Breeding investigations for salt tolerance in rice incorporating characterisation of salt affected soils and farmers perceptions and preferences for tolerant cultivars in north-eastern Tanzania.
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Date
2010
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Abstract
Rice (Oryza sativa L.) is the principal crop of North Eastern Tanzania but production is
threatened by salt affected soils, drought and the use of un-adapted cultivars, among other
constraints. Little research and hardly any breeding have been done on the aspects of salt
tolerance of the crop in sub Saharan Africa, leading to low yields and low production in
rice irrigation schemes under arid and semi arid conditions. A project was therefore
implemented in North Eastern Tanzania during 2007-2010 seasons to investigate the
possible breeding contributions to enhance productivity and production of the crop in salt
affected areas.
The objective of this study was to: a) determine farmers’ perceptions on both salt
problems and their effects on rice crop productivity as well as establishing farmers’
needs and preferences for rice varieties in the targeted irrigated environment; b)
determine the extent of salt problem in both soil and irrigation water in the available rice
irrigation schemes in the North-eastern Tanzania; c) identify the major physiological
mechanisms associated with salt tolerance in farmer-preferred native varieties and
landraces; and d) determine the mode of inheritance of salt tolerance in rice.
Participatory rural appraisal was conducted in Mkomazi and Mombo villages in Tanga
region with the aim of understanding characteristics of rice–based farm economy,
farmers’ perception of agriculture constraints and variety preferences in salt affected
areas of North-eastern Tanzania. This was followed by a preliminary study to understand
soil characteristics in relation to salt problems and its extent in selected nine rice
irrigation schemes. Studies was established under controlled conditions to assess the salt
tolerance of some rice farmers preferred rice cultivar and evaluate the putative traits in
the rice materials that contribute to the performance of a genotype under saline and
saline-sodic condition. Thereafter, genetic mechanism governing various morpho-physiological
parameters in selected Tanzania local farmers’ preferred varieties and salt
tolerant donors under saline and sodic soil conditions of North Eastern Tanzania were
determined.
A participatory rural appraisal (PRA) established that rice was a major staple food and
cash crop and rice farming was a major economic activity in the area. However, soil
degradation through increased salt affected soils was identified as the major factor
responsible for irrigated rice yield decline. Major varieties grown are salt sensitive, and
salt tolerant varieties were not available. The study also revealed that most farmers’
preferred traits of rice cultivars were high yield potential, aroma, early maturing, medium
plant stature, tolerance to salt and drought. Improvement of these characters in new salt
tolerant varieties would increase food production in fields with low or zero productivity
and the well-being of the poor farmers.
A soil characterisation study indicated the magnitude of the problem, whereby, seven out
of nine studied irrigation schemes were affected and sodic and saline-sodic conditions
were the dominant types of soils. Poor irrigation canals and management of irrigation
water were the driving factors that contributed to salts accumulation causing a decline in
productivity.
Experiments were established under controlled environments to evaluate the tolerance of
10 and 11 rice genotypes under saline and saline-sodic stresses, respectively. Significant
variation between genotype and significant interactions between genotype and salt
treatment (P<0.001) were observed for all characters studied. Genotype Pokkalli, IR
67076-2B-21-2 and IR 56 showed superior performance under saline, whereas CSR 27,
Nerica 2 and IR 56 had superior performance under saline-sodic. The study therefore
established that, all the local farmer preferred cultivars except IR 56 performed poorly
under both salt stress environments. High seedling vigour, less leaf injury, less Na+ and
high K+ accumulation in leaves, low Na+/K+ ratio of ion uptake, high spikelet fertility,
increased grains per panicle and 1000 grain weight were considered as the desirable
characteristics therefore can be used in developing lines for salt tolerance for production
under saline and saline - sodic conditions.
Gene action and combining ability studies for nine morpho-physiological traits were
studied under normal, saline and sodic soil environments. The rice populations were
generated through 7 x 7 full diallel crosses and advanced to F2. The parents comprised
two donors for saline tolerance, one donor for sodic tolerance and four salt sensitive
farmers preferred varieties. Both additive and non-additive gene effects were important in
the inheritance of the characters studied in all soil environments. However, additive
effects were more important for the number of tillers, shoot Na+, Na+/K+ ratio and plant
height. Both additive and non-additive gene effects were important for spikelet fertility,
days to 50% flowering, number of grains per panicle, 1000 grain weight, and grain yield;
however the magnitude of additive gene effect was higher than non additive effects.
Amongst the parental lines, the best general combiners for yield along with other traits
were TXD 306 and IR 67076-2B-21-2 under normal non-saline/sodic condition; IR 56,
Pokalli and TXD 306 under saline condition and CSR 27 and TXD 306 under sodic
conditions. The overall results from this study indicated the possibility of improving both
yield and salt tolerance from this set of germplasm; therefore contributing to increasing
rice yields in the marginal salt affected environments.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.
Keywords
Rice--Tanzania., Rice--Breeding--Tanzania., Rice--Varieties--Tanzania., Rice--Yields--Tanzania., Rice--Effect of salt on--Tanzania., Rice--Tanzania--Genetics., Soils, Salts in--Tanzania., Saline irrigation--Tanzania., Salt-tolerant crops--Tanzania., Theses--Plant breeding.