Chlorophyll fluorescence emission and gas exchange rates in two cacao genotypes infected with withces' broom. In: Agrotrópica v. 13 p. 1-8, 2001.

Abstract

Parameters of chlorophyll (Chl) fluorescence in the fast phase and gas exchange rates were studied in seedlings of Catongo and SCA6 x Catongo, a susceptible and a tolerant to witches’ broom cacao cultivars, respectively. The seedlings were inoculated with a mixture of isolates of Crinipellis perniciosa, causal agent of the disease. Kinetics of fluorescence induction of Chl was evaluated during leaf ontogeny using a portable system PAM-2000. Gas exchanges were measured in mature leaves using a Portable Photosynthetic System LI-6200. Inter- and intra-genotypic statistical differences were found in relation to minimum (F0 ) fluorescence values at the B (7 to 14-days-old), C (15 to 20 days-old) and E (> 30 days-old) stages of leaf development. The variable fluorescence (Fv ) differed inter- and intra-genotypically only at the C stage, however, the differences were only intergenotypic at the B and E stages. Also, statistical differences were found between infected and control genotypes for maximum potential quantum yield of PS II (Fv /Fm ) and the ratio of variable fluorescence [(Fm -F0 )/F0 ] at the B, C, D (21 to 30 days-old) and E leaf developmental stages. On the other hand, the rates of the photosynthetic electron transport to PS II (Amax ) differed interand intra-genotypically only at the C and E stages. At the B stage these differences were only inter-genotypic. No inter- and intragenotypic differences during leaf ontogeny were found between Amax and Fm and Amax and Fv /Fm in spite of a highly positive correlation between these parameters. Furthermore, inter- and intra-genotypic statistical differences were found for carbon exchange rate (CER), stomatal conductance (Cs), internal CO2 concentration (Ci) and transpiration rates (TR) measured in mature leaves of infected and non-infected seedlings. The infected Catongo seedlings showed the lower gas exchange values. Catongo, the most affected genotype by the C. perniciosa infection, showed damages in the reaction centers of PS II associated with alteration in the biochemical reactions of photosynthesis.