Factors influencing seed yield and quality of dwarf French bean (Phaseolus vulgaris L.) in Canterbury

Abstract

Treatments designed to influence seed quality were imposed on dwarf French bean plants during crop development and maturation. The effects of the treatments on seed yield and yield components, as well as the suitability of Canterbury for growing this crop as a seed crop, were determined.

In 1996/97 plants were grown under 50 % shade at a population of 82 plants/m² without fertiliser or irrigation (high competition, HC) or without shade at 14 plants/m² with irrigation and fertiliser (low competition, LC). Other treatments were total defoliation or no defoliation, and 50 % pod removal or no pod removal. The effects of shading and defoliation were studied in more detail in a glasshouse experiment in the same season. In 1997/98, duration and timing of crop development were studied. Seeds were sown on 3 dates (29/10/97, 12/11/97 and 26/11/97) at 3 plant populations (40, 70 and 100 plants/m²) and plants were either desiccated approximately 9 days before one of 3 harvests (800°Cdays, 860 °C days or 920 °C days) or were harvested at the same times, without desiccation. The influences of weathering and mechanical threshing on seed quality were also studied.

In 1996/97, HC increased total dry matter (TDM) from 194 g/m² to 311 g/m² and seed yield from 70 to 151 g/m² while reducing thousand seed weight (TSW) from 367 to 339 g. The higher plant population reduced TSW. Defoliation reduced yield from 141 to 80 g/m² because of a reduction of pods/plant and TSW, the latter being reduced from 388 to 318 g. Removing 50 % of the pods reduced seed yield by only 30 % but did not increase TSW. In the following year, increasing plant population from 41 plants/m² to 94 plants/m² did not change TDM, but seed yield increased from 266 g/m² to 311 g/m². There was no change in TSW.

In the cold 1996/97 season, the high plant population accelerated crop development and the HC crop reached maximum TDM 30 days earlier than the LC crop. The LC crop was killed by an early autumn frost and many seeds were harvested pre-maturely. In the warmer 1997/98 season, no differences in time to maturity were found among the 3 plant populations or 3 sowing dates. Late October sown plants produced more TDM (834 g/m²), a lower seed yield (264 g/m²) and a higher TSW (252 g) than any November sown plants, which yielded on average 760 g/m² TDM and 308 g/m² seed with a TSW of 238 g.

Carbohydrate supply strongly influenced TSW, which influenced seedling dry weight. However, no relationships were found between TSW and germination, strong seedlings, accelerated ageing (AA) or electro conductivity (EC). Despite all the treatments given to the plants, the only other seed quality changes found were related to climatic conditions at maturation and harvest. In 1996/97 frosted LC plants produced seed with a reduced germination (from 90 % in early maturing HC crops to 84 % in LC crops which had not matured by late April). Since 1997/98 was one of the warmest seasons in the last 40 years, there was no scope for under-canopy temperature increase or accelerated maturation in high-density crops. Desiccation reduced the time to harvest maturity by ca. 10 days and reduced TSW in the early harvests from 250 to as low as 195 g. Seed quality was not affected by the size of the seed, but green seeds harvested were not viable. The optimum seed moisture content (SMC) for mechanical threshing was between 15 and 25 %.

Despite favourable maturation conditions in the second year, 22 mm of rain which fell over a period of a week in mid-March before harvest maturity had been reached, did affect seed quality. Seeds that still had a SMC of 25 % or higher during that week were not affected. However, despite showing only a small reduction in seed germination, seeds that, at the time of the rain, had a SMC of 20 % or less had lower vigour (as assessed by AA, strong seedlings and EC).

While it proved almost impossible to reduce seed quality by applying severe treatments to the mother-plant, both mechanical threshing and mild weathering reduced germination from over 90% to 80 %, while AA and strong seedlings were decreased by 15- 25 %. To improve bean seed quality it is more important to look at environmental conditions during maturation and harvest time, seed weathering and harvesting techniques, than at crop husbandry and conditions during seed development.