Invloed van vog- en ligstremming op die ontwikkeling en bioproduktiwiteit van Glycine max (L) Merr. genotipes van verskillende volwassenheidsgroeperings

Abstract

Trials were conducted during the 1997/98 and 1998/99 growing seasons to determine (1) the influence of drought and low light stress on soybean development and productivity, (2) to set up a data base to calibrate computerised crop models and (3) to identify possible drought avoident culitivars as well as the mechanism of drought avoidance. During the 1997/98 season cultivars from an early, intermediate and late maturity group namely Columbus, A5409 and Ibis respectively, were planted under dryland and irrigation conditions at Brits and Potchefstroom. During the following season A5409 and Ibis were planted under rain shelters at Roodeplaat ARC Institute for Soil, Climate and Water. The treatments consisted of optimal plots, low-light-plots, rain-only-plots and water deficit plots. The optimal and low light plots were kept at field capacity with the low light plots only receiving 20% of the normal light intensity during the entire duration of the season. The rain-only-plots received rain as only means of irrigation, while the water deficit plots received neither rain nor irrigation and were only irrigated when plants started to die off. Growth-analysis techniques in conjunction with physiological and water studies were used to achieve the above mentioned goals. Results indicated that less than 450 mm seasonal soil water caused yield losses to the extend of the economical threshold value of 1,5 tons/ha. The yield components that contribute to this loss were pods per plant, seeds per pod and seed mass. The cultivar Ibis was less susceptible to drought than A5409, because off its ability to extract available soil moisture much deeper and more effectively. Low light intensity caused hastening of flower and pod formation and a loss of 50% biomass for A5409, but 0% loss for Ibis. Loss in yield due to low light was 52% for A5409 and 58% for Ibis respectively although low light intensity had no significant effect on seed mass. Chlorophyll fluorescence data indicated that drought deactivates photosystem II reaction centres with an concommitant increase in the efficiency of the remaining reaction centres. Low light intensity caused an increase in the density of reaction centres and resulted in an increase in the quantum use efficiency.