Effects of soil water on gas exchange and water consumption of seedling leaves of Camellia oleifera

The effects of soil water availability on gas exchange characteristics and water consumption of seedlings of Camellia oleifera were analyzed, including the influence of different soil moisture gradients and weather conditions. Photosynthetic characteristics, water consumption and environmental factors related to the growth of seedlings were measured in the field with a Li-6400 portable photosynthesis system, an electronic balance and a Hobo Weather Station in Hunan Province of southern China. Four soil water gradients were analyzed, i. e. W1 (soil moisture content was set at 95% - 100% of field capacity (FC)), W2 (80% -85% of FC), W3 (65% -70% of FC) and W4 (45% -55% of FC); for each of these four treatments, we measured the diurnal courses of stomatal conductance (Gs ), net photosynthetic rate (Pn ), and transpiration (Tr ). Gs, Pn and Tr were graphed and showed unimodal curves. Pn and Gs peaked in 10:00 to 12:00, and Tr and water consumption peaked in 12:00 to 14:00. During the study period, Pn and Gs for each treatment were W2 W1 W3 W4. Under the W4 treatment, Pn, Gs, and water consumption reduced greatly, the peaks of above were lower than observed in the other treatments with more soil moisture, and low soil moisture in W4 prevented the natural growth of seedlings. Intrinsic water use efficiency decreased significantly during the W4 treatment. Our results indicated that the W2 treatment was more suitable for seedling growth because seedling productivity was greater under W2 than under W3 conditions. The maximum water consumption rate of seedling leaves was 153.7 g/(m2hour) and their average daily water consumption rate was 85.3 g/(m2hour) with adequate water. Weather conditions and soil moisture had a combined effect on the water consumption of seedlings. When soil moisture changed from high to low, water consumption of unit leaf area decreased unevenly, and its diurnal course became flat, which may be a result of the reduction of soil water availability or existence of a critical moisture point, which limits the seedling growth.