Aims Our objective was to investigate the effect of sulfur acid rain (SAR), nitric acid rain (NAR) and their mixture (MAR) on the physiology, growth and yield of the crop rape (Brassica napus).
Methods Fifteen 4 m × 5 m plots were distributed stochastically for exploring SAR, NAR and MAR stress, with a roughly 0.6 m spacing between plots to avoid interference. We used three replicate treatments of pH = 7.0, 1.5, 3.1, 4.1 and 5.1 and examined growth and development at 5-day intervals from budding to the end of flowering (March 10 to April 25, 2006). On April 12 for the peak period of flowering, we measured leaf cell membrane permeability, contents of malondialdehyde (MDA) and photosynthesis pigment in leaves, photosynthesis speed, leaf injury rate and leaf area as physiological and growth parameters. On May 10, we measured the yield index during the ripening stage.
Important findings These kinds of acid rains can inhibit physiology, growth and yield with effects differing among treatments. At pH ≤ 4.1, SAR, NAR and MAR damage the membrane of rape leaves, decreasing photosynthetic pigment content and photosynthetic speed, thereby diminishing photosynthesis of the crop. For pH ≤3.1, leaf area is decreased and the leaf injury rate increases greatly. The value of pH = 4.1 can be used as the threshold for impact on rape yield. Over the range of pH = 1.5-7.0, leaf membrane permeability, MDA content and leaf injury rate follow the pattern of NAR > MAR > SAR and for the photosynthetic speed, content of photosynthesis pigment, leaf area and yield the order is SAR > MAR > NAR. At pH > 4.1, the differences between these treatments of acid rain are insignificant. For pH ≤ 3.1, the stress differences become large (p < 0.05), with greater differences at stronger acidities, following the pattern NAR > MAR > SAR. This indicates the first two treatments have stronger inhibiting effects on the physiology, growth and yield of rape.