Plant-available nutrients in soils are usually distributed in a heterogeneous or patchy manner. Plant responses to low levels of phosphorous (P) are not uniform across and within species. In this study, we examined the adaptive role of physiological plasticity (increased rate of nutrient uptake in localized zones) to the heterogeneous distribution of P in the soil, and whether low P stress transcends to the shoot and triggers similar biochemical changes that enhance tolerance. Two Chinese fir clones with high P efficiency (M1, which is tolerant to low P, and M4 which is able to decouple fixed P) were chosen as the research materials and their physiological responses to low P stress were examined using a sand culture experiment. For both clones, there was no significant difference in nutrient concentration between P-replete and P-deficient patches. Heterogeneous P supply did not affect the allocation of nutrients to the above-ground parts of the plants. The activity of acid phosphatase (APase) and malondialdehyde (MDA) content increased initially but declined with increasing duration of stress, while the content of soluble protein and total chlorophyll contents remained unaffected by the heterogeneous P supply. We conclude that physiological plasticity plays no role in adaptation to low P stress in these clones, while the changes in APase activity and MDA content in needles suggest functional metabolic processes are involved in enhancing P-efficiency in these clones.
Chinese fir (
Moreover, many of the sites where Chinese fir plantations have been established are deficient in available phosphorus (P), albeit high level of total P and the available P rapidly form insoluble complexes with cations, particularly aluminum and iron (
The most sustainable method of P management is to screen out P-efficient genotypes that are effective at acquiring P from the soil and its subsequent utilization (
There is a need for further research to understand, whether physiological plasticity plays a role in adaptation to the heterogeneous distribution of P in soil, and whether the stress transcends to the shoot and triggers biochemical changes that enhance tolerance to low P stress. The main objective of this study was to examine the effects of heterogeneous P supply on nutrient uptake, translocation, and biochemical changes in the needles of two Chinese fir clones with high tolerance to low P (M1) and the ability to decouple fixed P (M4). The hypotheses of the study were: (1) heterogeneous P distribution has no marked influence on the rate of nutrient uptake and its subsequent translocation (lack of physiological plasticity); (2) membrane lipid oxidation, measured by malondialdehyde (MDA) content, is the same for P-stressed and P-replete seedlings; (3) the activity of acid phosphatase (APase) in needles is higher in response to low P stress; and (4) low P stress reduces the content of soluble protein and total chlorophyll, when compared to normal P supply conditions.
Two Chinese fir clones with high P-use efficiency (M1 and M4) were chosen as test materials. M1 can tolerate low P levels while M4 has the ability to activate fixed phosphate (Fe-P) and make it available for plant use (
A sand culture experiment with heterogeneous P supply was carried out in the greenhouse at the College of Forestry, Fujian Agriculture and Forestry University, China. Specially designed glass pots (50 cm length, 25 cm width, 43 cm height) were made for P stress simulation. Each pot was partitioned into two compartments (a P-poor patch and a P-rich patch -
In order to meet the other nutrients requirements for growth of the Chinese fir cuttings, macro-nutrients were added to each pot during the stress period with a modified Hoagland solution (
During the 2 months of stress period, needles were sampled every 20 days to measure the following physiological variables: malondialdehyde (MDA) content, acid phosphatase (APase) activity, soluble protein content, and chlorophyll content. The MDA content was determined by coloration method using thiobarbituric acid (
APase activity was determined following the method of (
Soluble protein content was determined using the Coomassie Brilliant Blue method (
The total chlorophyll content was analyzed using ethanol-acetone extraction method (
The cuttings in each treatment were harvested after 2 months of stress treatment by flushing the fine sand through the opening at the bottom of the pot with water. The roots in each pot patch were bundled together before pulling out the cuttings. All roots from the different patches were harvested separately, cleaned with distilled water, quickly dried with paper towels, and the fresh mass was determined. The fresh masses of stems and leaves were also determined separately. For nutrient analyses, oven-dried plant samples (leaf, stem, and root in each pot) were ground to pass through a 1 mm mesh, and triplicate samples were analyzed for P, Nitrogen (N), Potassium (K), Calcium (Ca) and Magnesium (Mg). The P and K contents were extracted by wet ashing of 0.2 g plant material in an acid mixture consisting of 10 ml H2SO4, 3 ml HNO3, and 1 ml HClO4, following the method of
The nutrient concentration of the P-poor and P-rich roots was calculated as the nutrient content of the samples divided by the dry mass of the roots. Aboveground nutrient concentration was determined as the nutrient content of needles and stems divided by their respective dry masses. A two-way ANOVA was performed to examine differences in nutrient concentration between patches and P treatments for each clone. When the interaction effect was significant, a one-way ANOVA was performed for each patch and clone. In addition, physiological plasticity was determined by dividing the P concentration of roots (root P divided by root dry mass) in the P-rich patch by that in the P-poor patch, and a one-way ANOVA was performed to examine differences among heterogeneous P treatments.
For biochemical changes in the needles of the two Chinese fir clones in response to heterogeneous P supply over time, a repeated measures ANOVA was performed using the following general linear model (
where
For both clones, significant differences (p < 0.01) were detected in the concentrations of P, K, and Ca in the roots between P-poor and P-rich patches, but not significant difference were found for N and Mg. Roots of both clones absorbed more P, K, and Ca in the P-poor than in the P-rich patch, which varied with P treatments. For the low P-tolerant clone (M1), the concentrations of P and K in partly P-starved roots (P-poor patch) were significantly higher with the insoluble P supply than with the normal P supply, whereas the concentrations of N, Ca, and Mg remained unaffected by the heterogeneous P supply (
For the clone adapted to decouple fixed P (M4), the concentration of P in partly P-starved roots was significantly higher with the high insoluble P supply (P2) than the low insoluble P supply (P1) and P-deprived patch (P0) but was comparable similar to the normal P supply (P3 -
Nutrient accumulation in the needles and stem showed differential responses to heterogeneous P supply (
Nutrient accumulation in the stems varied significantly between clones and P treatments for P, Ca, and Mg, but accumulation of K in the stems varied significantly with respect to heterogeneous P supply, while N accumulation in stems remained unaffected by P treatment. The clone M1 accumulated more P in the stem than the M4 clone, while the reverse was found for Ca accumulation in stems particularly with an insoluble P supply (
The biochemical content of needles exhibited marked temporal variation in response to heterogeneous P supply. The APase activity in needles varied significantly with respect to within-subject factors (stress period and its first and second order interactions with P treatment and clone - p < 0.0001), as well as for between-subject factors (clone - p < 0.0001), but not P treatments (p = 0.58). During the first 20 days of stress, the mean APase activity did not change much for any of the P treatments for either clones, but the activity sharply increased in the heterogeneous P supply treatment with insoluble P after 40 days, and the increase was markedly higher for the M4 clone than the M1; activity sharply declined thereafter in all P treatments (
The MDA content also varied significantly with respect to within-subject factors (stress period and its first and second order interactions with P treatment and clones, p < 0.0001), as well as with between-subject factors (clone and P treatment, p < 0.0001 for both). The mean MDA content sharply increased during the first 20 days of the treatments arranged with no P and normal P supply in each patch, thereafter the content declined sharply for the M1 clone (
The content of soluble protein also varied significantly with respect to within-subject factors (stress period and its first and second order interactions with P treatment and clone - p < 0.05), as well as with between-subject factors (clone - p < 0.0001), but not for P treatments (p = 0.375). The mean soluble protein content sharply increased during the first 20 days of the experiment in all P treatments, and the increase was markedly higher for the M4 than the M1 clone (
Heterogeneous P supply had no marked influence on the rate of nutrient uptake by P-efficient Chinese fir clones, thus root physiological plasticity plays no adaptive role in P acquisition from localized P-replete zones. Phosphorous acquisition was more significant in terms of growth benefits than P utilization, under low P conditions (
There is strong evidence in this study that rhizospheric low P stress induces a number of biochemical changes in the needles of Chinese fir clones with high phosphorus efficiency. APase activity sharply increased after 40 days in the heterogeneous P supply involving insoluble P for both M1 and M4, the increase being markedly high for the latter. APase participates in various metabolic processes in plants, particularly in hydrolyzing orthophosphate monoesters into more mobile orthophosphate anions, Pi (
Our study also showed that membrane lipid oxidation, as measured by increased MDA content, occurred shortly after the stress was induced (20 days), but declined rapidly thereafter. Environmental stresses are often followed by increased production of reactive oxygen species that are known to damage membrane integrity. However, plants adapted to such stresses produce a suite of antioxidants to scavenge the elicited Reactive Oxygen Species (
There is no evidence in the present study that rhizospheric insoluble P affects the synthesis of soluble protein. This can be related to effective translocation of absorbed P to the needles and efficient utilization of P, as can be seen from similar concentrations of P in the needles in both the heterogeneous supply with high fixed P and homogeneous supply of normal level of P treatments. It should be noted that these two clones produced similar quantities of shoot and root biomass in P-starved and non-starved seedlings suggesting high P-utilization efficiency (
The results of the present study did not show an increased rate of nutrient uptake in P-replete patches; thus, we conclude that root physiological plasticity does not play a role in adaptation to a heterogeneous P supply by enhancing P-acquisition efficiency in P-efficient Chinese fir clones. In addition, a heterogeneous P supply had no negative impact on the uptake and translocation of other nutrients to the shoots. The most interesting part of this study is the variation in nutrient uptake within the same root system depending on P availability. The part of the root system that was deprived of available P had a higher concentration of nutrients (P, K, and Ca) than the part of the root system supplied with normal P levels. This suggests that determinate root development can occur even within the same root system in response to spatial variation in the availability of P around the root system. The changes in APase activity and MDA content in needles suggest functional metabolic processes involved in enhancing P efficiency in these clones. Since the two Chinese fir clones appeared to be adapted to low P stress, mainly
This research was financially supported by the National Natural Science Foundation of China (U1405211 and 31370531) and by the Science and Technology Plan Project of Fuzhou City, China (2017-N-35).
The design pot used for the heterogeneous phosphorus stress experiment. (a) P-poor patch; (b) P-rich patch.
Nutrient accumulation (mg g-1·per plant, Dry Mass) in needles and stems of low P-tolerant Chinese fir genotypes, M1 and M4, in response to heterogeneous P supply (mean ± SE).
Temporal variation in APase, MDA content, soluble protein and total chlorophyll content of low P-tolerant Chinese fir clone, M1 and M4, in response to heterogeneous P supply over time (mean ± SE).
Effect of heterogeneous P supply on root nutrient concentration (mg g-1, Dry Mass) of low P-tolerant Chinese fir clone M1 together with physiological plasticity (mean ± SE). (P0): No P+P; (P1): Low Fe-P + P; (P2): High Fe-P + P; (P3): Normal P + P. For each nutrient element and main effect of patches, means followed by the same letter(s) within a row are not significantly different (p > 0.05).
Nutrients | Treatments | P-poor patch | P-rich patch | Plasticity |
---|---|---|---|---|
P | No P + P | 0.66 ± 0.02 a | 0.45 ± 0.01 a | 0.68 ± 0.03 a |
Low Fe-P + P | 0.57 ± 0.02 b | 0.46 ± 0.02 a | 0.80 ± 0.04 a | |
High Fe-P + P | 0.63 ± 0.02 ab | 0.46 ± 0.01 a | 0.72 ± 0.04 a | |
Normal P + P | 0.48 ± 0.01 c | 0.41 ± 0.01 a | 0.84 ± 0.04 a | |
Mean (patch) | 0.59 ± 0.02 a | 0.44 ± 0.01 b | - | |
N | No P + P | 11.48 ± 1.21 a | 12.03 ± 0.43 a | 1.08 ± 0.14 a |
Low Fe-P + P | 13.16 ± 1.44 a | 13.23 ± 0.97 a | 1.01 ± 0.05 a | |
High Fe-P + P | 10.70 ± 0.35 a | 12.97 ± 0.22 a | 1.22 ± 0.06 a | |
Normal P + P | 12.23 ± 0.52 a | 12.81 ± 0.75 a | 1.05 ± 0.09 a | |
Mean (patch) | 11.89 ± 0.51 a | 12.76 ± 0.31 a | - | |
K | No P + P | 18.57 ± 0.12 b | 8.73 ± 1.00 a | 0.47 ± 0.05 a |
Low Fe-P + P | 17.92 ± 0.21 b | 13.60 ± 0.93 b | 0.76 ± 0.05 b | |
High Fe-P + P | 18.64 ± 0.28 b | 12.99 ± 0.86 b | 0.70 ± 0.04 ab | |
Normal P + P | 14.12 ± 0.75 a | 14.85 ± 0.40 b | 1.06 ± 0.08 c | |
Mean (patch) | 17.31 ± 0.59 a | 12.54 ± 0.78 b | - | |
Ca | No P + P | 0.79 ± 0.04 a | 0.49 ± 0.03 a | 0.62 ± 0.07 a |
Low Fe-P + P | 0.70 ± 0.01 a | 0.56 ± 0.03 ab | 0.81 ± 0.04 ab | |
High Fe-P + P | 0.67 ± 0.06 a | 0.77 ± 0.05 c | 1.18 ± 0.14 b | |
Normal P + P | 0.71 ± 0.01 a | 0.70 ± 0.03 bc | 0.99 ± 0.06 b | |
Mean (patch) | 0.72 ± 0.02 a | 0.63 ± 0.04 b | - | |
Mg | No P + P | 0.12 ± 0.01 a | 0.11 ± 0.01 a | 0.96 ± 0.16 a |
Low Fe-P + P | 0.12 ± 0.01 a | 0.11 ± 0.003 a | 1.02 ± 0.15 a | |
High Fe-P + P | 0.13 ± 0.01 a | 0.11 ± 0.01 a | 0.83 ± 0.07 a | |
Normal P + P | 0.10 ± 0.01 a | 0.11 ± 0.003 a | 1.10 ± 0.09 a | |
Mean (patch) | 0.12 ± 0.01 a | 0.11 ± 0.002 a | - |
Effect of heterogeneous P supply on root nutrient concentration (mg g-1, Dry Mass) of Chinese fir clone M4 adapted to decoupling fixed P together with physiological plasticity (mean ± SE). (P0): No P+P; (P1): Low Fe-P + P; (P2): High Fe-P + P; (P3): Normal P + P. For each nutrient element and main effect of patches, means followed by the same letter(s) within a row are not significantly different (p > 0.05).
Nutrients | Treatments | P-poor patch | P-rich patch | Plasticity |
---|---|---|---|---|
P | No P + P | 0.33 ± 0.02 a | 0.29 ± 0.003 a | 0.89 ± 0.04 a |
Low Fe-P + P | 0.39 ± 0.02 ab | 0.39 ± 0.02 b | 1.00 ± 0.10 a | |
High Fe-P + P | 0.49 ± 0.01 c | 0.37 ± 0.003 b | 0.77 ± 0.02 a | |
Normal P + P | 0.44 ± 0.02 bc | 0.43 ± 0.02 b | 0.99 ± 0.07 a | |
Mean (patch) | 0.41 ± 0.02 a | 0.37 ± 0.02 b | - | |
N | No P + P | 12.43 ± 0.26 a | 14.00 ± 0.02 a | 1.13 ± 0.02 a |
Low Fe-P + P | 12.06 ± 0.31 a | 13.35 ± 0.17 a | 1.11 ± 0.04 a | |
High Fe-P + P | 15.06 ± 1.15 a | 13.10 ± 0.61 a | 0.88 ± 0.04 b | |
Normal P + P | 14.88 ± 0.74 a | 12.07 ± 1.01 a | 0.81 ± 0.05 b | |
Mean (patch) | 13.61 ± 0.51 a | 13.13 ± 0.33 a | - | |
K | No P + P | 17.18 ± 0.47 a | 12.66 ± 0.07 a | 0.74 ± 0.02 a |
Low Fe-P + P | 16.75 ± 0.50 a | 14.05 ± 0.70 a | 0.84 ± 0.02 a | |
High Fe-P + P | 17.13 ± 0.84 a | 14.09 ± 0.11 a | 0.83 ± 0.05 a | |
Normal P + P | 14.91 ± 1.05 a | 14.49 ± 0.72 a | 0.98 ± 0.04 b | |
Mean (patch) | 16.49 ± 0.43 a | 13.82 ± 0.30 b | - | |
Ca | No P + P | 0.66 ± 0.02 a | 0.33 ± 0.03 a | 0.49 ± 0.04 a |
Low Fe-P + P | 0.89 ± 0.07 bc | 0.39 ± 0.04 a | 0.44 ± 0.08 a | |
High Fe-P + P | 1.02 ± 0.05 c | 0.73 ± 0.06 b | 0.71 ± 0.07 ab | |
Normal P + P | 0.78 ± 0.01 ab | 0.71 ± 0.06 b | 0.91 ± 0.09 b | |
Mean (patch) | 0.84 ± 0.04 a | 0.54 ± 0.06 b | - | |
Mg | No P + P | 0.10 ± 0.00 a | 0.09 ± 0.00 a | 0.88 ± 0.03 a |
Low Fe-P + P | 0.10 ± 0.01 a | 0.10 ± 0.003 a | 0.97 ± 0.10 a | |
High Fe-P + P | 0.11 ± 0.01 a | 0.10 ± 0.01 a | 0.92 ± 0.11 a | |
Normal P + P | 0.10 ± 0.01 a | 0.09 ± 0.00 a | 0.91 ± 0.11 a | |
Mean (patch) | 0.10 ± 0.003 a | 0.09 ± 0.003 a | - |