SO₂:Nutrient or toxin for Chinese cabbage

Chinese cabbage (Brassica pekinensis) is one of the most important high-yield vegetable crops in China, and is often cultivated around big cities. Atmospheric SO2 pollution may affect Chinese cabbage, which is usually produced under intensive farming practice with low-sulfur or even sulfur-free fertilizers. In this thesis, the interaction between atmospheric SO2 and sulfate nutrition of the root was studied in order to evaluate whether SO2 may be considered as toxin or nutrient for Chinese cabbage and to what extent sulfur fertilization in the field needs to be adjusted to the level of atmospheric SO2 pollution. In Chapter 2, SO2, NOX and acid deposition problems in China and their impact on agriculture are summarized. China has experienced an unprecedented period of rapid economic growth resulting in a high demand for energy. The enhanced energy consumption has resulted in high levels of air pollution viz. SO2, NOx, particulates and in acid rain. As a consequence of the increase in air pollution, visible injury and growth and yield reductions have been observed in and around several industrialized areas in China. Vegetables, fruit trees and agricultural/horticultural crops appear most at risk, since most of them are grown close to the densely populated areas. In Chapter 3, the interaction between atmospheric SO2 exposure (0.06 to 0.18 μl l-1) and sulfate nutrition of the root was investigated with a Dutch cultivar of Chinese cabbage (Brassica pekinensis, cv. Kasumi F1). The results demonstrated that in the absence of sulfate in the root environment, Chinese cabbage was able to utilize atmospheric SO2 absorbed by the shoot as sulfur source for growth. Chapter 4 describes the impact of various SO2 concentrations on growth, sulfur and nitrogen metabolism and the relevance of toxic and nutritional effects of SO2 in two cultivars of Chinese cabbage, a Dutch cultivar (Brassica pekinensis, cv. Kasumi F1) and a local Chinese cultivar (Brassica pekinensis, cv. Beijing 3). In both cultivars, there was a linear relation between the uptake of SO2 and atmospheric SO2 over a wide concentration range. The impact of SO2 on Chinese cabbage seemed to be paradoxical. On the one hand, SO2 taken up by the shoot was beneficial when sulfate was absent in the root environment, since an atmospheric SO2 level as low as 0.06 μl l-1 appeared to be sufficient to cover the plants’ sulfur requirement for growth. On the other hand, Chinese cabbage appeared to be rather susceptible to SO2. Shoot biomass production was reduced upon prolonged exposure to ≥ 0.1μl l-1. Furthermore, SO2 exposure resulted in an increased level of sulfur metabolites (sulfate, thiols and total sulfur) in the shoot with the atmospheric concentration. The absence of sulfate in the root environment resulted in a shift in shoot to root biomass partitioning during growth in favor of that of the root, which was not alleviated when SO2 was used as sulfur source for growth. The possible consequence of a change in shoot to root partitioning for Chinese cabbage growing under field conditions is discussed. In Chapter 5, the ability of Chinese cabbage to utilize SO2 as sulfur source in relation to the sulfur status of the plant was investigated in more detail by exposing seedlings at different developmental stages and levels of root sulfate nutrition. If seedlings of Chinese cabbage were transferred to a nutrient solution without sulfate directly after germination, plants became rapidly sulfur deficient, and plant development was impaired. Plant biomass production was decreased, dry matter content of the shoot increased and the shoot to root biomass ratio decreased. The sulfate uptake capacity of the root was strongly increased. In the absence of sulfate in the root environment only leaves, which were already present at the start of the exposure, benefited from the utilization of the absorbed SO2. Furthermore, SO2 exposure did not affect sulfate uptake by the roots. In Chapter 6, the sulfur status of Chinese soils and the response of Chinese cabbage to sulfur fertilization in field experiments in the Beijing area are presented. During recent years, sulfur deficiency has become a major problem in agricultural crops throughout China, due to an imbalance of sulfur in relation to N, P and K in the fertilizers applied. One-fourth of the tested Chinese soils appeared to be sulfur deficient. Pot experiments in the field in the Beijing area showed that shoot biomass production of Chinese cabbage was significantly enhanced upon sulfur fertilization of the soil. A level of fertilization of 15 to 30 kg S ha-1 was sufficient to optimize yield. Still, the level of fertilization in other regions in China might have to be adjusted to the level of local atmospheric sulfur deposition. In Chapter 7, the relevance of toxicity versus metabolism of SO2 for Chinese cabbage and the significance of shoot to root signaling in sulfur uptake and assimilation are discussed. The impact of atmospheric SO2 on crops is ambiguous and complicates sulfur fertilization recommendations, since it may act as both toxin and nutrient. Despite the fact that SO2 absorbed by the shoot can be used as sulfur source for growth, there was a poor shoot to root signaling in the uptake of sulfate by the root upon SO2 exposure. Furthermore, SO2 hardly affected the shift in shoot to root biomass partitioning, which occurred in the absence of sulfate in the root environment