Sunflower (Helianthus annuus L.) and maize (Zea mays L.) were chosen as C₃and C₄ crop plants and assessed for the impact of enhanced CO₂ (700 ppm) and its interaction with drought stress in open top chambers (OTCs). The ameliorative effect of higher CO₂ concentration (eCO₂) under drought stress was quantified. It is interesting to note that the C₃ crop responded significantly and positively with eCO₂ under both well-watered and drought stress treatments for root: shoot ratio while C₄ crop showed a better response only with the drought stress environment. Root volume showed a positive significant response with CO₂ concentration enhanced over ambient level and the increment in root volume was 146% and 340% in sunflower and maize crops, respectively. The leaf water potential, stomatal conductance and transpiration showed a decreasing trend in both the crops with drought stress and eCO2 showed an ameliorative effect leading to higher P_n rates in sunflower crop under drought stress treatment. The findings reveal that improvement of root traits is worth attempting for the future crop behavioral responses under eCO₂ and drought stress environments. The study confirmed the beneficial effect of eCO₂ in maize and sunflower by ameliorating the adverse affects of drought stress.

The response of blackgram (Vigna mungo L. Hepper) to two levels of elevated carbon dioxide (550 and 700 ppm) in terms of growth and yield was investigated and compared with ambient CO 2 level (365 ppm) using open-top chambers. The growth parameters viz., length and weight of root and shoot, root:shoot ratio, leaf area and weight significantly increased at 700 ppm CO 2 when compared with 550 ppm. The percentage increase in total biomass at 700 and 550 ppm CO 2 was 65.4% and 39%, respectively compared to the ambient (chamber) control. The increase in total seed yield at 700 ppm (129%) was due to an increase in number of pods per plant and 100 seed weight, whereas at 550 ppm (88.7%) it was due to an increased number of pods/plant and seeds/pod. The results indicate variable responsive effects at different levels of CO 2 emphasizing the pertinence of research on elevated CO 2 in various agroecological inhabitations all over the world. The indication of higher responses for root and leaf at initial growth stages at the higher elevated level of CO 2 (700 ppm), which leads to better root establishment, achieving early photosynthetic efficiency and also better biomass production, and its improved partitioning can be reckoned as a positive aspect of increasing concentrations of CO 2 in atmosphere. The harvest index increased significantly to 35.7 and 38.4% at 550 and 700 ppm, respectively; it is a very important phenomenon in pulses for breaking the yield barrier.

Growth and yield responses of castor bean (Ricinus communis L.) to two elevated CO 2 levels (550 and 700 ppm) were evaluated up to the maturity of first order spikes in open top chambers (OTCs). The growth characteristics -root and shoot lengths, root volume, root:shoot ratios, leaf area, dry weights of different plant parts, leaf area duration and crop growth rate increased with 550 and 700 ppm of CO 2 levels compared with ambient control. The spike length, pod and seed yield of first order spikes increased under enhanced CO 2 levels over ambient control. Elevated CO 2 levels significantly increased the total biomass and yield of castor bean, however enhanced CO 2 levels per se did not changed the content and quality of the castor oil. A positive response of castor bean to increased CO 2 concentrations is a good indication for its future existence in potentially changed climatic conditions.

Pigeon pea (Cajanus cajan L. Millsp.) is an important grain legume crop of the semi arid tropics and is a major dietary protein source. The extra short duration cultivar of pigeon pea ICPL 88039 was evaluated at ambient<br />(370 &mu;mol/mol) and twice the ambient (700 &mu;mol/mol) concentrations of CO₂ in open top chambers (OTCs). The results showed that the crop recorded a significant positive enhanced response for total biomass, fodder yield, grain yield, number of pods and seeds per plant, test weight and HI at elevated CO₂. The ANOVA revealed significant differences in response of the characteristics to CO₂ concentrations. Under elevated CO₂ the total biomass recorded an improvement of 91.3%, grain yield 150.1%, fodder yield 67.1%. The major contributing components for improved grain yield under elevated CO₂ were number of pods, number of seeds and test weight which recorded an increase of 97.9%, 119.5% and 7.2%, respectively. The crop maintained a significant positive increase of harvest index (HI) at elevated CO₂ with an increment of 30.7% over ambient values. This increase in HI was due to its improved pod set and seed yield under enhanced CO₂concentration thereby emphasizes this crop for sustained food with nutritional security under climate change scenario. &nbsp;

Under the present global scenario of CO 2 increase (IPCC 1996), it has become pertinent for researchers all over the world to find solutions for future. Firstly, research can help to identify the crops which respond to the above situation and those which do not. Secondly, among the crops which respond, those with relatively a higher magnitude are to be identified initially to address the food and feed self-sufficiency followed by the soil improvement.Various reviews on the response of different crops revealed that an increase in CO 2 has a positive effect on the plant biomass. Kimball (1986) obtained an average increase of 21% in biomass in response to elevated CO 2 when he analyzed 94 observations of different plants. Cure (1985) and Cure and Acock (1986) reported that sorghum showed a stimulation of 5% increase in biomass with elevated CO 2 levels (scaled to 550 µmol/mol). Venkteshwara Rao (1999) observed that in groundnut cv. TMV-2 biomass production was 29% higher in elevated CO 2 (660 ppm) than in ambient CO 2 . In sunflower the growth was affected at elevated CO 2 by increasing net CO 2 assimilation rate (Tezara et al. 2002). It was observed that with elevated levels of CO 2 (using the FACE technology) there was a greater stimulation of belowground than aboveground biomass (Kimball et al. 2002). Under ample water and nutrients the root growth of C 3 grasses was stimulated by about 47% as compared with the 12% of shoots whereas in clover (C 3 legume) the root growth stimulation (25%) was nearly same as that of shoots (24%).In this paper an attempt was made to present the response of certain rainfed crops up to vegetative stage to enhanced levels of CO 2 and were compared with chamber control at ambient level CO 2 , which actually takes care of the deviation in temperature and relative humidity due to the OTC. The crops were raised directly in the field to avoid the root restriction in pots. In enhanced CO 2 condition, the CO 2 levels were elevated to set ppm continuously 24 h from the day of sowing to final sampling. The response reported that the actual effect of CO 2 with all other conditions maintained similar for both the treatments. MATERIAL AND METHODSThe seeds of sorghum (Sorghum bicolor L . The results showed significant differences between crops, conditions and time intervals, as well as the single and double order interactions for all the characters studied viz., total dry weight, stem dry weight, root dry weight, leaf dry weight, shoot length, root length and leaf area. Total dry weight and its components viz., stem dry weight, root dry weight and leaf dry weight along with leaf area showed a significant increase under enhanced CO 2 conditions. Among the four crops studied the overall results showed the highest response to elevated CO 2 by blackgram while the lowest response by sorghum.