Calcium chloride improves photosynthesis and water status in the C-4 succulent xerophyte Haloxylon ammodendron under water deficit

摘要

Our results showed that in addition to sodium (Na+), succulent xerophyte Haloxylon ammodendron was more inclined to absorb and accumulate large amount of calcium (Ca2+) than potassium (K+) as an important osmotica for osmotic adjustment (OA) under arid environments. To further reveal the possible physiological mechanisms of Ca2+ accumulation involved in drought resistance of H. ammodendron, 8-week-old plants were treated with a series concentration of external Ca2+ (calcium chloride, 1 x 10(-4) to 8 x 10(-4) g CaCl2 g(-1) dry soil) by potted sandy soil culture experiments in field. Plants were also subjected to water deficit (30% of field water capacity) in the presence or absence of Ca2+ (4 x 10(-4) g CaCl2 g(-1)dry soil) by potted sand culture experiments in greenhouse. The results showed that 4 x 10(-4) g CaCl2 g(-1) soil resulted in optimal plant growth and effectively mitigated harmful impacts of water deficit on the growth of H. ammodendron, by inducing a significant drop in PB water potential and, concomitantly, increasing PB turgor pressure and chlorophyll concentration resulting in an enhancement of overall plant photosynthetic activity. Furthermore, 4 x 10(-4) g CaCl2 g(-1) soil had no effects on the photosystem II photochemistry, but mitigated the inhibitory effect of water deficit on the activity of photosystem II in H. ammodendron. The contribution of Ca2+ to the total osmotic potential varied from 6.2% in the control to 11.4% in plants subjected to water deficit and, surprisingly, to 21.6% in plants grown in the presence of 4 x 10(-4) g CaCl2 g(-1) soil under water deficit; however, the contribution of K+ significantly decreased from 12.1 to 7.0%. These findings suggest that, under arid environments, H. ammodendron is able to accumulate high concentration of Ca2+ in its PB and use it directly for OA, which was coupled with an improvement in PB hydration and photosynthetic activity.