Photosynthetic activity of the calyx, green shoulder, pericarp, and locular parenchyma of tomato fruit

Photosynthesis of tomato fruit was studied using green fruit from six heritage cultivars of Lycopersicon esculentum Mill. and one of Lycopersicon pimpinellifolium. Chlorophyll concentrations in the green shoulder, pericarp and locular parenchyma of the fruit were determined and the apparent photosynthetic electron transport activity (ETR) and chlorophyll fluorescence quenching characteristics of these tissues and the calyx were compared. In all cultivars, green shoulder formation, apparent as intense pigmentation of the proximal pericarp shoulder, was positively related to the degree of shading of the fruit during development. Appearing as a photosynthetic adaptive trait for increasing the photoautotrophic capacity of fruit grown under low light, the green shoulder contained 17-57% of the total pericarp chlorophyll content. The pericarp below the green shoulder had lower chlorophyll a+b concentrations than in the locular parenchyma. The proportion of light-harvesting chlorophyll to active centre chlorophyll was also lower in the pericarp than in the locular parenchyma as indicated by higher ratios of chlorophyll a/b. At a photon flux density (PFD) of 1200 mu mol m(-2) s(-1), different fruit tissues were found to have different levels of ETR. In 'Yellow Pear', the upper surface of the calyx had an ETR of 154 mu mol m(-2) s(-1), while the lower surface had an ETR of 88 pmol m(-2) s(-1). On the green shoulder, ETR was 203 mu mol m(-2) s(-1), whereas in the pericarp distal to the green shoulder, ETR was 97 pmol m(-2) s(-1.) In the locular parenchyma, ETR was 66 mu mol m(-2) s(-1). This trend towards a lower ETR in distal and internal fruit tissues appeared to indicate a shift towards a more shade-type photosynthesis. Concomitant with this shift were changes in chlorophyll fluorescence quenching characteristics. Generally when tissues displayed reduced levels of ETR they also displayed a faster decrease in the photochemical quenching coefficient q(P) and a more rapid diversion of absorbed photon energy to non-photochemical pathways which was seen by the rise in the non-photochemical quenching coefficient q(N). However, the response of the locular parenchyma was an exception, under equivalent levels of PFD the q(P) levels in this tissue were much lower than those in the pericarp, but q(N) levels were similar. This finding indicated that the lower ETR of the locular parenchyma was due to a lower capacity to utilize electrons generated from water splitting rather than a greater diversion of absorbed photon energy to nonphotochemical pathways. The levels of photosynthetic activity found in the calyx, green shoulder, pericarp, and locular parenchyma suggest that all of these tissues have significant roles in CO2 scavenging and the provision of carbon assimilates. The potential role of fruit photosynthesis in influencing the fruit acid to sugar ratio and hence fruit quality is discussed.