Threshold effects of photosynthetic efficiency parameters of wild jujube in response to soil moisture variation on shell beach ridges, Shandong, China

Abstract

To investigate the threshold effects of photosynthetically active radiation (PAR) and soil mass water content (MWC) on photosynthetic efficiency parameters of Ziziphus jujuba Mill var. spinosa and to understand the adaptability of Z.jujuba to light and soil moisture variation, we determined optimal MWC and PAR for Z. jujuba which maintained higher net photosynthetic rate (PN) and water use efficiency (WUE). Using a Li-6400 portable photosynthesis system, we measured light response of PN, transpiration rate (E), WUE, and other gas-exchange parameters of 3-year-old Z. jujuba shrubs in a range of soil moisture conditions. The results showed that the leaf photosynthetic rate and WUE of Z. jujuba had a significant response to MWC and PAR. Given increases in the MWC (7.1–17.6%), the plant's light compensation point decreased and its light saturation point (LSP), apparent quantum yield, and maximum PN increased. When MWC was at 17.6%, the low and high light use efficiency of Z. jujuba was all maximal. PN obviously increased with increasing MWC (9.2–17.6%). However, PN decreased when MWC was too high or low. When PAR ranged from 800 to 1200 μmol m− 2 s− 1, PN and WUE were higher and the LSPs of PN and WUE ranged between 706 and 1209 μmol m− 2 s− 1. These data indicate that Z. jujuba possessed higher adaptability to light conditions. Based on photosynthetic efficiency parameters, the soil moisture availability and productivity of Z. jujuba were classified and evaluated. For Z. jujuba woodland, MWC < 9.2% and MWC>21.5% resulted in low productivity and medium WUE, 19.8–21.5% of MWC resulted in medium productivity and low WUE, 9.2–11.2% of MWC resulted in medium productivity and medium WUE, and 11.2–19.8% of MWC resulted in high productivity and high WUE. The optimum high productivity and high WUE of MWC were at 17.6%, and the corresponding optimum PAR was 1209 μmol m− 2 s− 1.