Growth and Topological Changes of Citrus limon (L.) Burm. f. `Eureka' in Response to High Temperatures and Elevated Atmospheric Carbon Dioxide

Growth and topological indices of `Eureka' lemon were measured after 6 months in well-watered and well-fertilized conditions and factorial combinations of moderate (29/21C day/night) or high (42/32C day/night) temperatures and ambient (350 to 380 μmol·mol) or elevated (constant 680 μmol·mol -1 ) CO 2 . In high temperatures, plants were smaller and had higher levels of leaf chlorophyll a than in moderate temperatures. Moreover, plants in high temperatures and elevated CO 2 had about 15 % higher levels of leaf chlorophyll a than those in high temperatures and ambient CO 2 . In high temperatures, plant growth in elevated CO 2 was about 87% more than in ambient CO 2 . Thus, high CO 2 reduced the negative effect of high temperature on shoot growth. In moderate temperatures, plant growth in elevated CO 2 was only about 21% more than in ambient CO 2 . Irrespective of temperature treatments, shoot branch architecture in elevated CO 2 was more hierarchical than those in ambient CO 2 . Specific shoot extension, a topological measure of branch frequency, was not affected by elevated CO 2 in moderate temperatures, but was increased by elevated CO 2 enrichment in high temperatures-an indication of decreased branch frequency and increased apical dominance. In moderate temperatures, plants in elevated CO 2 had fibrous root branch patterns that were less hierarchical than at ambient CO 2 . The lengths of exterior and interior fibrous roots between branch points and the length of second-degree adventitious lateral branches were increased >50% by high temperatures compared with moderate temperatures. Root length between branch points was not affected by CO 2 levels.