番木瓜樹冠結構與全樹碳能供需之研究

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

王仁晃(Ren-Huang Wang)

指導教授 ： 張龍生

國立臺灣大學/生物資源暨農學院/園藝學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.00431

摘要

番木瓜具有高產的潛能，但是全樹碳需求與供應模式尚未被建立。本研究首先探討於田間網室栽培下，番木瓜樹冠內光合作用能力與光度分布的輪廓，並以樹冠立體結構模型，說明葉片發育期間葉片角度的調整對樹冠內光線分布的影響。接著以自組氣體開放式果實光合作用測量系統，調查不同生育時期離體果實CO2氣體交換率，並計算出果實碳需求量及果實光合作用對果實碳需求的貢獻。最後，再以可移動式開放氣體交換系統量測2.5、4.5及6.5個月株齡番木瓜全樹光合作用，以瞭解番木瓜植株可供應的碳量。
番木瓜樹冠穿透係數值（extinction coefficient value, k）高達1.68，顯示位於樹冠上方的成熟葉間無明顯遮陰，但對下位葉造成顯著的遮陰效應。由於上方樹冠層葉面積指數（leaf area index, LAI）值0.3-1.4 m2 m-2 (約為樹冠總葉面積的46%)能夠維持較高的光合光通量密度（photosynthetic photon flux density, PPFD）及最大光合作用率(maximum net CO2 assimilation in saturated light, ACO2)，因此建議一個理想的木瓜樹冠，於接近採收期間，必須設法讓LAI值0.3-1.4 m2 m-2 (大約為11 – 29 葉位)的成熟葉，擁有最大的PPFD以維持樹冠最大光合作用能力。
番木瓜連續葉序為符合黃金角度的基礎螺旋排列，番木瓜葉片發育期間，葉柄傾角（φP）則逐漸由幼葉的垂直角度（φP ≒ 90o）調整為下位葉的平面角度，葉柄與葉面的夾角隨著φP持續進行調整，使得葉面維持於平面狀態。可藉由φB之調整與葉柄伸長，將第21葉位（48天葉齡）以前成熟葉維持於上層樹冠層，並擁有最大的光合光通量密度。而利用葉片發育介量可繪製樹冠立體圖形，並準確地模擬樹冠光截取及分布量，試驗結果亦說明豐產番木瓜樹冠必須擁有高光截取效率與維持完整樹冠結構的特性。
番木瓜果實以單位鮮果重為基礎，番木瓜果實授粉後二週發育初期暗呼吸作用率（dark respiration rate, RD）、淨光合作用速率（net photosynthetic rate, RL）及果實總光合作用率（gross photosynthetic rate, PG）較高，隨著果齡增加，RD及RL分別於12周及6周果齡逐漸降至平緩，其中RL值接近0 μmol kg-1 hur-1。若以單粒果重為基礎，則RD、RL及PG隨著果實發育，呈逐漸增加的趨勢；以單位面積為基礎，PG於不同果齡間介於2-3 μmol m-2 s-1。田間果實氣體交換率日變化趨勢，於光飽和點以下RL隨著光強度增加而增加，夜間RD及果實水份損失則與溫度有關。
果實以單位乾重為基礎， CO2氣體交換率隨著溫度增加而升高，以25-35℃間增幅較大。果實CO2氣體交換率仍以幼果時期較高，隨著果實發育而逐漸下降。果實時期碳需求量包含果實碳累積量與呼吸損耗量，果實碳累積量於第10週果齡起迅速累積，於採收前維持約310 - 400 mg fruit-1 day-1C碳累積量。果實光合作用對果實碳需求貢獻量，若分別以25/15℃及35/25℃（日/夜溫）為條件，發育期間分別平均約貢獻15.4%及17.3%，果實光合作用於果實發育期間，有利於維持果實碳水化合物的需求。
番木瓜全樹光合作用日變化趨勢與光度有關，2.5個月株齡全日固定CO2量為1.6 molCO2 plant-1 day-1，4.5-6.5個月株齡介於4.1-4.7 molCO2 plant-1 day-1；全樹蒸散作用率受到季節溫度變化的影響變動較大，介於378.5-810.4及molH2O plant-1 day-1間。番木瓜全樹淨CO2交換率（net carbon dioxide exchange rate, NCER）與葉面積指數(Leaf area index, LAI)及樹冠幅的大小有關，因此，當樹冠幅達到最大後，較高的LAI並無法增加NCER。利用本研究所設計的全樹氣體交換量測系統，適用於評估番木瓜樹冠氣體交換特性，並有助於建立果實碳需求量的評估。

關鍵字

番木瓜 ； 光合作用 ； 光截取 ； 果實光合作用 ； 碳需求 ； 全樹光合作用

並列摘要

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.

並列關鍵字

Carica papaya L. ； Photosynthesis ； Light interception ； Fruit photosynthesis ； Carbon requirement ； Whole canopy photosynthesis

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