Leaf age and light intensity affect gas exchange parameters and photosynthesis within the developing canopy of field net-house-grown papaya trees

Leaf age and light intensity dramatically affect the leaf photosynthetic capacity of the developing canopy of erect herbaceous plants. However, little is known regarding this response in papaya trees. The objective of this study was to determine how leaf age and light intensity affect the gas exchange characteristics within a developing papaya canopy by examining the distribution of maximum net CO2 assimilation in saturated light (ACO2) and photosynthetic photo flux density (PPFD) of papaya trees cultivated in a field net-house. To measure changes in relative gas exchange capacity with leaf age, a comparison of the gas exchange parameters of individual tagged leaves and the leaf position with maximum ACO2 within the developing canopy was carried out at a leaf age between 14 and 98 days in two cropping seasons (2008-2009 and 2009-2010). At or before the time of full leaf expansion (leaf age 26-36 days), the total chlorophyll concentration, relative ACO2, transpiration rate (E), stomatal conductance (gs) and PPFD reached maximum levels. In leaves aged 26-62 days, the relative ACO2 remained at above 90% of the maximum ACO2 and above 60% of PPFD. As the leaves aged and gradually became over-shaded within the canopy, the relative ACO2 gradually declined to 45-55% of the maximum. The degree of decline in gs was larger than that in ACO2 and E, but no significant differences were observed in Ci among the various leaf ages. Therefore, our results imply that ACO2 was limited by coordinated changes in both stomatal and nonstomatal factors. To access the photosynthetic capacity and light intensity profile in the papaya canopy, we investigated the ACO2 and PPFD at each leaf position within the canopy near time of harvest. The observed high extinction coefficient value (1.68) for field net-house-grown papaya at a high solar elevation indicated that the mature leaves in the top layer did not cover each other in the upper strata but effectively shaded leaves in the lower strata. The mature leaves in the upper layer of the canopy with a LAI of 0.3-1.4 m2 m−2 (46% of the total leaf area of the canopy) were able to maintain high PPFD and ACO2. The study suggests that an ideal papaya canopy should be exposed to a LAI of 0.3-1.4 m2 m−2 (approximately the 11th-29th leaf position) to acquire the maximum amount of PPFD and maintain photosynthetic capacity during mid-day measurements near harvest.