Interactions of CO2 enrichment and temperature on cotton growth and leaf characteristics

Studies on the interactive effects of atmospheric CO2 and temperature on growth and leaf morphology, particularly on stomatal index and density are limited. Upland cotton was grown in naturally-lit plant growth chambers at 30/22°C day/night temperatures from planting until squaring or the fifth or sixth leaf emerged. Five growth chambers were maintained at ambient (350 μl l−1) CO2 and another five at twice ambient (700 μl l−1) CO2 throughout the experiment. Day/night temperature treatments of 20/12, 25/17, 30/22, 35/27 and 40/32°C were imposed at each CO2 treatment for 42 days after squaring. The plants were irrigated with half-strength Hoagland's nutrient solution three times per day. Growth of plant parts was determined at the end of the experiment. Stomatal characteristics, nonstructural carbohydrates and specific leaf weight were measured on the fully expanded tenth mainstem leaf. Stomatal density and index were not affected by elevated CO2. Stomata and epidermal cell numbers per leaf increased in high CO2 and were positively correlated with final leaf sizes irrespective of CO2 level. Our results suggest that plants do not acclimate to elevated CO2 by changing stomatal density within a single generation. Leaves had greater area and accumulated more biomass when grown in high CO2. Growth stimulation expressed as dry weight at 700 μl l−1 over dry weight at 350 μl l−1 CO2 was uniform across temperatures. Temperature optimum for vegetative and reproductive growth was 30/22°C and was not altered by CO2 enrichment. Fruit retention was severely curtailed at the two higher temperatures compared to 30/22°C in both CO2 environments. Increased carbohydrate storage in leaves may be an added advantage for initiation and growth of vegetative structures such as branches at all temperatures. However, it is unlikely that high temperature effects on flower abortion will be ameliorated by high CO2. Species/cultivars that retain fruits at high temperatures would be more productive both in the present-day cotton producing environments and are even more desirable in the future warmer world.