The expansion of mungbean production in Australia is severely limited by the susceptibility of mungbeans (Vigna radiata - green gram; V. mungo - black gram) to pre-harvest sprouting and to reductions in quality and viability following rainfall. Imrie (1983) showed that simple selection for resistance to weather damage using visual measurements in the field was unsuccessful . The objective of this present study was to improve the breeding process for weathering resistance by (a) developing a reliable screening procedure based on an understanding of the causes and effects of weathering, (b) using this procedure to measure the variation in response to weathering, and then (c) identifying and further investigating the trait/s associated with the weathering response.
Initial studies on the role of water absorption showed that weather damage was cumulative and dependent upon the timing of rainfall and stage of pod development. Therefore, all material for screening was produced in a glasshouse where it was protected from prior exposure to wetting. Symptoms of weather damage were most successfully induced by suspending dry racemes inside a small plastic chamber and exposing them to five consecutive cycles of wetting and drying at a constant temperature. Weather-damaged pods were typically darkened and split, and the testas of seeds were discoloured and cracked. Weathering also reduced cell membrane integrity, and seed weight, vigour, and viability.
A total of 302 lines was screened for response to weathering, and the degree of damage was expressed using indices of water absorption (WI) and seed death (SDI). The screening procedure successfully reproduced known differences in response between the relatively resistant black gram cv. Regur, the moderately susceptible green gram cv. Celera, and the extremely susceptible green gram cv. Berken. Some twentyfive lines of green gram were identified with weathering resistance as high as that of cv, Regur. Thus, an efficient, controlled screening method for weathering resistance was described for use in the routine evaluation of experimental lines and/or new introductions. Indeed, data derived from this screen contributed to the subsequent release o f one selection as cv. Satin (R.J. Lawn, pers. comm.).
Variation among lines for weathering response was subsequently related to that in some 26 pod and seed traits potentially involved in weathering. No single trait was strongly associated with resistance to weathering in the screened material. Nevertheless, a weathering-resistant ideotype for mungbean was described as possessing the combination of a large pod wall weight to surface area (PW/SA), high levels of persistent hardseededness, and a large pod diameter. Both PW/SA and hardseededness appeared to reduce weather damage by increasing the resistance to water absorption by seeds within pods. Although models based on these traits were highly significant (P<0.01)), they accounted for a limited proportion of the variation in response .
The results of the screening and earlier studies showed that hardseededness can potentially reduce the susceptibility of mungbean to weather damage. Further studies involving this trait s howed that lines differed in the duration between the development of germinabilty and hardseededness, and in the persistence of seed impermeability during immersion. Hardseededness was regulated by the strophiole which, in most seeds, became permeable after immersion for five to 20 minutes in hot (60°C) water. Used commercially, this treatment would be expected to reduce storage costs a nd maximize sprouting uniformity. Furthermore, selection for high proportions of impermeable seeds would not necessarily preclude the use of seed for sowing or sprouting.
No hardseeded accession was agronomically acceptable and hybridization became necessary. The inheritance of hardseededness ranged from dominant, to partially dominant, to recessive, depending on the parental source and the duration of immersion. Frequency distributions of progeny in the F2 and F 3 generations were not distinct, and heritabilities were low to moderate. This may reflect variation in other traits associated with hardseededness , and/or the influence of the environment. Nevertheless, for crosses using the wild progenitor of mungbean , V. radiata ssp. sublobata, hardseededness appeared to be conditioned by a single major gene with complete dominance. Therefore, simple backcrossing techniques have the potential to transfer the hardseeded trait into more acceptable germplasm.
Hardseededness was however, only one of the traits described for the weather-resistant ideotype. Additional studies should investigate the other traits, particularly PW/SA, identify further traits which affect weathering, and complete the description of the ideotype. Nevertheless, this study has improved the process of breeding for weathering resistance and will contribute toward the production of a mungbean cultivar which can better resist the effects of pre-harvest rainfall.