Differential Effects of Cold Stress on Chloroplasts Structures and Photosynthetic Characteristics in Cold-Sensitive and Cold-Tolerant Cultivars of Sugarcane

Abstract

Freezing often results in a significant loss of sugarcane production. An investigation was conducted in two sugarcane cultivars, GT28 (Guitang 28, cold tolerant) and YL6 (Yuanlin 6, cold sensitive) to understand the cold tolerance mechanisms. The plants at early ripening stage, 245 days after planting, were exposed to cold stress (0 °C) for 0, 2, 4, and 6 days. The changes in chloroplast structure, microtubule, and physiological parameters under cold stress were determined. The morphology and chloroplast ultrastructure of the cultivar GT28 were observed normal while those of the cultivar YL6 was severely damaged and accumulation of starch grains in chloroplasts was evident under cold stress. At the beginning of cold stress, the microtubules in the cultivar GT28 was more obviously depolymerized than those in the cultivar YL6. With extended cold stress, the microtubules in the cultivar YL6 decreased noticeably and the chloroplast membrane became indistinct, while the microtubule fluorescence in the cultivar GT28 was intensified, indicating that the structure of microtubules was reorganized. Consequently, the cultivar GT28 showed a greater photosynthetic activity than YL6 under cold stress. The cold tolerance of the cultivar GT28 appeared to be related to the integrity of chloroplast structure, the stability of microtubule structure, and balanced physiological metabolisms. These results revealed that more stable chloroplast structure and microtubule structure under cold stress are the important physiological foundation of cold-tolerant sugarcane cultivars.