Print version ISSN 0103-8478
Cienc. Rural vol.26 no.3 Santa Maria Dec. 1996
PERFORMACE AGRONÔMICA DE GENÓTIPOS MACHO-ESTÉREIS E FÉRTEIS DE MILHO EM DUAS DENSIDADES DE SEMEADURA
This experiment was conducted in 1994 at Ames, Iowa, US to test whether cytoplasmic male-sterility can be used to decrease barrenness and to increase grain yield of maize at two plant populations. Four genotypes were tested: a hybrid (NK 6330) and an inbred, wifh sterile and fertile counterparts. Each genotype owas evaluated at plant populations equivalent to 25,000 and 75,000 pl. ha-1. Hybrids produced higher grain yield than inbreds at both plant populations. Gram yield was higher at 75,000 than at 25,000 pl. ha-1. No difference in gram yield, number of ears per plant, number of grains per ear, tassel length, and tassel number of branches was found between sterile and fertile counterparts of the inbred and hybrid, regardless of plant population. Fertile genotypes bore heavier tassels at anthesis than their sterile counterparts. Adequate precipitation distribution and high fertility level in the soil probably decreased competition between tassel and ears, mitigating potential yield benefits of suppressing genetically pollen production.
Key words: Zea mays, male-sterility, stress, gram yield.
Este experimento foi conduzido em Ames, Iowa, US durante o ano agrícola de 1994, tendo como objetivo avaliar se a macho-esterelidade genético-citoplasmática pode ser utilizada para aumentar o rendimento de grãos de milho em diferentes populações de planta. Quatro genótipos foram utilizados: um híbrido (NK 6330) e uma linhagem, ambos em suas versões fértil e macho-estéril. Cada genótipo foi avaliado em duas densidades de semeadura, equivalentes a 25.000 e 75.000 pl.ha-1. Os híbridos produziram maiores rendimentos de grão do que as linhagens nas duas populações utilizadas. O rendimento de grãos por área foi maior em 75.000 do que em 25.000 pl.ha-1. Nenhuma diferença significativa em termos de rendimento de grãos, número de espigas por planta, número de grãos por espiga, comprimento e número de ramos do pendão, foi observada entre genótipos férteis e macho-estéreis, independentemente da população de plantas. As versões férteis apresentaram pendões mais pesados do que os genótipos macho-estéreis. A adequada distribuição da precipitação e a alta fertilidade do solo possivelmente diminuíram a competição entre o pendão e as espigas, minimizando os benefícios da supressão genética da produção de pólen sobre o rendimento de grãos.
Palavras-chave: Zea mays, macho-esterilidade, estresse, rendimento de grãos.
Maize is a unique species among the economically important grasses due to its monoecious floral organization and protandrous partem of development. The male inflorescence (tassel), which is derived from the shoot apical meristem, is differentiated first and has developmental priority over the ears, located at the tip of axiliary branches (CHENG & PAREDY, 1994).
The tassel can dominate the ears and thus limit grain yield by three different mechanisms: (1) shading of the upper leaves (DUNCAN et al., 1967; HUNTER et al., 1969); (2) acting as a competitive sink (ANDERSON, 1972) and; (3) modifying the supply of growth regulators (specially auxins) and CO2 acceptors (SEYEDIN et al., 1980).
The degree of competition between tassel and ear development is highly related to the plant's environment. Under favorable conditions (water, light and nutrients) there is less competition between the male and female inflorescences. Under less favorable conditions, particularly in dense plantings or with drought stress, apical dominance is increased and ear development decreased. The net result of this protandrous pattem of development is an increase in barrenness and a decrease in grain yield.
A potential tool to decrease tassel dominance over the ears, and to increase maize tolerance to stresses at flowering, is male-sterility. Cytoplasmic male-sterility was used most extensively during the 50s and 60s to eliminate detasseling in the production of com hybrid seed. The Texas male-sterile cytoplasm (TMS) was carried in female inbreds, while a nuclear gene located in the male parent restored pollen fertility of the hybrid seed.
In addition to preventing pollen production, reports of CHINWUBA et al. (1961), SCHWANKE (1965) and MEYER (1970), indicated that TMS affected other agronomic characteristics of singie cross hybrids and seemed to reduce the detrimental effects of high plant densities on grain yield, raising the optimum stand level for maximum productivity of a genotype. A possible explanation for that behavior is that cytoplasmic male-sterile cultivars have small tassels that intercept less solar radiation, require less input of energy in their formation, and produce a lower amount of auxins.
The reports showing that male-sterility might be a way to reduce barrenness, especially under stressful conditions, generated interest in using cytoplasmic effects per se, not only to facilitate production of hybrid seeds but also to promote higher commercial grain yield. The following production system involving male-sterility was envisioned in the early 70s: the farmer would grow a blend of 95% sterile pius 5% fertile seeds of a hybrid at a population 20% higher than normal. This system would increase grain yield by 10% for the average hybrid. Moreover, it would provide the farmer a hybrid to resist stresses better than the fertile version at the lower population. However, the utilization of TMS was suddenly interrupted in 1973 due to development of the Helminthosporium maydis epidemic, which particularly affected the materiais containing the T-cytoplasm. After that, hybrid seed companies substantially decreased use of cytoplasmic male-sterility in their breeding programs, and the idea of utilizing that mechanism as a tool to decrease barrenness and improve population tolerance was abandoned.
Several other sources of male-sterility have been developed since the blight of 1973. There are also a number of new technologies in various stages of development which can achieve male-sterility through non-traditional means, including chemical sprays and genetic transformation. In addition, com hybrids have been considerably improved in the last 30 years. Hybrids developed in the 80s and 90s show a greater tolerance to barrenness than materiais grown in the 60s (RUSSELL, 1991).
Given lack of information about effects of male-sterility on agronomic traits of current genotypes, this experiment was carried out to test whether genetic suppression of pollen production would increase grain yield and decrease barrenness at different plant populations.
MATERIALS AND METHODS
The study was conducted during 1994 in Ames, Iowa, US. Study site soil was a Nicollet loam (Fine-loamy, mixed, mesic Aquic Hapludoll). The experimental design was a split piot with four replications of each treatment. The main plot consisted of four different genotypes: the hybrid Northrup King (NK) 6330, in sterile and fertile versions, and the inbred used as the female parent of NK 6330, in sterile and fertile versions. The source of male-sterility was the S cytoplasm. Each genotype was evaluated at two plant populations in the split plot: 25,000 and 75,000 pl. ha-1. Each split piot consisted of three rows, spaced 0.75 m equidistantly. Individual plot rows were l0m long.
The experiments was hand-planted on May 3. Strings containing marks at appropriate distances were used to assure the planting pattem desired for each plant density tested in the experiment. Three seeds were dropped per hill. Two weeks after emergence, when plants were at stage V4, thinning was performed to adjust population to desired levels. Plots were also hand-hoed and wheel-hoed to control post-emergence weed competition.
Plant evaluations were performed in the central row of each split plot. Tassel dry matter was analyzed at two phenological stages, at V 17 and when at least 50% of plants in fertile plots were shedding pollen. Four tassels were randomly picked for each stage. Tassels were placed in a dryer at a temperature of 65°C for 72 hours. Tassels collected during anthesis were used to determine tassel length and number of branches before they were dried.
Plant and ear insertion heights were determined at anthesis by taking tive plants at random and measuring the distance from the base of the stem to the tip of the tassel and the point of insertion of the lowest fully developed ear on the stalk. The total number of leaves per plant was determined at anthesis using four plants randomly chosen. Intemode length was evaluated indirectly by taking average plant height, subtracting tassel length, and dividing the result by total number of leaves produced per plant. Harvesting was performed on September 28, after leaves had senesced entirely. Ears were placed in a dryer at a temperature of 70° C, dehusked and shelled to determine grain weight, grain yield and yield components.
Daily meteorological data were collected, using an automated weather station (Campbell Scientific Corp., Logan, UT). Meteorological instrumentation was located approximately 4km from the experimental field. Values of temperature and precipitation were expressed as an average of each two-week period from May to September.
Analysis of variance was performed using the General Linear Models procedure of the Statistical Analysis System (SÃS). F values for main treatment effects and their interactions were considered significam at the 0.05 level. Whenever a particular factor or interaction of factors signifícantly influenced a variable, means were separated using Fischer's LSD test at the 0.05 probability level, following methodology presented by LITTLE & HILLS (1978).
The growing season had two distinct thermal trends when compared to normal (Table 1). During the most critical period for grain yield determination (from June 28 to August 22), the mean temperature was 1.8°C lower than normal. On the other hand, from August 22 to the end of the growing season, the growing season was warmer than average. The overall precipitation was slightly below average.
Grain yield and number of ears per plant were significantly affected by cultivar and plant population. Hybrids produced higher yield per área than the inbreds (Table 2). Male-sterile cytoplasm did not promote signifícant increment in yield relative to fertile counterparts (Table 2). Grain yield per hectare was 67% greater at 75,000pl. ha-1 than at 25,000pl. ha-1 (Table 3). Hybrids produced more ears per plant and grains per ear than inbreds, which contributed to their higher grain yield per área (Table 2). Cytoplasmatic male sterility had no effect on those variables. All genotypes had greater number of ears per plant at the lower plant population (Table 3).
Hybrids had heavier grains than inbreds, regardless of plant population tested (Table 4). Introduction of a sterile cytoplasm did not signifícantly change the weight of l,000 grains for the hybrid. Lack of viable pollen production in the inbred resulted in heavier grains, particularly at the higher plant population. The sterile version of the inbred was able to tolerate the increase in plant population without significantly decreasing the weight of 1,000 kernels, something that did not occur with its fertile counterpart.
Male sterile cytoplasm did not impact tassel dry matter at V 17 (Table 5). However, fertile versions of hybrid and inbred had tassels significantly heavier at anthesis than their sterile counterparts. Tassel dry matter of the fertile inbred was 33% higher at anthesis than at V 17. In the case of the sterile version, the increment was only 11%. The largest weight gain occurred with the fertile hybrid, whose tassels at anthesis were 77% heavier than at V 17. The fertile hybrid also showed the largest absolute values of tassel dry matter. Cultivar average tassel dry matter was significantly lower at 75,000 pl.ha-1 than at 25,000 pl.ha-1 (Table 3). Averaged across plant densities, hybrids had signifícantly larger tassels than inbreds (Table 5). Male sterility did not affect tassel length. Hybrid plants were taller and had ears at higher positions than inbreds (Table 6). Male sterility contributed to decrease plant height for the hybrid but it did not affect this variable for the inbred. There was no signifícant effect of male sterility on height of ear insertion for both inbred and hybrid. Higher population density promoted increase in plant and ear insertion height of the genotypes (Table 3). The final number of expanded leaves ranged between 19.2 and 19.8, depending on the cultivar (Table 6). The fertile version of the hybrid produced more leaves than its sterile counterpart. In contrast, no effect of male sterility on the number of expanded leaves of the inbred was observed.
Hybrids had longer intemodes than inbreds explaining their greater plant height (Table 6). No negative effect of male sterility on average intemode length of either cultivar was noted. In fact, the opposite trend was observed for the inbred, with the sterile version having longer intemodes than its fertile counterpart.
Contrary to lhe results of CHINWUBA et al (1961), SCHWANKE (1965) and MEYER (1970), cytoplasmic male-sterility did not promote signifícant improvements in grain yield or yield components, regardless of genotype and plant density (Table 2). The work done by those authors has indicated that this mechanism might be used to reduce the size of the tassel, decreasing apical dominance and improving grain yield, particularly at high plant populations. In the present report, the male-steriles did have lower dry matter tassels at anthesis when compared to fertile counterparts (Table 5). However, this was not sufficient to promote an increase in yield, even at 75,000pl. ha-1.
A combination of lack of water stress, fertile soil, and density-tolerant cultivars with small fertile tassels, probably contributed to lessen potential increments in yield that might have been promoted by cytoplasmic male-sterility. Competition between fertile tassels and developing ears is usually higher under conditions of high temperature, water deficit and low soil fertility. None of these three factors was signifícant during the trial.
Benefits from preventing pollen production in terms of decreasing barrenness at high plant densities have been greater for genotypes that produce large fertile tassels because there is a positive correlation between tassel size and silking delay (MOCK & BURREN, 1972; BURREN et al., 1974). The values of tassel dry matter and number of branches at anthesis observed in this experiment are considerably lower than those reported by ANDERSON (1972) and MOCK & SCHUETZ (1974) for genotypes develooed in the early 70s.
Considering the differences in size of the main inflorescence, a hypothesis could be drawn. Another reason for the failure of male-sterility to improve yield was the small size of the tassel of genotypes used in the experiment. Plants with small tassels require less photosynthate to develop the male inflorescence (ANDERSON, 1972), have fewer problems with apical dominance (SEYEDIN, 1980) and are potentially less yield responsive to genetic suppresion of pollen production (MEYER, 1970). Most breeding programs used to develop new inbreds and hybrids do not use tassel size per se as a criterion to select future parents (HALLAUER, personal communication - 1995). However, almost ali of them emphasize the ability of the potential progenitors to withstand higher than conventional plant population without showing pronounced barrenness. Since small tassels are usually positive ly correlated with tolerance to high density, it is possible that many commercial hybrids have this characteristic as a side-effect of the selection process that is used during development and evaluation of inbreds.
One of the most consistent effects of male-sterility is a reduction in plant height (MEYER, 1970). This was observed only in the hybrid (Table 6). The smaller tassels and lack of fertile pollen production in male-steriles may have reduced the amount of IAA produced by the shoot apex. The reduction in the amount of auxin being produced in the growing point may have promoted less cell elongation, which would result in an irregular shortening of the upper intemodes of the stalk (SARVELA & GROGAN, 1965). The fertile version of the hybrid had heavier tassels at anthesis in comparison to its sterile counterpart, which may have contributed to the increase in plant height (Table 5 and 6). However, no signifícant influence of male-sterility in decreasing intemode length was observed (Table 6). The apparent contradiction between the impacts of male-sterility on plant height and intemode length may be explained by the way the second variable was estimated. No direct measurement of individual intemode length was made. As SARVELA and GROGAN (1965) pointed out, the smaller plant height of male-stelile genotypes arises from the non-sequential shortening of specifíc intemodes probably caused by a temporary block of the plant hormones regulating cell elongation or cell division.
Cytoplasmic male-sterility did not increase maize grain yield, regardless of genotype or plant population. Male-sterile genotypes produced heavier tassels, higher weight of 1,000 grains and taller plants than their fertile counterparts. No difference was found between fertile and male-sterile counterparts in terms of number of grains per ear, tassel lenght and number of branches.
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1 Engenheiro Agrônomo, PhD., Professor of the Department of Crop Production, Santa Catarina State University, Caixa Postal 281, 88520-000 - Lages, SC, Brazil. Author for corespondence.
2 Engenheiro Agrônomo, PhD., Professor of the Department of Agronomy, Iowa State University.
Recebido para publicação em 19.03.96. Aprovado em 24.07.96