The manipulation of sugar beet (Beta vulgaris. L.) growth and development by paclobutrazol and mefluidide

Abstract

Effects of the plant growth regulator paclobutrazol were measured on crops of sugar beet (Beta vulgaris L. cv. Vytomo) at Lincoln College, Canterbury, sown on 10 September 1982 and 16 September 1983. In 1982-83, on a Wakanui silt loam soil, paclobutrazol at 0, 1 and 2kg ai.ha⁻¹ was applied when either 5 or 12 true leaves were present at plant populations of 80 000, 120 000, or 160 000 plants.ha⁻¹ drilled in 500nm rows. On a Templeton site loam soil in 1983-84 1.5kg ai.ha⁻¹ paclobutrazol was compared with a control when sprayed at the 12 leaf stage on plant populations of 82 000, 167 000, 231 000, 328 000, or 415 000 plants.ha⁻¹. Plant populations were in 500, 350, 290, 250, or 220nm rows respectively. Measurements were made on crop yield, radiation interception, sugar content, and plant morphology.

The only interaction between paclobutrazol and plant population occurred in 1983-84 when harvest index with paclobutrazol was maintained at 0.848 but declined linearly with increasing population when not treated. Throughout the trial there were seldom any differences between 1 and 2kg ai.ha⁻¹ paclobutrazol or between the application times. Shoot weight decreased when paclobutrazol was used and this was associated with a decrease in petiole weight. The amount of dead material was reduced in both years but root weight was diminished only in the first year. Dry matter percentages generally showed an increase in the above ground components with the use of paclobutrazol of 0.6 to 1.3% at the final harvest but there was no change in either the root dry matter percentage or sugar percentage.

Radiation interception was decreased by 5 to 9% with paclobutrazol but LAI decreased by between 20 and 30% at its peak. Crop growth rates were related to intercepted PPFD and were not significantly different when paclobutrazol was used. Maximum efficiency of conversion of radiant energy to dry matter production was between 2 and 3g.MJ⁻¹ but decreased towards the end of the season as crop weight increased.

Paclobutrazol reduced leaf length and width by between 20 and 30% but thickness increased by up to 50%. Treated plants had more leaves per square metre and a lower number of dead leaves. Immediately after treatment, leaf death increased slightly in both years but leaves formed after spraying lived longer. Chlorophyll content per gram fresh weight of leaf lamina was unchanged in January but was increased later in the season by paclobutrazol. However, leaf area reductions meant that the amount of chlorophyll per leaf stayed the same or declined. The number of stomata per unit leaf area increased but on a per leaf basis remained the same.

Higher plant population increased shoot weight during the mid-season but by the final harvest on 1 June 1984 the peak population was 167 000 plants.ha⁻¹. Root yield measured in March 1984 was greatest at the higher plant populations but at the final harvest root yield was greatest at 160 000 and 167 000 plants.ha⁻¹ in the 1982-83 and 1983-84 seasons respectively. There was more dead material as plant population increased, while less of the total shoot weight was allocated to leaf lamina at higher populations. The dry matter percentage of above ground components increased with population but root dry matter was unaffected. Both sugar content and yield were unchanged by plant population.

Above 160 000 plants.ha⁻¹ LAI remained similar. However, higher populations intercepted more radiation than lower ones until February and again from April to the end of the season. The lowest population showed an increase in radiation interception until late March but interception at higher populations peaked in late January. Both live and dead leaf numbers increased linearly with increasing population. Increasing plant population decreased the number of leaves per plant but had only minor effects on leaf longevity. All leaf dimensions were reduced by increases in plant population.

In a further trial in 1982-83 the use of both paclobutrazol at 0.75kg ai.ha⁻¹ and mefluidide at 1.5kg ai.ha⁻¹ to control spring bolting of an overwintered crop of sugar beet (cv. Monoire) was tested. The crop was sprayed in mid May, June or July and crop yield monitored until late October.

Bolting started in late August and by mid September 56.5% of control plants and 20% of treated plants had visible signs of bolting. By late October all plants showed visible signs of bolting. Overall, paclobutrazol was more effective in reducing stem elongation than mefluidide.

Shoot dry weight declined during winter to 280g.m⁻² then increased throughout September and October. Both chemicals retarded shoot yield during the spring, while root dry weight was not significantly affected by chemical application, and peaked at 1 678g.m⁻² In mid September, over 100g.m⁻² greater than in May and July. Sugar content remained high until mid September (17.9%) but then declined to late October (15.2%). Sugar yields peaked at maximum of 1 280g.m⁻² in September, 150g.m⁻² greater than in May or July. Chemical treatments had no effect on sugar yield.