Physiology and Agronomic Use of Azolla Species in Rice Culture

Daniel, Joshua Nallathamby
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Although the value of Azolla species as a green manure for lowland agriculture is well established, environmental and management constraints have limited its use to a few countries. Cultivation of Azolla species may be extended to other areas if species and cultivars adapted to specific agricultural envirorunents can be identified. Five accessions from three species of Azolla were identified as being relatively tolerant of high light and temperature. The nitrogen accumulation potential and physiological characteristics of these accessions were evaluated in a series of experiments. The nitrogen and biomass accumulati on by Azolla caroliniana (Brazil), A. caroliniana (Ohio, U.S.A.), A. microphylla (Galapagos), A. pinnata (Indonesia) and A. pinnata (Taiwan) were evaluated during summer in Taiwan. Although average air temperatures were above 30 C, A. caroliniana from Ohio, U.S.A. and A. pinnata from Indonesia accumulated more than 40 kg ha-1 of nitrogen in 20 days when intercropped with rice. Rice in plots fertilized with Azolla species had greater plant height, shoot weight, leaf area index, tiller number, and grain yield than the control plots. Rice growth and yield were not increased by treatments which included 25 kg ha-1 of fertilizer nitrogen with Azolla. The leaf area index and grain yield of rice fertilized with Azolla nitrogen were less than those obtained with a similar quantity of fertilizer nitrogen. The lower response to Azolla may be due to the delay in release of nitrogen by mineralization. The relative growth rate of A. caroliniana (Ohio), A. microphylla (Galapagos) and A. pinnata (Indonesia) in greenhouse studies was negatively related to initial biomass when the biomass exceeded 600 g m-2. An initial biomass of 600 g m-2 of A. caroliniana, A. microphylla or A. pinnata produced more fresh weight and nitrogen after 21 days of growth than did initial biomasses of 200 or 400 g m-2• Fresh weight and nitrogen accumulation of all three species declined progressively with decreasing growth light level. With initial biomasses of 250 or 500 g m-2, the two species accumulated almost the same biomass and nitrogen after 20 days when both were grown at 35% light. At higher light levels, A. microphylla accumulated significantly more fresh weight than A. caroliniana and A. pinnata, but had a significantly lower dry matter and nitrogen content. Growth of A. caroliniana and A. microphylla in controlled temperature chambers was greater at 20 C than at 33 C when harvested 10 days after planting (DAP); only the growth of A. microphylla was greater at 20 C than at 33 C at 20 DAP. Growth declined with decreasing light and the effect was generally much greater at 33 C than at 20 C. The carbon dioxide exchange rate per unit weight (CERw) of A. caroliniana was greater than rates measured for A. microphylla and A. pinnata. The greater CERw was attributed to a greater specific leaf area and specific chlorophyll content. CERw increased curvilinearly with increasing photosynthetic photon flux density up to about 1500 umoles m-2 s-1• The CERw decreased as the biomass increased, probably due to increased mutual shading.
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