Plant growth and yield in beet in response to reduced plant numbers

The main object of this study was to investigate the effect of poor plant establishment on growth and yield of beet. Two experiments were undertaken with the approach of establishing poor plant stands by reduction of plant numbers from normal crops. In the first experiment, one fodder beet and two sugar beet cultivars were grown in populations of 75,000 and 150,000 plants ha⁻¹, and the stands were reduced by one-third and two-thirds respectively at a seedling age of 10 weeks. The second experiment was conducted with only one sugar beet cultivar; the crops were initially established in populations of 80,000 and 160,000 plants ha⁻¹ and half the plants were removed from both densities at 6, 10 and 14 weeks after germination in December, January and February respectively. The sugar beets showed significantly higher root dry matter and sugar yield than fodder beet. The superiority of sugar beet was attributed to its higher leaf area index (LAI), higher dry matter and sugar content in the roots, and a longer growth period. In both experiments the increase in population by double the optimum did not improve economic yields; individual root weight decreased by increasing population. Plant reduction from the stands had a substantial effect on growth and yield of the remaining plants. Reduction of one-third of the plants from an optimum population (75,000 plants ha⁻¹) , at the age of 10 weeks did not result in any significant loss in root dry matter and sugar yields compared with the normal stand. Similarly, on average, yield losses due to reduction of half the plants from 80,000 and 160,000 plants ha⁻¹ at six weeks were only slight; the differences were not significant. The effect of plant loss on yield was more drastic when the reduction occurred gradually at an older age of the plants; root and sugar production declined linearly at the rate of 1.3 and 0.9 t ha⁻¹ respectively as plant reduction became progressively later. However, the decreases in yield due to the loss of plants were not proportionate to the degree of reduction in plant numbers in the stand. Additional root growth achieved by each individual plant in reduced stands compensated for yield losses by missing plants. The amount of compensation depended on the initial population of the stand, the proportion of plant loss, and the age of the crop at the time of plant loss and hence the length of the remaining growing period. The extra growth in individual roots following plant removal was mainly due to increase in cell size of the parenchyma tissue and resulting increases in the cambial ring distances; these did not affect the dry matter or sucrose content of the roots. The results confirmed that beet plants have considerable potential for additional growth under poor establishment conditions to compensate for yield losses due to plants missing from the stand.