| In view of the situation of poor soil moisture and nutrients, people are paying more and more attention to effective use of water and nutrient resources. Root growth and uptake capacity play a fundamental role in nutrient and water acquisition. In particular, the root system is known to be extremely plastic in response to heterogeneous soil environment, both in morphology and in physiology. It is currently a scientific hotspot problem being focused on by researchers how to regulate the interaction between plants and soil enviroment, and to make use of the potential plant response to soil water and nutient stresses, in order to realize the continuable and stable crop yield and high use efficiency of water and nutrients. Recently, there were many research about the response of crop to water and nutrient stress and partial water and nutrient supplies. Although the morphology, phosiology and struture function of crop showed different response to different water and nutirent conditon, but there was little research about the influence factor of crop growth and development under partial water and nutrient supplies.Thus, in this project, using a split-root technique, the hydroponic experiments on maize will be carried out, where the water stress was stimulated by the osmotic potential of a nutrient solution(PEG 6000) and N stress was stimulated by different N level. There were three water stress levels(-0.2 MPa,-0.4 MPa,-0.6 MPa), N stress levels(2 mM: N2, 1 mM: N1, 0 mM: N0), water and N stress levels(mild:-0.2 MPa+2 mM, moderate:-0.4 MPa+1 mM, severe water and N stresses-0.6 MPa+0 mM) and a control treatment(CK, both sides of the root zone supplied with sufficient water and N). Isotopic 15 N tracer was used to distinguish root nutrient uptake in two sub-root systems. Plants were sampled at the 0, 0.25, 0.5, 1, 3, 5, 7, 9th days of water and/or N stress to partial root system. By evaluating the dynamic changes of root growth, water and N absorbing capacity under partial water and N supplies, the effect of stress severity and duration, previous water and N status on root growth and absorption may be described. Moreover, root anatomical structure was studied under partial water and N supplies. Furthermore, the relationship of root growth, absorbing capacity and root anatomical structure was revealed. The main results were as follows:(1) The dynamic change of root water absorbing capacity was investigated for previous different water and N status under partial water and N supplies. It was found that partial water and N had large influence on root hydraulic conductance in non-stressed and stressed sub-root systems, and could stimulate the compensatory effect of root water absorbing capacity in non-stressed sub-root system. The compensotory effect was closely related to the severity and duration of water and N and previous water and N status under partial water and N supplies.The hydraulic conductance per root length in the non-stressed root zone for the treatment-0.2 MPa increased by 10.1% compared to CK after 0.5 d, and recovered to CK level after 1 d. Nine days after the experiment onset, the hydraulic conductance per root length in the non-stressed root zone for the treatment-0.4 MPa was significantly greater by 9.0% than that for CK, indicating this treatment had an obvious compensation effect on root water uptake. However, the corresponding value for the treatment-0.6 MPa reduced after 0.5~3 d, which recovered to CK after 5 d. Partial water resupply significantly enhanced the root hydraulic conductance in non-stressed sub-root compared with those of previous water stress and stressed sub-root. The hydraulic conductance per root length in the non-stressed root zone for the-0.2 MPa treatment recovered to the CK level at 5 d, suggesting that partial water resupply had an obvious compensatory effect on root water absorbing capacity in non-stressed sub-root system, but it was related to the severity and duration of osmotic stress. Increasing the severity and duration of water stress could result in the disappearance of compensatory effect.The hydraulic conductance per root area in each sub-root was significantly decreased by 21.6%~45.7% compared with CK during 0.5~1 d under partial N stress. With the increase of stress duration, there was always no significant difference between N2 and CK, while for N1 and N0 it was significantly higher than that of CK at 1 and 3 d, respectively, indicating that partial N stress could induce a significant compensatory effect on root water absorbing capacity in non-stressed sub-root. Moreover, the occurrence of such compensatory effect was closely related to the severity and duration of N stress. Increasing the stress severity delayed the the occurrence of such compensatory effect, but it sustained a longer time. Under partial N resupply, the hydraulic conductance per root area in non-stressed sub-root increased significantly compared with that in stressed sub-root at N2, N1 and N0 treatments, and was significantly higher than CK at 0.25 d, 0.5~3 d and 0.5~1 d, respectively. With prolonged the resupply duration, could result in the disappearance of compensatory effect.Root total hydraulic conductance, hydraulic conductance per root length and area in non-stressed sub-root was significantly larger than that of CK under partial mild water and N stress(-0.2 MPa+2 mM) during 5~7 d, but moderate(-0.4 MPa+1 mM) and severe stress treatments(-0.6 MPa+0 mM) had a reverse trend, suggesting that increasing the stress severity and duration would lead to the disappearance of compensatory effect. The recovery extent and time of root hydraulic conductance was relationship with water and N stress under partial water and N resupplies. Stress severity delayed the recovery of root hydraulic conductance. Furthermore, compared with previous full-strength water and N, previous water and N stress stimulated the recovery of root water uptake ability more slowly, but remained longer time.(2) In this study, isotopic 15 N tracer was used to distinguish root nutrient uptake in two sub-root systems under partial water and N stress. By evaluating the dynamic changes of root N inflow rates under different severity of partial water and N stress, the influence factors of root absorption may be described. It was revealed that partial water and N stress could stimulate compensatory effect of root N absorbing capacity in non-stressed sub-root. Partial water and N resupplies significantly increased root N inflow rate in non-stressed sub-root and improved maize growth and N use efficiency, but which was relationship with stress severity and duration.Root N inflow rate in non-stressed sub-root was 295.5% higher than that of CK and increased significantly compared with that in stressed sub-root at 0.25 d after N1 treatment under partial N stress. After 0.25 d, root N inflow rate in non-stressed sub-root uniformly reduced at N1 treatment, whereas N2 treatment showed a reverse trend. At 1~3 d, N2 treatment significantly increased root N inflow rate in non-stressed sub-root compared with that of CK. Afterward, root N inflow rate gradually decreased. For N0 treatment, root N inflow rate gradually increased after 1 d, and then reached its maximum value at 5 d and was significantly higher than that of CK during 1~9 d. Therefore, partial N stress can simulate compensatory effect of root N uptake capacity in non-stressed root and the occurrence time of compensatory effect may be attributed to difference of N level in stressed root. When sub-root system was resupplied N after previous N stress, the root N inflow rate rapidly increased, but the increase rate was closely related to previous stress severity and duration of partial resupply. Increasing stress severity delayed recovery of N inflow rate. Under previous N1 stress, root N inflow rate in non-stressed sub-root significantly increased compared with that of CK at 0.25 d, and had an obvious compensatory effect, though no significant compensatory effect was found in other treatments. Partial 2 mM N stress could improve N use efficiency and shoot dry mass, and even previous N stress also had same results.For all treatments, root N inflow rate in non-stressed sub-root was 34.3%, 51.8% and 123.2% larger than that of CK during 0~1 d under partial water and N stress, respectively, indicating that partial water and N stress can simulate compensatory effect of root N uptake capacity in non-stressed root. With the increase of water and N stress severity, the compensatory effect became more and more significance, and even during 7~9 d, root N inflow rate in non-stressed sub-root was 928.0%, 499.4% and 263.5% higher than that of CK, respectively. Moreover, root N content and accumulation in non-stressed sub-root maintained CK level even was higher than that of CK, thus, which leaded to no significant difference in plant N use efficiency among mild(-0.2 MPa+2 mM), severe water and N stress(-0.6 MPa+0 mM) and CK and a higher plant N use efficiency in moderate water and N stress(-0.4 MPa+1 mM). For non-stressed sub-root, partial water and N resupplies significantly enhanced the root N inflow rate compared with previous water and N stresses during 0~1 DAT. When water and N stresses did not exceed moderate stress level(≦-0.4 MPa+1 mM), partial water and N resupplies significantly increased root N inflow rate compared with control treatment during 0~1 and 7~9 DAT. However, during 3~7 DAT, the root N inflow rate was similar to or lower than control treatment. The root N content and accumulation at mild and moderate stress treatments(≦-0.4 MPa+1 m M) returned to control level at 1 and 5 DAT, respectively, which resulted in similar plant N use efficiency to control treatment. Compared with previous water and N stress, previous full-strength water and N had more advantageous to the occurrence of compensatory effect.(3) Partial water and N supplies had significant effect on root growth in each sub-root system, and obviously promoted root growth in non-stressed sub-root. Moreover, under partial water and N resupplies, after the early stage of treatment, root growth in non-stressed sub-root recovered obviously, and sometimes even exceeded CK, showing compensation effect of root growth. However, the occurrence and strength of such compensatory effect was closely related to the severity and duration of water stress and previous water and N status.In-0.2 MPa treatment, at 0.25~0.5 d, the root area growth rate in non-stressed sub-root was significantly higher than that of CK, but there was no significant difference after that period. In-0.4 MPa treatment, at 1~3 d, the root area growth rate in non-stressed sub-root was significantly higher than that of CK, and later significantly less than that of CK after that period.-0.6 MPa treatment was obviously larger than that of CK during 5~9 d. These results indicated that partial water stress could effectively stimulate the compensatory effect of root growth in non-stressed sub-root, and the occurrence of such compensatory effect delayed with the increasing water stress, but the duration extended with the increase of water stress. After the early stage of the stress(0~0.5 d), for all treatments, root average growth rate in non-stressed sub-root was distinctly higher than that in stressed sub-root under partial water resupply, and could return to the level of CK, and even surpassed CK. But root length and dry weight were more sensitive than root length during the initial stage of treatment. At 0~1 d, average growth rate of root length in non-stressed sub-root was 35% higher than that of CK at-0.4 MPa treatment. At 1~5 d,-0.2 and-0.6 MPa treatments were significantly greater that of CK, and-0.6 MPa treatment was significantly higher than-0.2 and-0.4 MPa treatments. So, after the early stage of treatment, partial water resupply could promote the compensatory effect of root dry weight, but the degree of compensatory effect depended on severity and duration of water stress. Also, root system in severe water stress(-0.6 MPa treatment) responded significantly to water supply. Compared with previous water stress, previous full-strength water was more conducive to the occurrence of compensatory effect for root system in non-stressed sub-root and lasted longer.Partial N2 and N1 stress clearly stimulated the compensatory effect of root length, area and dry weight; for partial N0 stress, root area and dry weight in supplied area also showed compensation phenomenon, but this compensatory effect was related to the duration of stress. The compensatory effect of N2 level disappeared after 5 d, while N1 level disappeared after 1 d and N0 level disappeared after 5 d. Under partial N resupply, at early stage of treatment, the average growth rate of root length, area and dry weight in non-stressed sub-root was significantly higher than that in stressed sub-root. At 0.25 d, the average growth rate of root length, area and dry weight at N2 treatment could return to and even surpass CK level, generating compensatory effect of root growth, but the compensatory effect disappeared after 5 d. For N1 stress level, the average growth rate of root length, area and dry weight was significantly higher than that of CK at 0.5~1 d, 0.25~3 d and 1~3 d, respectively. For N0 stress level, the average growth rate of root length and area was significantly higher than that of CK at 5~7 d and 0.25~5 d, respectively. Compared with previous full-strength N, previous N stress was more conducive to stimulate the compensatory effect of root growth, but the effect could not last longer.With partial water and N stress, the average growth rate of root dry weight in non-stressed sub-root at mild(-0.2 MPa+2 mM) and moderate stress(-0.4 MPa+1 mM) treatments was same as that of CK, and even significantly surpassed CK level at 5~7 d, which resulted in occurrence of compensatory effect of root growth. For severe stress level(-0.6 MPa+0 mM), at 0~1 d and 3~5 d, the average growth rate of root dry weight in non-stressed sub-root was significantly lower than that of CK, but it obviously exceeded CK after 5 d. Under partial water and N resupplies, mild stress treatment(-0.2 MPa+2 mM) increased quickly average growth rate of root dry weight in non-stressed sub-root at the early stage of treatment, acting out compensatory effect of root growth, but for moderate(-0.4 MPa+1 mM) and severe stress(-0.6 MPa+0 mM) treatments, the compensatory effect occurred at 5 and 3 d, respectively. Under different previous water and N conditions, root growth compensatory effect of each treatment could keep to the end of the test. Compared with previous full-strength water and N, previous water and N stress was more conducive to stimulate the compensatory effect of root growth, and the occurrence of the effect was earlier.(4) The dynamic change of leaf water status under partial water and N supplies was discussed. The relationship between root water absorption ability and leaf water status was established.Under partial water stress, when-0.2 MPa treatment had lasted for 1 d, leaf water potential was significantly higher than that of CK, and then it was same as CK. For-0.4 MPa level, there was no obvious difference compared with CK at 0~7 d, but at 9 d, leaf water potential was 18.20% higher than that of CK. Under partial water resupply, leaf water potential of-0.2 MPa treatment returned to the level of CK at 5 d, and enlarged stress severity could obviously affect crop water status. For all treatments, there was close relation between leaf water potential and root hydraulic conductance in each sub-root system. The correlation coefficient in non-stressed sub-root was much higher than that in stressed sub-root, but at-0.2 MPa treatment, there was no significant difference between the correlation coefficients of two sub-root systems, showing that two sub-root systems had same contribution to crop water absorption.During 0.25~0.5 d of partial N stress, compared with CK, leaf water potential at all treatments was remarkably decreased, and during 0.5~3 d, leaf water potential of N1 and N0 treatment was obviously smaller than that of CK, but then leaf water potential at all treatments could recovery to CK level, and even exceeded CK, showing that root water absorbing capacity was closely related to leaf water status. When 1/2 root system was resupplied N, for all treatments, root hydraulic conductance in non-stressed sub-root obviously recovered and even exceeded CK, but the compensatory effect could not maintain longer time, and root hydraulic conductance in stressed sub-root was significantly lower than that of CK, so leaf water potential of each treatment was significantly lower than that of CK, and could not return to CK level at 9 d, and leaf water potential declined with the increase of the stress.At 3 d of partial water and N stress, leaf relative water content at all treatments was significantly higher that of CK, and decreased with the extension of treatment duration, and the decreasing percentage was related to the stress severity. Under partial water and N resupplies, for mild stress treatment, the leaf relative water content was same as CK at 3 d, but it was obviously decreased with the increase of stress severity.(5) It was found that partial-0.2 MPa water stress, N1 stress and mild water and N stress(-0.2 MPa+2 mM) could effectively stimulate compensatory effect of root growth, water and N absorbing capacity in non-stressed sub-root.In the condition of partial water stress, root hydraulic conductance and the average growth rate in non-stressed sub-root were both higher than that in stressed sub-root, and the difference between the two sub-roots was significantly increased with the increase of stress duration, and even root hydraulic conductance and average growth rate in non-stressed sub-root was higher than that of CK, which showed compensatory effect of root water uptake and growth. But with the increase of stress severity, the compensatory effect occurred later, and when the severity exceeded-0.4 MPa, the compensatory effect of root water uptake disappeared. When a half of root system was resupplied water, root growth and water absorbing capacity in non-stressed sub-root quickly recovered, and were remarkably higher than that in stressed sub-root and CK.-0.2 MPa treatment could stimulate the compensatory effect of root growth and water uptake, and with the increase of stress severity, the degree of root growth compensatory effect increased, but compensatory effect of root water uptake disappeared.Partial N stress could effectively stimulate the compensatory effect of root hydraulic conductance, root growth and root N inflow rate, and with the increase of stress severity, the compensatory effect of root hydraulic conductance and root N inflow rate both occurred later and also disappeared later. When a half of root system was resupplied N, root growth and root hydraulic conductance in non-stressed sub-root also showed the similar trend, but 1 mM N stress treatment could effectively stimulate the compensatory effect of root N uptake capacity in non-stressed sub-root.Partial water and N stress could show compensatory effect of root growth and root N absorbing capacity, but only mild stress treatment(-0.2 MPa+2 mM) showed compensatory effect of root water uptake capacity during 5~7 d. Under partial water and N resupplies, all stress treatments could stimulate the compensatory effect of root growth and water absorption capacity. Moderate and mild stress level( ≦-0.4 MPa+1 mM) was conducive to the occurrence of compensatory effect of root N absorption capacity.(6) It was discovered that partial water and N supplies have significant influence on root anatomical structure. Furthermore, the physiology mechanism of root growth and absorbing capacity compensatory effect was revealed.Compared with CK, partial-0.2 MPa water stress significantly increased root diameter and vessel diameter and decreased root cortex thickness and cortex thickness/diameter ratio in non-stressed sub-root at 1, 5 and 9 d, which was benefit to increase root water absorbing capacity. When water stress duration was prolonged, root diameter in non-stressed sub-root was significantly reduced, but root cortex thickness, cortex thickness/diameter ratio and root vessel diameter showed a reverse trend. Under partial water resupply, root diameter and vessel diameter was significantly higher that of CK at 1 d, but no significant difference in root cortex thickness/diameter ratio was found. With the increase of treatment duration, root diameter and vessel diameter was gradually reduced and was significantly lower than of CK at 9 d, but root cortex thickness/diameter ratio was 25% higher that of CK.For non-stressed sub-root, partial N stress significantly increased root diameter, vessel diameter and root cortex thickness compared with CK, but no significant difference in cortex thickness/diameter ratio was found at 1 and 5 d. At 9 d, root cortex thickness/diameter ratio was higher than of CK. For stressed sub-root, root diameter and cortex thickness was markedly lower that in non-stressed sub-root. When stress duration was prolonged, there was no significant difference in root anatomical characters in stressed sub-root compared with that of CK. Under partial N resupply, root diameter in non-stressed sub-root was significantly lower than of CK at 1 and 9 d, whereas root cortex thickness, vessel diameter and cortex thickness/diameter ratio maintained, or even exceeded CK level. With the increase of treatment duration, root anatomical characters had a decrease trend.Partial water and N stress significantly increased root diameter in non-stressed sub-root compared with that of CK, while decreased root cortex thickness/diameter ratio during 1~9 d, suggesting that partial water and N stress can improve root anatomical structure in a long time and promote the compensatory effect of root water and N absorbing capacity. Under partial water and N resupplies, root cortex thickness/diameter ratio recovered to CK level after 5 d, which might result in root hydraulic conductance recovering to CK level at 7 d, suggesting partial water and N stress was helpful to stimulate root compensatory effect, whereas partial water and N resupplies decreased root absorbing capacity. |
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