The response of sugar beet (Beta vulgaris L.) to planting method and sowing date

Abstract

Effects of transplanting and agronomic treatments (plant arrangement, plant population, sowing date) were investigated on crops of sugar beet (Beta vulgaris L. cv. Amazon) during 1981/82 and 1982/83 seasons at Lincoln, Canterbury. In addition, the influence of transplanting at different growth stages (cotyledon, 2-leaf, 4-leaf) was also examined. Measurements of crop physiological and environmental variables were made to establish the causes underlying the variations in sugar yield associated with treatments.

The development of leaf area index (LAI) was examined in different planting methods during the 1981/82 season. Detailed measurements of leaf number, leaf death and leaf area were made and the expansion growth of individual leaves (leaves 5, 10, 15, 20, 25, logistic curves fitted to the parameters of growth duration of expansion) (final 30, 35) was studied using primary data to estimate the leaf size, mean rate and duration of expansion).

Rates of leaf appearance and leaf death were linearly related to thermal time (accumulated temperature above 2°C), and were similar irrespective of planting method. The greater number of leaves plant⁻¹ in the transplanted beet compared with the seed-sown beet was related to the method of plant establishment.

Individual leaf size varied from position to position -increasing in area up to the 10th leaf and then progressively decreasing. Planting method significantly affected the final size of individual leaves, particularly up to leaf 20. Variations in final leaf size were differentially affected by changes in the determining components. The rate and duration of expansion were attributable to the temperatures experienced by individual leaves prior to unfolding. The rate and duration of expansion of individual leaves were strongly negatively related. Final leaf size tended to be a more stable characteristic than either of its components.

The expansion of LAI up to a maximum was a function of thermal time (above 4°C) in all the planting methods. None of the treatments significantly affected the thermal rate (pL) or the duration (dL) of LAI increase. It was concluded that variations in LAI increase between planting methods were, therefore, caused by the number of leaves plant⁻¹, which were, in turn, due to method of plant establishment.

Sugar yield of the transplanted beet was significantly greater than the seed-sown beet in both seasons. The average response to transplanting was 252 g m⁻² (22%) in 1981/82 and 209 g m⁻² (16%) in 1982/83, above the yields obtained from the seed-sown beet. This was due mainly to the production of heavier roots. The final sugar percentage did not vary with treatment. Plants transplanted at the 4- leaf stage increased root and sugar yields more than those transplanted at the 2-leaf stage or the cotyledon stage. In the 1981/82 experiment there were no major differences between square and rectangular planting arrangements when the population was 10 plants m⁻².

Crops sown in mid August (S1 sowing) produced 15.4% more sugar than those sown in mid September (S2 sowing). This yield improvement in the Sl sowing resulted from greater root fresh weight as well as higher sugar percentage. A population of 10 plants m⁻² (P2 population) had a significantly greater sugar yield than a population of 6 plants m⁻² (P1 population), due to greater root weight per unit area. The sugar percentage was similar for the two plant populations. Final sugar yield was 19% greater at the higher density. There was a significant interaction between plant population and planting method (P x T), at the final harvest, for both root and sugar yield. The response to transplanting, especially at the 4-leaf stage, varied with plant population, being greater at the P2 population that at the P1 population.

Variations in total dry matter (TDM), root DM and sugar yield were analysed in terms of the amount of photosynthetically active radiation (PAR) absorbed and its efficiency of use (ε, gMJ⁻¹) in DM or sugar production. The relationship between TDM, root DM or sugar yield and accumulated PAR was linear in both seasons. The slope of the regression line represented a constant mean seasonal ε in DM or sugar yield. In addition, the efficiencies of radiation conversion to TDM (εT), root DM (εR) and sugar (εS) production were estimated at various harvests during the seasons.

The value of εT peaked to 1.79 g MJ⁻¹ in March (1981/82) and 2.01 g MJ⁻¹ in February (1982/83) respectively; but decreased thereafter until the final harvest. The value of εR increased throughout the growing season; giving an average value of 1.38 g MJ⁻¹ in 1981/82 and 1.55 g MJ⁻¹ in 1982/83. A stable value of 1.0 g MJ⁻¹ for the εS was observed in both seasons.

Transplanting, the Sl sowing and the P2 population were all important in ensuring maximum radiation absorption. Differences in DM or sugar yield between the treatments were related to the amount of PAR absorbed, its ε or both. Transplanted beet increased both the amount of PAR, by enhancing the size of the canopy early in the season, and the ε into DM or sugar production over seed-sown beet in both seasons. Similarly, plants established at the 4-leaf stage were superior to those at the 2-leaf stage or the cotyledon stage in the amount of PAR absorbed; the ε being inconsistent between different growth stages of transplants during the seasons. Seasonal differences in DM or sugar yield between sowing dates were entirely associated with the radiation absorption. Between the two plant populations, differences in yield were due to the amount of PAR absorbed and to changes in ε.

Although TDM yield was greatest with transplanting, early sowing and high plant population, the proportion of DM in the roots was not affected by the treatments. Both the Sl sowing and the transplanted beet increased root: shoot ratio over the S2 sowing or the seed-sown beet respectively. The ratio was similar for the two plant populations. An extension of the radiation interception analysis to evaluate the partitioning of TDM to root DM or sugar yield also indicated the same phenomenon. The ratios εR/εT (partitioning of TDM to root DM) and εS/εT (partitioning of TDM to sugar) were significantly affected by transplanting and the Sl sowing, but not by plant population. In contrast, the ratio εS/εR (partitioning of root DM to sugar) was similar in all the treatments in both seasons.

This study examined the effects of treatments on bolting and its effects on sugar yield. Sowing dates covered a range from mid August to early October in the two seasons. Few bolters were found when the crop was sown in mid September (1982/83) or early October (1981/82). However, mid August sowing (1982/83) increased the incidence of bolting to 4%. The incidence of bolting was variable between different stages of transplants. Transplanting at the 4-leaf stage was especially at risk, giving about 7% bolting compared with 2.3% for plants transplanted at the 2-leaf stage.

Bolters, especially the early bolters (plants with true seed-stalk), significantly decreased sugar yield by reducing both root fresh weight and sugar percentage. Conversely, root yield of the late bolters (leafy seed-stalk) was higher than for the normal plants. Sugar percentage was decreased more in the former than the latter. Harvest index (HI) was significantly reduced by bolting compared to normal plants.

The results suggest that higher sugar yield and greater economic benefits will be obtained under Canterbury conditions when the transplanted beet, especially at the 4- leaf stage, is sown early in spring (September) at a plant population greater than 6 plants m⁻².