Matter transport, photosynthetic characteristics and fluorescence dynamics of 2 Brassica rapa cultivars with different freezing-tolerance in Northwest China.

To clarify the relationship between material transport, photosynthesis, fluorescence dynamics and cold tolerance of winter turnip rape (Brassica rapa L.) and to further provide theoretical basis for rape breeding and evaluation of cold resistance, 2 cultivars Longyou 7 (strong freezing-tolerance) and Tianyou 4 (weak freezing-tolerance) were determined in natural temperature falling. Effects of low temperature on dry matter distribution, photosynthetic performance and kinetic curve of chlorophyll fluorescence induction were investigated under field conditions. It was shown that dry matter and distribution rates of dry matter in roots of both cultivars increased, but photosynthetic rate, transpiration rate, Fo (fluorescence at O-step at 20 microseconds), F_k (fluorescence at K-step, 300 microseconds), F_j (fluorescence at J-step at 2 millisecond) and Fm (dark-adapted maximum fluorescence at P-step) of chlorophyll fluorescence induced kinetics curves of O-J-I-P decreased, with the decreasing temperature. Thus the decline of photosynthetic performance might help accumulation of dry matter in roots at low temperature. Photosystem damage and photo inhibition occurred, which presented with falling of PSII receptors (area), photosynthetic performance index (PI), maximum fluorescence (Fm) and maximum photochemical efficiency (Fv/Fm), except the rising of initial fluorescence (Fo). Longyou 7 had lower Pn, Fm, Fv/Fm, and had lower potential activity of PSII (Fv/ Fo) and reduced capacity of Q_A and Q_B. It showed weaker photosynthetic capacity and stronger photo inhibition in Longyou 7 than those of Tianyou 4 under low temperatures. These indicated that the photosynthetic capacity of Longyou 7 decreased at low temperature before winter, which might be due to non-stomatal factors, and low temperature might lead to enhanced photoinhibition. In summary mary, the cold resistance of winter turnip rape was related to leaf photosynthetic characteristics and dry matter accumulation under low temperature. Weak photosynthesis could reduce growth activity of aboveground parts and root matter consumption, which was conducive to dry matter accumulation of rape root to enhance its cold resistance. [ABSTRACT FROM AUTHOR]