Effects Of Elevated CO₂ On The Root Growth And Root Exudation In Tomato (Lycopersicon Mill.) Plants Under Different Phosphorus Supply

Global atmospheric concentration of carbon dioxide (CO₂) has been increased year after year due to a huge quantity of carbon released from human activities into air. According to forecast, the atmospheric concentration of CO₂ will increase from 350μL L^-1 at present to 600μL L^-1 by 2050. The increase of CO₂ concentration of will bring a deep impact on human being subsistence, plants growth, and so on, which attributed scientists to pay more and more attention on the researches. Nevertheless, less work has been done on its effects on plant root growth and nutrient uptake. It is well known that the changes of root morphological structure will significantly affect nutrient uptake, especially to phosphorus. Therefore, the effects of CO₂ concentration elevated from 350μL L^-1 to 800μL L^-1 on the changes of root morphological structure and biochemical and physiological parameters tomato seedlings treated with different phosphorus level were studied by solution culture and pot experiments in present research to further reveal the mechanisms of its effects on nutrient uptake and response to P deficiency. The main results obtained from experiments are as follows:1. Effects of CO₂ elevation on the root morphological characteristic and nutrient uptake in tomato plantThe effects of CO₂ elevated from 350μL L^-1 to 800μL L^-1 on the biomass, root characteristics and nutrient uptake of tomato seedlings were studied by carrying out a hydroponics experiment with conditions of P deficiency and normal P supply. The results showed that elevated CO₂ significantly increased the dry matter accumulation of shoot and root, and root/shoot ratio of tomato seedlings no matter P was deficient or not. The effect of elevated CO₂ on promoting root growth was mainly indicated by increase of root volume and surface area under P deficient conditions while by increase of not only root volume and surface area but also root forks under normal P supply conditions, which was beneficial to form a strong root system. Under both conditions of P deficient and normal P supply, elevated CO₂ showed a diluting effect with different degrees on N, P and K contents in different organs of tomato plant. However, the total accumulation of N, P and K in tomato plant was markedly increased by elevated CO₂. Moreover, an obviously significant positive interaction between CO₂ concentrations and P supply on N, P and K accumulation in tomato plants was also observed. It was discovered that elevated CO₂ treatment also significantly improved root hair development and elongation, which can enhance nutrient uptake. Increased indole acetic acid (IAA) concentration in plant tissues and ethylene (ET) release in the elevated CO₂ treatment may have resulted in enhanced root growth, and root hair development and elongation.2. Effects of CO₂ elevation on root exudation of tomato and its mobilization to insoluble phosphate.Tomato (Lycopersicon esculentum ) grown hydroponically was conducted to study the effects of elevated atmospheric CO₂ concentrations from 350μL L^-1 to 800μL L^-1 on the exudation quantities of organic carbon and organic acid of roots and their activation of insoluble P in the rhizosphere under phosphorus normal and deficient conditions. The results showed that P deficiency might result in an obvious increase of root/shoot ratio, which could also be observed by the elevated CO₂ treatment. CO₂ elevation greatly increased the total carbon secretion from solo plant, but this CO₂ effect was eliminated when expressed per unit dry mass of low-P-grown plants. Percentage of organic acid to total exudated C was enhanced by P deficiency although the total exudated C of low P treatment was lower than the normal P treatment, and the ratio was increased by CO₂ elevation. It can be concluded that the mechanism of tomato adaptation to P-deficiency is not by increasing total quantity of exudation but by increasing the ratio of functional fraction. Among the Organic acids, the amount of citrate acid accounted for more than 90% of the total exudation. Root exudation had positive effect on insoluble P activation. Mobilized P was significantly correlated with citrate acid, malate acid and succinate acid, but was insignificantly correlated with total exudated C. It can be speculated that tomato adapted to P-deficiency by increasing citrate acid and its ratio in total exudated C. Moreover, Normal CO₂ treatment strengthened the anti-deficiency capacity of tomato.3. Effects of CO₂ elevation on the utilization of insoluble phosphate by tomato plantThe effect of CO₂ elevation on insoluble phosphate uptake by tomato seedlings was also studied. Here, quartz sand was employed as substrate, and Al-P, Fe-P and Ca-P as the only phosphorus supply, respectively. The results showed the uptake of these three forms of phosphorus could be enhanced along with the increasing concentration of CO₂, especially for Al-P. P concentration in the plant and the accumulation of P in solo-plant were increased by 220% and 330% compared with the control, respectively. Furthermore, there was significant positive correlation between the uptake of insoluble phosphate and root lengths, root surfaces, root volumes, root forks and root tips, respectively, whereas no correlation was found with the root diameters.Synthetic ferrihydrate with different P saturation was employed as the only phosphorus source to further investigate the effects of CO₂ elevation on the utilization of different adsorbent-P. The results indicated that although P concentration in roots, P uptake by shoots and roots increased significantly, P concentration in shoots decreased. P accumulation in the roots treated with 25% and 50% saturation of ferrihydrate-P were enhanced by 2.1 and 2.3 folds by CO₂ elevation, respectively. Hence, it suggested that P uptaked by tomato seedlings from ferrihydrate would not be transported to the shoot. It might be attributed to the decrease of stomata conductance and transpiration under Normaler CO₂ concentration, and P transportation within the plant tissues due to the uptake of Fe.4. Effects of CO₂ elevation on the activity of acid Phosphatase in tomato plant and root exudationThe increase of Phosphatase exudations by roots and its activity in plant tissues has been considered as a kind of stress response by plants when P is deficient. In present study, the activities of acid Phosphatase in plants and root exudation during different growth periods were investigated to explore the impacts of CO₂ elevation. The results showed that acid Phosphatase activities of leaves, roots and root exudations were mainly affected by 25 d of P deficiency, which were increased along with time. However, acid Phosphatase activities were stable at normal P supply level, and no changes were found by CO₂ elevation. However, it was found Phosphatase activities also increased with time passing under normal P supply treatment if calculated with per unit tomato root or root-exucated acid phopatase. Therefore, P supply level might be the dominant factor in affecting acid Phosphatase activity. In conclusion, the activity of acid Phosphatase could be increased at Normaler CO₂ concentration, especially under P deficiency conditions. The response of the acid Phosphatase activity to P supply is more sensitive in root exudation than that inside the root.