Comparative Study on Photoinhibition Characteristics and Adaptation Mechanisms of New Current-Year Needles of Pinus densiflora var. zhangwuensis and Pinus sylvestris var. mongolica in Sandy Land Environments

Whether photoinhibition occurs in tall trees of Pinus densiflora var. zhangwuensis and P. sylvestris var. mongolica in summer and their response mechanisms to strong midday light in sandy land are not yet clear. Therefore, three standard trees were selected for each species as biological replicates by randomly setting up standard plots. Daily changes in the chlorophyll a fluorescence parameter of current-year needles (CYNs) and one-year needles (OYNs) in different parts of the standard tree crowns (in sun branches and shade branches) of the two species were comparatively studied, and the potential hydraulic transport capacities of the sample branches were also evaluated. The results are as follows: (1) The daily changes in the maximal photochemical efficiency of PSII (Fv/Fm) in the sun CYNs of P. sylvestris var. mongolica obviously presented a descending–ascending trend, and its lowest value was observed at noon. The maximum non-photochemical quenching (NPQ) of the sun CYNs in this species was observed at noon, and an NPQ of greater than 1 lasted for 4–5 h. (2) The midday Fv/Fm of the sun CYNs in P. densiflora var. zhangwuensis was close to the morning and shade CYN levels. Compared with P. sylvestris var. mongolica, the NPQ of the sun CYNs of P. densiflora var. zhangwuensis remained low under the condition of high light at noon, but the number of active reaction centers per unit cross-sectional area (RC/CSm) and the electron transfer efficiency (Fm/Fo) were higher. Further analysis of the variation in the light energy absorbed by each reaction center (ABS/RC) showed that the variation in the NPQ in the sun CYNs of the two species was very similar to that of ABS/RC, and the two indices had a very significant positive correlation (the correlation coefficients of P. sylvestris var. mongolica and P. densiflora var. zhangwuensis were 0.7535 and 0.7193, respectively, p < 0.001). (3) Through the comparison of the tracheid morphologies, our study also found that the outer and inner diameters of the pit of P. densiflora var. zhangwuensis were 1.26 and 1.28 times those of the pit of P. sylvestris var. mongolica, respectively, and the theoretical hydraulic conductance (Kt) in the sun branches of the former was 6.32 times that of the latter. The above results suggested that photoinhibition might also be involved in the tall trees of P. sylvestris var. mongolica, while P. densiflora var. zhangwuensis had a more effective mechanism to avoid photoinhibition.