Three contrasting maize (Zea mays L.) genotypes- DHM-117 (single cross hybrid), Varun (synthetic) and Harsha (composite) with different yield potentials were selected to assess their growth and yield performance at ambient (390ppm) and elevated (550ppm) CO₂ condition in Open Top Chamber (OTC) facility. The phenology, biomass accumulation, grain yield and HI was quantified of these three maize genotypes at both CO₂ levels. The phenology of flowering was early by 1.5 to 2days, while the anthesis silking interval (ASI) was not influenced by elevated CO₂ in DHM-117 and Varun, where as it was reduced by two days in Harsha. Response of selected three maize genotypes was different to elevated CO₂ (550ppm) condition in terms of biomass, grain yield and HI. The improvement in biomass ranged from 32% to 47%, grain yield 46% to 127% with 550ppm CO₂ as compared with ambient control. The improvement in grain yield was due to increased grain number (25-72%) as well as improved test weight (8-60%). The overall response of less efficient maize genotype Harsha with elevated CO₂ concentration was found to be significantly high especially the grain yield and its components. Elevated CO₂ also improved the maize HI (11% to 68%) indicating that influence of elevated CO₂ was there on partitioning of biomass of this C₄ crop.

Keywords: maize, elevated CO₂, genotypes, ASI, grain yield, grain number, HI

The changing climatic conditions are expected to increase the atmospheric CO₂ concentration, temperatures and alter the precipitation pattern. Atmospheric CO₂ concentration is predicted to reach 550ppm by 2050, and probably exceed 700ppm by the end of this century.¹ These changes are anticipated to affect the production and productivity of agricultural crops and influence the future food security. The impact analysis of climate change on global food production discloses a 0.5% decline by 2020 and 2.3% by 2050.^2,3 The development of climate ready germplasm to offset these losses is of the upmost importance.⁴

The C₄ grass maize (Zea mays L.) is the third most important food crop globally in terms of production and its demand is predicted to increase by 45% from 1997 to 2020.⁵ Studies with maize response to double the ambient CO₂ showed varying effects on growth ranging from no stimulation of biomass⁶ to 50 % stimulation.⁷ These studies reveal that C₄ plants do have the potential to respond to elevated CO₂. The basis for the observed enhancement of growth of C₄ plants under elevated CO₂ is not as clear as in C₃; plants. The present study was aimed to assess the response variability of maize genotypes at elevated CO₂ condition in terms of phenology, biomass and yield components.

The seed material of the maize genotypes DHM-117, Varun and Harsha were obtained from DMR Regional station at Hyderabad and raised in open top chambers (OTCs) at ambient (390ppm) and elevated (550ppm) CO₂ levels during post rainy season (Rabi) 2012. The OTCs having 3m x 3m x 3m dimensions lined with transparent PVC (polyvinyl chloride) sheet having 90% transmittance of light were used. The elevated CO₂ of 550ppm was maintained in two OTCs and other two OTCs without any additional CO₂ supply served as ambient control. The CO₂ concentrations within the OTCs were maintained and monitored continuously throughout the experimental period as illustrated by Vanaja et al.⁸

Each chamber had 6 plants of each genotype planted in two rows of 1.0m with 0.35m spacing within row and 0.75m between rows. The recommended dose of fertilizers 60 kg N ha^-1 and 60 kg P ha^-1 as diammonium phosphate, 30 kg K ha^-1 as muriate of potash was applied as basal dose; second dose of 30 kg N ha^-1 at knee- high stage and third dose of 30 kg N ha^-1 as urea and 30 kg potassium ha^-1 as muriate of potash was side dressed at tasseling stage. The crop was irrigated at regular intervals and maintained pest and disease free with plant protection measures.

The phenological observations such as days to 50% tasseling, anthesis and silking and maturity were recorded. At harvest the observations on plant height, total biomass, stover weight, cob weight, grain yield, test weight and other yield contributing traits were recorded. The analysis of variance (ANOVA) was carried out to assess the significance of CO₂ levels and genotypes and their interaction.

The analysis of variance (ANOVA) revealed that the selected three maize genotypes- DHM-117, Varun and Harsha recorded significant difference (p < 0.01) for plant height, total biomass, stover weight, cob weight, grain yield, test weight and HI. The CO₂ levels were significant for total biomass, cob weight, grain yield, grain number and test weight at p < 0.01 level and for plant height and harvest index at p < 0.05 level (Table 1), whereas the interaction of genotypes x CO₂ levels was significant only for test weight.

The plant height of all the maize genotypes showed a significant (p < 0.01) increase with enhanced CO₂ concentration (550ppm) as compared with ambient grown plants (Table 1). Driscoll et al.⁹ observed increase in maize plant height by 23% at 700ppm CO₂ and affirmed that being a C₄ crop, maize plant can show improved performance to increased CO₂ concentration. Elevated CO₂ also influenced the phenology of flowering in maize and it was observed that day to 50% tasseling, anthesis and silking was early by 1.5 to 2 days as compared with ambient controls (Figure 1). However, anthesis-silking interval (ASI) in DHM-117 and Varun was not influenced by elevated CO₂ as both anthesis and silking were early, whereas in Harsha, elevated CO₂ could reduce only the days to silking and not anthesis there by ASI was shortened to the extent of two days. In the life cycle of plant, the flowering time is very critical stage and in many crops it determines the number of seeds and final yield¹⁰ and the environmental conditions which affect the plant growth tend to influence the flowering dynamics.¹¹ Review of 60 studies on flowering time and elevated atmospheric CO₂ by Springer and Ward ¹² revealed that this response is crop and variety specific. The enhanced CO₂ condition reduced the days to initiation and 50% flowering in castor bean,¹³ whereas a delay in phenology of flowering was observed in soybean.¹⁴ Leakey et al.¹⁵ from their FACE experiments reported that 550ppm CO₂ didn’t influenced the duration of anthesis and silking of maize cv 34B43 (Pioneer Hi-Bred International).

Figure 1 Days to tasseling (T), anthesis (A) and silking (S) of three maize genotypes- DHM-117, Varun and Harsha at ambient (390ppm) and elevated (550ppm) CO₂ conditions.

The response of selected maize genotypes was different to elevated CO₂ (550ppm) condition in terms of total biomass, grain yield and HI. Enhanced CO₂ concentration significantly improved the total biomass as compared with ambient condition and the response was maximum with Varun (47%) followed by Harsha (34%) and DHM-117 (32%). Studies on impact of elevated CO₂ on maize crop revealed varying effects from no stimulation of biomass⁶ to 3-6%,¹⁶ 20%,^17,18 24%,¹⁹ 36%²⁰ and up to 50%.⁷ This differences in magnitude of response of maize to elevated CO₂ could be due to genotypic variability,²¹ strength of source and sink, management of the crop such as water and nutritional status, duration of exposure, light intensity, temperature, and even pot size.^22,23

The increase in biomass can be explained by the ability of the high CO₂ grown plants to maintain elevated photosynthetic rates and there was a 1.5 to 2 fold increase in internal CO₂. Ghannoum et al.²⁴ proposed two major mechanisms that may be responsible for increasing C₄ plant growth under elevated CO₂. The first potential mechanism operates through CO₂-induced increases in net photosynthetic rates and second mechanism deals with CO₂-induced reductions in stomatal conductance which can improve overall plant water relations and facilitate greater biomass production. In addition, reductions in transpirational water loss may slightly increase leaf temperature, thereby stimulating rates of photosynthesis and biomass production. Increased photosynthetic rate to synthesize the more sucrose and starch, and to utilize these end products of photosynthesis to produce extra energy by respiration, may contribute to the enhanced growth of maize under elevated CO₂.

The impact of elevated CO₂ was observed to be different in enhancing the vegetative and reproductive biomass of selected maize genotypes. With enhanced CO₂ greater vegetative growth was recorded by Varun (36%), whereas reproductive biomass by Harsha (94%). It is interesting to observe that in all the genotypes the increased response of reproductive biomass was much higher with enhanced CO₂ condition than vegetative biomass and indicating its function in triggering the partitioning of biomass more towards cob or grain weight (Figure 2). The improved grain yield due to 550ppm CO₂ was 46% in DHM-117, 61% in Varun and 127% in Harsha as compared with respective ambient control (Table 1). The improvement in grain yield was contributed by both increased grain number to the extent of 34%, 25% and 72% as well as enhanced test weight by 8%, 29% and 60% in DHM-117, Varun and Harsha respectively. Elevated CO₂ also significantly improved the HI of maize genotypes to the extent of 11% (DHM-117 and Varun) to 68% (Harsha). The simulation study using CropSyst model on the impact of elevated CO₂ on major cereal crops revealed that maize response was more than C₃ rice and wheat as well as C₄ pearl millet²⁵ and even the increase in yield was observed up to 3°C rise in temperature under doubled CO₂ situation. It was concluded that the improved response of maize being a C₄ crop could be due to more efficient use of increased CO₂ than the other C₃ crops.

Figure 2 The impact of elevated CO₂ (550ppm) on increase (%) of biomass, grain yield and HI of three maize genotypes- DHM-117, Varun and Harsha over ambient control (390ppm).

The evaluation of maize genotypes at elevated CO₂ (550ppm) for their biomass and yield revealed that maize crop though having C₄ photosynthetic pathway was able to respond positively with enhanced atmospheric CO₂ concentration. It is also evident that there is a significant variability between maize genotypes in response to elevated CO₂. The positive and significant response of elevated CO₂ on maize HI was due to higher partitioning of biomass towards reproductive parts than vegetative parts makes this crop more climate resilient.

The present work at CRIDA, Hyderabad was funded by Indian Council of Agriculture Research (ICAR) under National Initiative on Climate Resilient Agriculture (NICRA).

The author declares no conflict of interest.

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