Study on Canopy Structure, Whole Tree Carbon Supply and Demand in Papaya

• Main Authors: Ren-Huang Wang, 王仁晃
• Other Authors: Loong-Sheng Chang
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/57817664518644152632

博士 === 國立臺灣大學 === 園藝學研究所 === 102 === Papaya has a tendency for high productivity. However, there is still unsatisfactory information on the carbon supply and demand of papaya trees. First, we accessed the photosynthetic capacity and light intensity profile within the developing canopy of field net-house-grown papaya trees. Second, to simulate the distribution of light intensity within the papaya canopy, we applied a simple statistical model for reconstructing three dimensional (3D) canopy structures and explained how adjustment of leaf inclination affects light intensity distribution within the developing canopy of papaya trees. Then, we designed an open-flow chamber system to measure papaya detached fruit CO2- and H2O-fluxes in developing stages and calculated the contribution of fruit photosynthesis to the carbon requirement of developing papaya fruits. Last, four flow-through chambers were built to measure gas exchange of whole papaya canopy at 2.5, 4.5 and 6.5 months after planting and to access the carbon supply of whole papaya tree. 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. The angular position of papaya leaves follows genetic spiral arrangements corresponding to the ''golden angle'' around the stem. The vertical petiole inclination angle (φP) with the newly leaf continually turned into horizontal with increasing leaf position. To maintain φB in horizontal situation with papaya mature leaves, the leaf angle between leaf blade and petiole could be gradually adjusted with φP. By progressively adjusting φB and elongating petiole length in the process of leaf development, the newly developed leaves were able to maintain the leaves within 21th leaf positions (leaf age 48 days) in the upper strata of papaya canopy and high photosynthetic photon flux density (PPFD). The parameter values obtained in this study were applied to draw three-dimension canopy architecture and accurately stimulated the PPFD distribution within the canopy. Our results imply that the high effective light interception with canopy and well-developed canopy architecture are the main requirement for a high productivity canopy of papaya. On a unit fresh weight basis, the dark respiration rate (RD), net photosynthetic rate ( RL) and gross photosynthetic rate (PG) were higher during the early developing stage of fruit growth. RD and RL decreased gradually until 12 weeks and six weeks fruit age, respectively. RL maintained in a stable level and close to 0 μmol kg-1 hur-1 until fruit maturity. On single fruit basis, fruit RD, RL and PG increased gradually with fruit weight. Furthermore, on a unit surface area basis, the value of PG was about 2-3 μmol m-2 s-1 in fruit developing stages. The daily RL trend of attached fruits followed the increase of irradiance under the light saturated point in the field experiment. The increasing in RD and net water loss of papaya fruits was related to the ambient temperature. On a unit dry weight basis, the net CO2 exchange rate was raised with the increasing temperature especially between 25-35 ℃. The net CO2 exchange rate was higher during the early developing stage of fruit growth and declined in maturing fruits. The carbon requirement of developing fruits included carbon accumulation and respiration loss of fruit development. Carbon requirement increased rapidly from 10 weeks after pollination and maintained in a stable level with 310 - 400 mg fruit-1 day-1C until fruit maturity. Photosynthesis of papaya fruit at 25/15℃and 35/25℃ (12/12 hurs and day/night temperature) provided 15.4% and 17.3%, respectively, of the total fruit carbon requirements during fruit development and maintained carbohydrate requirements during the growing season. Whole tree gas exchange closely tracked changes of solar radiation. Daily CO2 fixation rate was 1.6 molCO2 plant-1 day-1 at 2.5 month after planting and 4.1 to 4.7 molCO2 plant-1 day-1 between 4.5 and 6.5 month after planting. Vary temperature among seasons affected canopy transpiration rate, the daily rate was 378.5 to 810.4 mol H2O plant-1 day-1. In this study, we revealed that the whole tree net carbon dioxide exchange rate (NCER) with papaya depends on the relationship between the LAI and the diameter of canopy. Therefore, at high LAI, when the diameter reaches the largest canopy, further increases in leaf area (or LAI) would not lead to an increase in NCER. The gas exchange system presented here is a suitable design to assess the canopy gas exchange properties and estimate of carbon requirement for fruit development.