Effects of direct film crop cover and top dress nitrogen on earliness and yield of broccoli crop (Brassica oleracea var. italica Plenk)

摘要

Air temperature and mineral nitrogen are major environmental factors affecting broccoli growth, development, earliness and yield. In order to quantify these effects, broccoli (cv. Marathon) was grown from January to April 1999, outside and under non-woven polypropylene crop cover (17 g m~(-2)) for the first 7 weeks after planting. Five rates of ammonium nitrate (0, 60, 120, 180 and 240 kg N ha~(-1)) were supplied as top dressings. Commercial maturity was attained 87 days after planting (or 1047 accumulated day degrees d deg C) under polypropylene, 12 days before outside crop (1126 d deg C), however yield was lower. At the end of spear maturity (4 to 7 days after commercial maturity), spear diameter under cover was similar to outside crop. At thistime shoot fresh weight increased with increasing nitrogen up to 180 kg N ha~(-1), and spear diameter increased up to 120 kg N ha~(-1). At commercial maturity for the covered crop, neither spear diameter (cm) nor yield (t ha~(-1)) responded to more than60 kg N ha~(-1). Since excess mineral nitrogen may delay the time taken to reach commercial maturity, and the fact that the crops did not respond to more than 120 kg N ha~(-1), it could be implied that higher rates of nitrogen may cause nitrate leachingor luxury nitrogen uptake. The relationship between spear growth rate and earliness is discussed on the basis of the thermal time and nitrogen, to explain the potential risks of high nitrogen rates under conditions of low temperature and heavy rain.