Cold hardiness estimation of Pinus densiflora var. zhangwuensis based on changes in ionic leakage, chlorophyll fluorescence and other physiological activities under cold stress

Abstract

Pinus densiflora var. zhangwuensis grows fast, and its drought and salinity resistance are better than Pinus sylvestris var. mongolica. We compared cold hardiness and mechanisms of cold hardiness between the two species, to provide a theoretical basis for promoting and applying P. densiflora var. zhangwuensis in cold regions. A cold stress experiment was carried out on 3-year-old plantlets of P. densiflora var. zhangwuensis and P. sylvestris var. mongolica after hardening at five temperature regimes, 5, −10, −20, −40, and −60 °C, respectively. Some indices of needle samples for both species were measured, such as relative conductivity (REL), maximum photochemical efficiency (Fv/Fm), malondialdehyde (MDA), catalase (CAT), proline (Pro), soluble sugar (SS), and stomata density. REL and MDA values of both species after hardening had the same trend of increasing, but the trend was opposite in Fv/Fm value with increasing cold stress. Compared with P. sylvestris var. mongolica, the P. densiflora var. zhangwuensis had smaller increases in REL and MDA, and a smaller decline in Fv/Fm during cold stress. Compared to the control, REL growth of P. densiflora var. zhangwuensis and P. sylvestris var. mongolica at −60 °C were 0.41 and 0.60, and MDA growth was 29.94 mol g−1 FW and 47.80 mol g−1 FW, and Fv/Fm declines were 0.08 and 0.27. Half-lethal temperatures (LT50) calculated by logistic equation for P. densiflora var. zhangwuensis and P. sylvestris var. mongolica were −58.23 and −50.34 °C, respectively. These data suggest that cold resistance of P. densiflora var. zhangwuensis is stronger than that of P. sylvestris var. mongolica. Cold-resistance mechanisms of the two species differed. In response to cold stress, P. sylvestris var. mongolica had strong osmotic adjustment ability because of higher Pro and SS content, while P. densiflora var. zhangwuensis had strong antioxidant ability due to stronger CAT activity. Stomata density and diameter of P. densiflora var. zhangwuensis were smaller, as were single leaf area and number of leaves per plant, both characteristics promoting survival in a cold environment. Greater shoot height and total biomass of seedlings of P. densiflora var. zhangwuensis might be another reason for its stronger cold tolerance.