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 c ombinations of moderate (29/21C day/night) or high (42/32C day/night) temperatures and ambient (350 to 380 mu mol . mol(-1)) or elevated (co nstant 680 mu mol . mol(-1)) CO2. In high temperatures, plants were sm aller and had higher levels of leaf chlorophyll alpha than in moderate temperatures. Moreover, plants in high temperatures and elevated CO2 had about 15% higher levels of leaf chlorophyll alpha than those in hi gh temperatures and ambient CO2. In high temperatures, plant growth in elevated CO2 was about 87% more than in ambient CO2. Thus, high CO2 r educed the negative effect of high temperature on shoot growth, In mod erate temperatures, plant growth in elevated CO2 Was only about 21% mo re than in ambient CO2. Irrespective of temperature treatments, shoot branch architecture in elevated CO2 was more hierarchical than those i n ambient CO2. Specific shoot extension, a topological measure of bran ch frequency, was not affected by elevated CO2 in moderate temperature s, but was increased by elevated CO2 enrichment in high temperatures-a n indication of decreased branch frequency and increased apical domina nce, In moderate temperatures, plants in elevated CO2 had fibrous root branch patterns that were less hierarchical than at ambient CO2. The lengths of exterior and interior fibrous roots between branch points a nd the length of second-degree adventitious lateral branches were incr eased >50% by high temperatures compared with moderate temperatures, R oot length between branch points was not affected by CO2 levels.