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 herit age cultivars of Lycopersicon esculentum Mill. and one of Lycopersicon pimp inellifolium. Chlorophyll concentrations in the green shoulder, pericarp an d locular parenchyma of the fruit were determined and the apparent photosyn thetic electron transport activity (ETR) and chlorophyll fluorescence quenc hing characteristics of these tissues and the calyx were compared. In all c ultivars, green shoulder formation, apparent as intense pigmentation of the proximal pericarp shoulder, was positively related to the degree of shadin g of the fruit during development. Appearing as a photosynthetic adaptive t rait for increasing the photoautotrophic capacity of fruit grown under low light, the green shoulder contained 17-57% of the total pericarp chlorophyl l content. The pericarp below the green shoulder had lower chlorophyll a+b concentrations than in the locular parenchyma. The proportion of light-harv esting chlorophyll to active centre chlorophyll was also lower in the peric arp than in the locular parenchyma as indicated by higher ratios of chlorop hyll a/b. At a photon flux density (PFD) of 1200 mu mol m(-2) s(-1), differ ent fruit tissues were found to have different levels of ETR. In 'Yellow Pe ar', the upper surface of the calyx had an ETR of 154 mu mol m(-2) s(-1), w hile the lower surface had an ETR of 88 pmol m(-2) s(-1). On the green shou lder, 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, ET R was 66 mu mol m(-2) s(-1). This trend towards a lower ETR in distal and i nternal fruit tissues appeared to indicate a shift towards a more shade-typ e photosynthesis. Concomitant with this shift were changes in chlorophyll f luorescence quenching characteristics. Generally when tissues displayed red uced levels of ETR they also displayed a faster decrease in the photochemic al 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 loc ular parenchyma was an exception, under equivalent levels of PFD the q(P) l evels in this tissue were much lower than those in the pericarp, but q(N) l evels were similar. This finding indicated that the lower ETR of the locula r parenchyma was due to a lower capacity to utilize electrons generated fro m 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 prov ision of carbon assimilates. The potential role of fruit photosynthesis in influencing the fruit acid to sugar ratio and hence fruit quality is discus sed.