). For efficient source-sink relationship, the essential contributing factors are longevity and health of the leaf-canopy and minimal distance between the source and the sink. SP maize, having erect leaves near the top of the canopy, should synthesize food more rapidly than those strains having horizontally oriented leaves. Furthermore, the middle leaves and lower leaves are unable to photosynthesize because of chlorophyll breakdown and loss of functional chloroplasts. Thus SP maize possesses all the required parameters to increase the yield potential of maize.

Among all the contributing factors, the placement of the ear is the most important. Ears must occupy positions in the upper one-third of the plant height rather than the middle so that photosynthate is translocated right from subtending and upper leaves to the sink. Our analysis of the fifty strains of maize (as given earlier in the text) has shown that in modern maize, on an average, there are five internodes between the uppermost ear and the tassel, whereas in SP maize there are only two internodes between peduncle and the uppermost ear. Obviously the modern cultivars have inherent limitations in utilizing the photosynthesized product of the upper canopy.

In view of the above facts a hypothesis can be advanced that if the maize plant can be restructured and harnessed to have an upper bearing habit (rather than the present middle bearing habit) with two to three productive ears, a breakthrough in yield may be obtained. Many strains of the Northeastern Himalayan region possess these characteristics. SP maize specially can play a greater role in maximizing the yield potential of modern maize.

Figure 1. Different collection of Sikkim primitives showing drooping tassel, upper bearing habit, prolificacy and uniformity in ear size (L-R; strains from Kumaon Hills, Tripura, Sikkim, Meghalaya and Nagaland.

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