Leaf photosynthesis, photorespiration, and respiration in papaya genotypes (Carica papaya L.) with different leaf chlorophyll concentrations

The papaya genotypes 'Sunrise Solo' (SS) and 'Golden' (G) are the two most important genotypes of the 'Solo' group cultivated in commercial plantations in Brazil. Although the G genotype has a greater acceptance in the external market due to the quality of the fruit, this genotype presents a lower yield (90 t ha(-1)) compared with genotype SS (120 t ha(-1)). This may be related to photosynthetic rates, since the G genotype has a lower total leaf chlorophyll concentration than the SS (SPAD reading of 33.9 +/- 3.6 vs. 53.5 +/- 2.4, respectively). Additionally, differences in yield may be related to differences in both photorespiration and leaf respiration rates. Therefore, the aim of this work was to compare the photosynthesis, photorespiration, and rates of respiration in the dark and in the light between genotypes SS and G. Both genotypes were grown in 40-L pots in a greenhouse located in northern Rio de Janeiro State, Brazil. Soil was fertilized and watered as necessary. Gas exchange measurements were performed using a portable open-system infrared gas analyzer (LI-6400, Li-COR, Lincoln, NE, USA). The net photosynthetic rate as well as Rubisco carboxylation and oxygenation rates (V-c and V-o, respectively) were determined at 08:00 am and 12:00 pm. V-o and V-c rates, as well as rates of respiration in light and in dark, were estimated from the light response curves with 24 levels of photosynthetically active radiation. Measurements were performed in completely expanded leaves (approx. 27 and 21 cm in length for the leaf central vein of SS and G, respectively) (n=4). Although the G genotype had shown lower photorespiration rates than SS at both 8:00 am (similar to 17%) and 12:00 pm (similar to 26%), our results indicate that the reduced yield of G genotypes (containing lower chlorophyll concentration) may not be associated with both photosynthetic and respiratory rate.