Diversity and adaptation in rice varieties under static (ex situ) and dynamic (in situ) management |
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Authors: | HQ Tin T Berg Å Bjørnstad |
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Institution: | (1) Department of Biodiversity Conservation, Farming Systems Research and Development Institute, Cantho University, Vietnam;(2) Center of International Environment and Development Studies (Noragric), Agricultural University of Norway, Norway;(3) Department of Horticulture and Plant Science, Agricultural University of Norway, Norway |
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Abstract: | This study compares genebank-conserved and farmer managed populations of the same farmers' varieties of rice. Seven varieties
that had been collected twice, in the early 1980s and in 1991, were recollected in 1997 after having been grown continuously
in farmers' fields. Since the first genebank collection, rice cultivation in the Meking delta has been intensified with a
rather abrupt switch from single to double cropping, more use of chemical fertilisers, improved water management, and more
market oriented production. Many farmers' varieties have been maintained as a second crop but with a considerably delayed
planting time compared to previous practice. In this experiment, the ex situ materials represent adaptation to pre-intensification conditions while the in situ populations have been exposed to the intensive production system for a number of years. The materials were tested in the
wet season of 1997 under current farmers' management practices in an on-farm field experiment within the area where the varieties
originated. Agronomic, stress resistance and morphological traits and variation at 7 isozyme loci were observed in the field
or laboratory. Analysis of variance (ANOVA) and Principal Component Analyses (PCA) were used to study differences in agronomic
and morphological traits between ex and in situ populations. Isozyme variation was analyzed by Nei's diversity indices and Wright's F-statistics. Farmer-managed populations
showed a general trend of later flowering and maturity time, more uniformity of grain quality, lower frequency of undesired
off-types, and reduced drought stress tolerance compared with corresponding ex situ populations. There were no significant differences in grain yield or tolerance to biotic stresses. Allelic frequencies of
isozymes showed no consistent differences that could be related to changes of the farming system. These results are interpreted
as an adaptation to the changed farming system and include natural and farmers' selection for maturity time (all varieties
are photoperiodic)and market standards. The poorer drought tolerance may reflect the fact that such stress was common before
intensification but is not normally a factor under the current water management regime. For in situconservation strategies this case sheds some light on the survival of allelic diversity vs. adaptedness. Isozyme data indicate
maintenance of allelic diversity. Adaptedness, however, is at risk under on-farm conservation. Natural and intentional selection
will normally not remain constant over time. Consequent genetic changes include loss of adaptation to past conditions and
building up of adaptation to new. In this case such changes have happened surprisingly fast. However, changes are limited
to adaptation to factors of the environment and to market-relevant quality traits. Yield seems to be unaffected. Considering
needs for crop improvement this case has kept the materials `updated' with respect to adaptation and unchanged with respect
to yield potential.
This revised version was published online in August 2006 with corrections to the Cover Date. |
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Keywords: | adaptation Convention on Biological Diversity in situ management of genetic resources isozymes rice |
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