Atmospheric CO₂ concentrations, soil moisture, and photosynthetically active radiation are the major factors limiting plant growth and photosynthetic productivity in large areas of the world. These factors affect agricultural and forest production and the distribution and composition of vegetation. Human activities have resulted in increased CO₂ concentrations in the atmosphere, leading to changes in temperature, water cycling, and radiation. CO₂ is required for photosynthesis, and the ongoing increases in atmospheric CO₂ concentrations could eventually lead to higher temperatures and greater evaporative demands. Hence, droughts will be more frequent, intense, and erratic, and may affect regions that are not currently affected by drought. The effect of changes in CO₂ concentrations and soil water content on plants is an important theme in studies on global change. To date, few studies have assessed the effects of continuous increases in CO₂ concentration and soil moisture on photosynthesis. Securinega suffruticosa is one of the dominant drought-tolerant species growing on the shell islands in the Yellow River Delta region. To investigate the carbon and water cycles of the ecological system in this region, it is important to investigate the photosynthetic responses of S. suffruticosa to changes in CO₂ concentrations and soil moisture conditions. This has important practical significance both in terms of food safety and for constructing, improving, and managing a functional ecological environment as the global climate changes.
The study materials were 2-year-old plants of S. suffruticosa, originally from the shell islands in the Yellow River Delta. These plants were obtained from a nursery. The plants were subjected to seven different levels of water supply using an artificial water control system in a covered shed, and to a series of CO₂ concentrations controlled using a LI-COR 6400 portable photosynthesis system. The objective of this study was to investigate photosynthetic responses to CO₂ concentrations and determine whether photosynthetic productivity was quantitatively related to soil moisture content. The photosynthetic rate (P_n) and characteristic parameters of photosynthesis showed threshold-value responses to variations in soil relative water content (SRWC). S. suffruticosa plants were able to maintain higher photosynthetic productivity in the range of 50.3% to 83.2% RWC; in this range, the range of suitable CO₂ concentrations was 700 to 1100 μmol/mol, and P_n increased with increasing CO₂ concentrations. The most suitable SRWC was 70.5%, and P_n peaked at the CO₂ concentration of 900 μmol/mol. The characteristic parameters of photosynthesis showed different responses to changes in SRWC. Under normal and double CO₂ concentrations, the ranges of SRWC in which S. suffruticosa maintained higher photosynthetic productivity were 45.5%-90.0% and 47.0%-92.6%, respectively, indicating that S. suffruticosa was highly adaptable to changes in soil moisture content. The apparent maximum photosynthesis rate (P_max,c) and carboxylation efficiency (CE) increased with increasing SRWC, peaking at 70.5% SRWC and then decreasing slowly at higher SRWC. The CO₂compensation point showed the opposite trend to that of P_max,c and CE, and reached a minimum value (approx. 51.3 μmol/mol) at 70.5% SRWC. The photorespiration rate increased slowly with increasing SRWC and peaked at approximately 2.6175 μmol·m^-2·s^-1 at 50.3% SRWC.
We concluded that photosynthesis in leaves of S. suffruticosa is widely adaptable to changes in CO₂ concentrations and soil moisture, and that this species has the typical physiological characteristics of a water-tolerant and drought-tolerant plant. Thus, S. suffruticosa has the potential for cultivation on the shell ridge islands of the Yellow River Delta, China.