Oil Palm Bulletin No. 51 (November 2005) p10-19

Carbon dioxide enrichment in oil palm canopies and its possible influence on photosynthesis

HENSON, Ian Eugene , MOHD HANIFF Harun

This paper reviews past and recent work on at- mospheric concentrations of carbon dioxide (CO2)in and above stands of oil palm.Although data is limited, atmospheric CO2 concentrations both within and above stands,determined for several plantings,showed large diurnal variations with CO2accumulating to high levels over-night.Over much of the day, higher concentrations than are generally recognized as typical of global ambient levels were detected.

In recently planted immature palms, CO2levels,averaged over the morning,increased gradually over an 18-month period as the canopy developed. Although the increase was associated with enhanced rates of leaflet photosynthesis, other factors were considered more likely to be responsible for this.
In a developed oil palm stand with full leaf cover, large diurnal fluctuations in CO2 concentrations were detected both above and within the canopy. Levels increased overnight and then rapidly decreased after dawn as photosynthetic activity resumed. Concentration gradients developed which reflected the gradient in photosynthetic activity with heights during the day (with concentrations being minimum near the canopy top), and in respiratory activity and efflux from the soil during the night (with maximum accumulation of CO2 occurring within the trunk space).
In young palms in a dry region, both seasonal conditions (wet or dry)and wind speed were found to affect the amplitude of daily cycles of CO2concentration. Lower concentrations were found under dry than under wet conditions and CO2 only accumulated at wind speeds below about 1 m s-1.
Under controlled conditions in the field, leaflet photosynthesis rates of oil palm showed a curvilinear response to CO2 concentration, almost doubling over the range 300 to 900 µmol mol-1CO2. Although such a response suggests that dry matter production of oil palm should be considerably enhanced and its efficiency of water use improved as CO2concentrations rise, a simple additive model indicated there to be little immediate gain in daily net photosynthesis due to the presently observed overnight and early morning accumulation of the gas.
Abstrak: Artikel ini mengimbas kembali kerja-kerja baru dan lama mengenai kepekatan karbon dioksida (CO2)di dalam dan di atas kanopi sawit. Walaupun maklumat adalah terhad, kepekatan CO2atmosfera di dalam dan di atas kanopi untuk beberapa peringkat umur sawit didapati amat berbeza mengikut masa dan paras paling tinggi adalah pada waktu malam. Pada waktu siang, paras CO2didapati telah melebihi kepekatan biasa sedunia. Purata kepekatan CO2 pada pokok muda didapati meningkat pada waktu pagi untuk tempoh 18 bulan mengikut perkembangan kanopi sawit.Peningkatan ini mempunyai hubungan rapat dengan pertambahan kadar fotosintesis daun,tetapi faktor-faktor lain mungkin juga terlibat sama. Untuk tanaman sawit matang yang mempunyai kanopi tertutup, perubahan besar pada kepekatan CO2 dapat dikesan di atas dan di dalam kanopi.Paras meningkat pada waktu malam dan menurun selepas matahari terbit setelah aktiviti fotosintesis berlangsung. Pembentukan kecerunan kepekatan CO2 mencerminkan kecerunan aktiviti fotosintesis dengan paras ketinggian pada waktu siang (paras minimum di atas kanopi) dan aktiviti pernafasan dan penghasilan dari tanah pada waktu malam (paras maksimum pada ruang di antara kanopi dan tanah).
Untuk pokok muda di kawasan kering, keadaan musim (basah atau kering) dan had kelajuan angin didapati mempengaruhi amplitud kitaran harian kepekatan CO2.
Dalam keadaan terkawal di ladang, kadar fotosintesis daun sawit menunjukkan tindak balas kurvilinear kepada kepakatan CO2, iaitu hampir dua kali ganda pada paras 300 ke 900 µmol mol-1 CO2. Walaupun tindakbalas ini menunjukkan pengeluaran berat kering pokok sawit boleh bertambah dan keupayaan penggunaan air meningkat apabila kepekatan CO2meningkat berikutan kesan pemanasan global, model ringkas menunjukkan bahawa terdapat sedikit penambahan pada hasil fotosintesis harian akibat dari pengumpulan CO2 pada waktu malam.

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