The effects of plant population, growth regulators and irrigation on development and yield of spring sown field beans (Vicia faba L.)

Two field experiment were conducted during the years 1982/83 and 1983/84 to investigate the effect of two foliar applied plant growth regulators (PGRs) TIBA and paclobutrazol (PP333), on field beans (Vicia faba L.) cv. Maris Bead. In the first experiment, the PGRs were applied at the 6 and 9-10 (flowering) leaf stage. Plant populations were 50, 70 and 90 plants m⁻². In the second season PGRs were applied only at the 9-10 leaf stage. Sprinkler irrigation was introduced as a treatment and plants were grown at population of 40, 80 and 100 plants m⁻². In both seasons dry matter (DM) production was strongly and linearly related to absorbed photosynthetically active radiation (PAR) up to pod set. Thereafter the amount of OM produced per unit of absorbed PAR declined. Plants treated with both PGRs intercepted a similar amount of PAR and therefore total OM production did not differ compared with the control. The time of PGR application had no effect on crop growth and yield. Irrigation increased total OM production by 35% and this was associated with a better conversion of PAR into plant OM rather than in increased absorption of PAR by the leaf canopy. In 1983/84 the plants were taller than in 1982/83. Plant height was greater at the higher plant populations until pod set. This was associated with an increase in mean internode length. Paclobutrazol decreased plant height and stem OM in both seasons by reducing both internode length and weight. TIBA also reduced plant height and weight when conditions were favourable for production of taller plant (1983/84 season). Irrigation increased plant height by increasing both internode length and weight. Application of paclobutrazol increased flower set per node from 4 to 5 possibly by improving assimilte supply. Paclobutrazol treated plants bore flowers and pods up to node 20 compared with node 19 on control and TIBA treated plants. However, mature pods did not form on node 20. Paclobutrazol also increased pod set on branches possibly by the diversion of assimilates from mainstems which were lighter and shorter. Fewer pods were set on the mainstem of TIBA treated plants although the number of flowers formed was unchanged. The yield in 1983/84 of 2.5 t ha⁻¹ was lower than the yield of 3.3 t ha⁻¹ in 1982/83. This was associated mainly with fewer seeds per pod 2.2 in 1983/84 compared with 2.9 in the 1982/83. Grain yield did not increase with increased plant population within the range of 40-100 plants m⁻². This was due to decline in the number of branches at high plant populations. Both TIBA and paclobutrazol increased grain yield from 2.9 to 3.2 t ha⁻¹ in 1982/83 when pod abscission was high under dry conditions. However, when soil and weather conditions produced considerable vegetative growth in 1983/84, TIBA increased branch survival but had no significant effect on seed yield. Higher branch survival was apparently achieved by the diversion of assimilates from the mainstem. Paclobutrazol increased grain yield in 1983/84 from 1.9 to 2.7 t ha⁻¹. This was due to both increased branch survival and increased pod retention on branches. In both seasons, paclobutrazol increased the harvest index (HI). This effect was most marked in the 1983/84 crop when HI was 25% in the paclobutrazol treated plants compared with 18% in the controls. However, in 1982/83 paclobutrazol only increased HI from 45% to 50%. Irrigation increased seed yield from 1.8 to 2.5 t ha⁻¹ by increasing pods per plant from 5.0 to 5.9 and mean seed weight from 350 to 374 mg. However maximum yield of 3.2 t ha⁻¹ was achieved when irrigation was applied with paclobutrazol. It was concluded that paclobutrazol has the potential to reduce vegetative growth and increase grain yield. TIBA however had inconsistent effects on changing plant morphology or grain yield.