Photosynthesis, Respiration, Transpiration, and Growth of Acacia Koa Seedlings as Effected by Photosynthetic Photon Flux Density

Walters, Gerald Alan
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Acacia koa Gray is Hawaii's most valuable forest tree. It is considered a pioneering species because it becomes established rapidly after overstory removal if seeds are present in the soil. The two leaf forms of koa apparently allow it to gain and then maintain control of the site. Leaves and phyllodes of koa differ markedly in leaf orientation, morphology, and anatomy, and in the levels of chlorophyll, total soluble protein, and Ribulose-l, 5-bisphosphate carboxylase. Both leaf forms have similar CO2-exchange rates when determined on a leaf area basis. Mean maximum rates of photosynthesis for both leaf forms were about 24 mg CO2 dm-2h-1. Light saturation and light compensation for both leaf forms occurred at 1200 and 25 umol m-2s-1 , respectively. The CO2 compensation concentration was about 55 ppm for both leaves and phyllodes, indicating that both leaf forms fix CO2 via the C3-pathway. On an organ basis leaves have higher CO2-exchange rates than phyllodes. Leaves also have higher transpiration rates than phyllodes. Growth and development of koa seedlings was greatly affected by available light. Seedlings grown under the highest light levels were the tallest and had the largest stem diameter and total dry weight. All measured growth parameters decreased as light intensity decreased. As the seedlings grew and developed under the different light levels, roots as a percentage of the total dry weight remained a fairly constant 18 percent. Initially, leaves made up more than 50 percent of the total dry weight. The percentage of leaf dry weight decreased with time, while the percentage of stem dry weight increased. Phyllodes developed only on seedlings exposed to light of at least light-saturating levels. The data indicated that koa seedlings can survive and grow only at light levels equal to or greater than 25 percent of full sunlight. The vigor of koa seedlings grown at less than 25 percent full sunlight declined with time and it appeared that they would eventually die. This minimum light requirement accounts for the scarcity of natural koa reproduction in an undisturbed koa forest. Koa leaves and phyllodes readily adapted to changes in available light. Leaves and phyllodes grown in full sunlight developed the characteristics of sun leaves. Conversely, at 27 percent of full sunlight, both leaf forms developed the characteristics of shade leaves. Fully-developed leaves and phyllodes only adapted physiologically to changes in available light. Partially-developed leaves and phyllodes adapted both physiologically and anatomically to changes in available light. Seedlings with phyllodes grown in 27 percent of full sunlight for 6 to 8 weeks developed leaves at the terminals. Seedlings again produced phyllodes when placed in full sunlight.
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