Growth and chlorophyll fluorescence characteristics of mulberry trees in simulated environment of heterogeneous habitats of a rocky desertification area.

The tropic growth of mulberry roots in response to different water and nutrient conditions was studied in a simulated rocky desertification habitat. Four treatment groups were compared, including group H (variable moisture), group F (variable nutrients), group HF (both moisture and nutrients varying) and group CK (control), which were designed to simulate the heterogeneous soil conditions associated with rocky desertification. The experiment was conducted to study the influence of root tropisms on the growth and chlorophyll fluorescence characteristics of mulberry trees that grew in the stressful environment. The results showed that: 1) the roots of mulberry trees showed obvious hydro-tropism and fertilizer-tropism, but the fertilizer-tropism of mulberry roots did not occur in the condition of moisture shortage. 2) The mulberry roots in group H and HF both showed obvious tropic growth that promoted absorption to water for its growth, while in the group F and CK, roots showed no tropism characteristics and the trees grew under the drought conditions for a long term, maintaining a high root-shoot ratio. 3) Root surface area, total root length, number of root tips, root biomass, leaf area, plant height, basal diameter, crown width, and above-ground biomass of mulberry trees in each group were measured and compared, and these indices showed that the growth status of mulberry trees in group HF was the best, followed by those in group H. Group F and CK were significantly poorer. 4) The photosynthetic pigment contents, chlorophyll a, chlorophyll b and carotenoid of mulberry trees in group F and CK were both significantly lower than those in group H and HF. Chlorophyll fluorescence parameters (Fv/Fm, F′v/F′m, ΦPSII and ETR) of mulberry trees in group F and CK were significantly lower than those in group H and HF. The photochemical quenching coefficients (qP) of mulberry trees in group F and CK were significantly lower than those in group H and HF, by contrast with the non-photochemical quenching coefficients (NPQ), which were higher. These results indicated that in the rocky desertification area, water is the key factor that restricts live and growth of mulberry trees. Nevertheless, the mulberry trees can get enough water and nutrients due to root tropism and they show favorable growth characteristics.