Summary, Ingunn Molund Vågen

Instituttt for plante- og miljøvitenskap

The vegetable broccoli (Brassica oleracea var. italica) has become increasingly popular during the last decades, and has received much attention due to its high content of health-promoting substances. As broccoli is a nutrient demanding crop with a low harvest index, nitrogen fertilization practices are of primary concern for the development of environmentally friendly cropping practices. Specific information on nitrogen effects in broccoli under Norwegian climatic conditions have been more or less absent, and the objective of this study was therefore to explore the uptake and utilization of nitrogen under the local conditions throughout the growing season.

Eight field experiments were performed during the years 1999-2001 at Apelsvoll Research Centre, Division Landvik in Grimstad, South Norway (58°20’N). The experiments were conducted with an early and a late cultivar (Milady F1 and Marathon F1, respectively) during the first or latter part of the growing season (planting in May and July, respectively) and at three nitrogen application rates (0, 120, and 240 kg N ha-1). There was a high degree of variation among the experimental sites in the level of mineralized nitrogen (Nmin) at planting, thus the available N (Nmin at planting + applied N) ranged from 29 to 356 kg ha-1 in the topsoil (0–30 cm).

The current recommendation for broccoli production in Norway prescribes a total input of 200 to 250 kg N ha-1, adjusted for soil mineral nitrogen. The results of this study confirm this recommendation, leaving no doubt that an economically viable production of this short-lived crop is impossible without a high pool of readily available nitrogen in the soil. On average for all experiments, the fresh weight yield of broccoli heads was 6 230, 14 440, and 18 360 kg ha-1 at an N input of 0, 120 and 240 kg N ha-1, respectively.

The main effects of cultivar and planting time were generally small compared to those of nitrogen. Marathon tended to give the higher yield in the early plantings, while Milady was slightly superior in the late plantings. Because of an earlier head initiation, the growing period from planting to harvest was always shorter in the early plantings.

The response of broccoli to nitrogen is described in Paper I, which sees the yield formation in relation to the nitrogen dynamics of plants and soil over time, and analyses how efficiently nitrogen is utilized to produce a saleable yield. The actual N uptake of the crop appeared to be more critical for fresh weight yield than was the dry matter amount of biomass produced to support broccoli heads.

The average harvest index was 0.17, 0.23 and 0.26 at N rates of 0, 120 and 240 kg N ha-1, respectively. As the corresponding values for N fraction allocated to the heads were 0.28, 0.31 and 0.32, the broccoli heads acted as nitrogen sinks during inflorescence development. However, the fact that increasing N rate had a stronger impact on HI than on N fraction allocated to the heads also indicates that abundant supply of N during the latter part of the growing period reduced the natural relocation of N from stems and leaves to heads. On average for all treatments, only 30% of the nitrogen taken up was removed in broccoli heads, the remaining being left in crop residues in the field. The low harvest index and fraction of nitrogen found in heads is a large environmental challenge in broccoli production.

The growth analysis presented in Paper II shows that an underoptimal supply of nitrogen during the first half of the growing period, especially in combination with low temperatures, limited broccoli yields because of too slow development of an adequate leaf area. On the other hand, abundant supplies of nitrogen during the second part of the growing period resulted in a leaf area beyond what was necessary for maximal light interception and head yield. An evaluation of an exponential and a linear equation for calculating the critical N concentration for maximal growth (Nc) indicated that the latter was better with respect to level of estimates of relative values for biomass production, leaf area index (LAI), accumulated intercepted PAR, and radiation use efficiency (RUE).

Paper III explores the nitrogen productivity concept as a tool to analyze nitrogen utilization of broccoli. The average nitrogen use efficiency (NUE) declined from 50.2 g DM g-1 N at no N application to 33.0 and 26.7 g DM g-1 N at inputs of 120 and 240 kg N ha-1, respectively. This decline was partly due to a lower nitrogen productivity (NP), but, more importantly, to a much shorter mean residence time (MRT) for nitrogen on plots receiving nitrogen fertilizer. The nitrogen productivity declined with increasing biomass at the same rate as calculated critical N concentration. The NP analysis also revealed that MRT and plant N concentration (PNC) were negatively correlated, and there may well have been a negative correlation between MRT and net photosynthesis. Mean utilization time (MUT) is suggested as a more appropriate term than MRT in agricultural production systems, and low MUT values might be an indicator of excessive N uptake.

The present study gives no prospect that it is possible to maximise the saleable yield, and, at the same time, the NUE of a broccoli crop. By doing detailed experiments with more N rates and application times than were practised in the present study, it might, however, be possible to identify threshold values for NP and MUT that would allow a high yield without allowing excessive protein turnover and thus maintenance respiration due to luxury consumption of N. A reduction in the nitrogen input during the latter part of the growing period (>6 weeks after planting) may well enhance the NP, MUT and NUE of the system; reduce the amount of N-rich residues to be mineralized after harvest, and, at the same time maintain a high level of intercepted photosynthetic radiation (PAR), radiation use efficiency (RUE) and broccoli yield.

Vågen, I.M. 2005. Nitrogen uptake and utilization in broccoli (Brassica oleracea var. italica). Doctor Scientiarum Theses 2005:12. Norwegian University of Life Sciences. ISBN 82-575-0556-8.

Keywords: Biomass, critical N, fertilizer, growth analysis, nitrogen productivity, nitrogen use efficiency, Nmin, recovery, relative growth rate, vegetable, yield.


Oppdatert: 17.08.05
Utskriftsvennlig versjon : Arkiv : Tips en venn