Abstract: 【Objective】 The objective of this paper is to investigate the effects of low nocturnal temperature on photosynthetic apparatus of winter rapeseed (Brassica campestris L.) and the associated differences between different cultivars. 【Method】 An artificial climate chamber was used to simulate the effects of low nocturnal temperature on stomatal morphologies, chloroplast ultrastructure, photosynthetic parameters, and dry matter distribution and accumulation in seedling leaves of two winter rapeseed cultivars, Longyou-7 with ultra cold resistance and Tianyou-2 with weak cold resistance. 【Result】 At diurnal/nocturnal temperatures of 20/10℃ (control), the leaves of Louyou-7 had lower growing point than those of Tianyou-2, both showed a creep plant type, a deep green leaf color, and similar levels of leaf chlorophyll content, stomatal conductance (G_s), intercellular CO₂ concentration (C_i), and photosynthetic rate (P_n). After nocturnal temperature drops to 5°C for 7 days, both cultivars had decreased G_s, C_i, and P_n but increased leaf chlorophyll content and root/shoot ratio, indicating that more photosynthetic products were transported with priority to the roots. Tianyou-2 had curved leaf margin and tip with spotty water stains, showed obvious symptoms of cold damage, whereas Longyou-7 had a depended leaf color, a flat leaf shape, and a subsided growing point with no symptoms of cold damage. After treatment at low nocturnal temperature, both cultivars had increased leaf chlorophyll content but the extent of increase differed between Tianyou-2 and Longyou-7 (6.0% vs. 9.6%). During this period, the majority of stomata were closed or semi-closed in Tianyou-2, with higher C_i but lower G_s and P_n than Longyou-7. These observations suggested that at low nocturnal temperature of 5℃, photosynthesis was significantly inhibited in Tianyou-2, mainly because of non-stomatal limitation. On the contrary, most stomata remained open in Longyou-7 at low nocturnal temperature of 5℃, associated with minor decreases in C_i and P_n but no significant differences from data of the control (diurnal/nocturnal temperatures of 20/10℃). These results demonstrated that Longyou-7 maintained relatively high photosynthetic rate at low nocturnal temperature. Regarding chloroplast ultrastructure, the leaves of control plants at diurnal/nocturnal temperatures of 20/10℃ had orderly arranged chloroplasts close to the inner wall of cells, with neat and dense stacks of grana lamellae in an orderly arrangement; the chloroplasts displayed a spindle or single-sided concave shape, containing starch granules which were more and bigger in Longyou-7 than in Tianyou-2. When the diurnal/nocturnal temperatures dropped to 20/-5℃, leaves of Tianyou-2 had the lower part completely dried up and the green-holding leaves with water stains as the symptoms of cold damage, whereas Longyou-7 had interior leaves in a deep green color and the lower part of leaves in a flat shape, with massive red color at the blade surface. With respect to chloroplast ultrastructure, Tianyou-2 had adjacent chloroplasts fused together, with ruptured chloroplast membrane, released inclusions, dissolved grana, and broken stromal lamellae, indicating complete damage of the ordered structure of chloroplast membrane. In contrast, Longyou-7 had chloroplasts retaining complete and clear outer membrane and partial structure of grana, with complete structure of stromal lamellae containing a small amount of starch granules. After treatment at low nocturnal temperature of -5℃, both cultivars had increased C_i and decreased chlorophyll content, G_s and P_n. However, Tianyou-2 had significantly lower chlorophyll content than Longyou-7 and the P_n rapidly declined to 0.210 μmol CO₂·m^-2·s^-1, with 255.2% decrease compared to the control. Longyou-7 had the P_n of 0.434 μmol CO₂·m^-2·s^-1, which was 2.06-fold that of Tianyou-2. These results indicated that Longyou-7 maintained relatively high photosynthetic rate after treatment at low nocturnal temperature. The root/shoot ratio was significantly higher in Longyou-7 than in Tianyou-2, reflecting that the former transported more photosynthetic products with priority to the roots for storage. 【Conclusion】 Low nocturnal temperature caused damage to the structure of photosynthetic membrane of chloroplasts and reduction of P_n in leaves of winter rapeseed, thus influencing photosynthetic processes in this crop. The reduction of P_n was mainly related to stomatal limitation at diurnal/nocturnal temperatures of 20/5℃ and non-stomatal limitation at diurnal/nocturnal temperatures of 20/-5℃.

Key words: winter rapeseed, low temperature, photosynthetic characteristics, chloroplast ultrastructure