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The greenhouse (GH) effect has emerged as a major factor in changing cropping patterns and limiting crop yields. This study was conducted to determine the comparative growth and photosynthetic responses of selected heat-resistant (cv. Sadaf) and heat-susceptible (cv. Agatti-2002) cultivars of maize to simulated GH conditions during spring and autumn seasons at seedling, silking and grain filling stages in 2007. Fifteen day old plants were shifted to plexiglass-fitted canopies to create GH conditions and data were recorded at each growth stage. The results revealed that the seasons, GH conditions and cultivars had large effects on plant growth and photosynthetic attributes. Simulated GH conditions increased the canopy temperature 4–7 °C in spring and 3–5 °C in autumn, but increased relative humidity by 2–3% in spring and 5–9% in autumn season. Although GH reduced the growth of both cultivars, shoot dry mass was reduced more in spring grown heat-susceptible maize at all growth stages. Although the cultivars showed a decrease in growth and photosynthesis, GH conditions resulted in less damage to cv. Sadaf than cv. Agatti-2002 in both seasons. Major indicators of sensitivity to GH effect were loss of chlorophyll b and carotenoids, reductions in net photosynthesis and stomatal conductance, and possibly reduced ability of Rubisco to fix CO2 in sensitive maize.