M. Shahbaz Akhtar

Relative phosphorus utilization efficiency, growth response, and phosphorus uptake kinetics of brassica cultivars under a phosphorus stress environment

Abstract Plants grown in highly weathered or highly alkaline calcareous soils often experience phosphorus (P) stress but never a P‐free environment. Thus, applications of mineral P fertilizers are often required to achieve maximum yield, but recovery of applied P fertilizers is notoriously low. Phosphorus deprivation elicits a complex array of morphological, physiological, and biochemical adaptations among plant species and genotypes to enhance P acquisition and utilization efficiency. Ten Brassica cultivars were grown hydroponically to investigate their relative efficiency to utilize deficiently (20‐µM) and adequately (200‐µM) supplied P, using Johnsons modified solution. Cultivars differed significantly (P<0.001) in biomass accumulation. Orthophosphate concentration and uptake in shoot and root, absolute and relative growth rate, and P‐utilization efficiency (PUE) were also significantly different among various Brassica cultivars. Root‐shoot ratio and specific absorption rate were substantially increased in plants subjected to low P supply. Shoot and root dry‐matter yield as well as total biomass production correlated significantly (P<0.01) with their total P uptake and PUE. Cultivars, which were efficient in P utilization, were also efficient accumulators of biomass under adequate as well as deficient levels of P supply. As part of the study, kinetic parameters of P uptake were evaluated for six contrasting Brassica cultivars in PUE, grown in nutrient solution. The kinetic parameters related to P influx were maximal transport rate (Vmax), the Michaelis–Menten constant (Km), and the external concentration when net uptake is zero (Cmin). Lower Km and Cmin values were indicative of P‐uptake ability of the cultivars, evidencing their adaptability to P‐stress conditions. In another experiment, six cultivars were exposed to no P nutrition for 27 days after initial feeding on optimum nutrition for 14 days. All the cultivars retranslocated P from aboveground parts to their roots during growth in P‐free conditions, the magnitude of which was variable in different cultivars. Phosphorus concentration at 41 days after transplanting was higher in developing leaves than developed leaves. Translocation of absorbed P from metabolically inactive sites to active sites in plants growing under P‐stress conditions may have helped the tolerant cultivars to establish a better rooting system, which provided basis for tolerance against P‐deficiency stress and increased PUE.

Path and Correlation Analyses of the Factors affecting Biomass Production of Brassica Cultivars under Phosphorus-Deficiency Stress Environment

Abstract Path analysis is a statistical technique that partitions correlations into direct and indirect effects and distinguishes between correlation and causation, whereas correlation in general measures the extent and direction (positive or negative) of a relationship occurring between two or more variables. The estimates of correlation and path coefficients can help us to understand the role and relative contribution of various plant traits in establishing growth behavior of crop cultivars under given environmental conditions. Dependence of shoot dry‐matter (SDM) production of six hydroponically grown Brassica cultivars on various growth parameters and characteristics of P metabolism was investigated using the modified Johnsons nutrient solution to maintain deficient (10 µM) and adequate (200 µM) P levels. Root dry‐matter (RDM), total dry‐matter, P content in shoot, and P‐utilization efficiency (PUE) had significant and positive effects on production of SDM in a P‐deficient environment. Root–shoot ratio (RSR), however, negatively affected SDM of cultivars exposed to P‐deficient conditions and did not show any impact on SDM production in either of the two treatments. In a pot study, six Brassica cultivars were grown in a sandy loam soil that was deficient in NaHCO3‐extractable P (3.9 mg P kg−1 soil) for 49 days. Significant positive correlations were observed between SDM and some other plant traits such as RDM, leaf area per plant, P uptake, and PUE, at both genotypic and phenotypic levels. The correlations of SDM with RSR, however, were not observed, implying that relative partitioning of biomass into roots or shoots had little role to play in SDM production by Brassica cultivars under P‐deficiency stress. Path analysis revealed that favorable impact of RDM and leaf area on SDM production was indirect through positive effect of these parameters on P uptake and PUE. Thus, under P‐deficiency stress, better P acquisition and efficient P utilization by the cultivars for biomass synthesis collectively formed the basis of higher SDM production by the cultivars, evidencing that P uptake and utilization efficiency are two important plant traits for selecting P‐deficiency‐stress‐tolerant Brassica cultivars.

Genetic diversity of Brassica cultivars in relation to phosphorus uptake and utilization efficiency under P-stress environment

Phosphorus (P) plays a central role in plant cell metabolism. Therefore, low P availability is one of the major constraints for plant productivity in natural and agricultural ecosystems. The addition of phospahtic fertilizers is economically not feasible for resource-poor farmers as the efficiency of added P fertilizers is often low. Plants have evolved a myriad of adaptive mechanisms to increase P uptake and P-utilization efficiency (PUE). Plant efficiency for P uptake and PUE may contribute to improve crop-yield potential in resource-poor environments and selection of P-efficient cultivars can enhance productivity on low fertility soils and reduce fertilizer requirements. A controlled-climate chamber was used to investigate the performance of 14 hydroponically grown Brassica cultivars. Cultivars differed significantly in biomass accumulation (shoot and root dry matter), root-shoot ratio, P concentrations and contents in plant parts, relative reduction in biomass accumulation due to P stress or P-stress factor (PSF) and PUE indicating considerable diversity among the tested cultivars. PUE and the proportional increases in shoot dry matter (SDM) production (SDMmax/SDMmin) in response to the P levels assisted in differentiating plants into efficient and inefficient utilizers of the absorbed P. Cultivars were classified into efficient responsive (ER), efficient non-responsive (ENR), non-efficient responsive (NER) and non-efficient non-responsive (NENR). Efficient cultivars showed the lowest shoot P concentrations and PSF, and the highest biomass accumulation and PUE.

Genotypic Variations in Growth Response and P-acquisition Efficiency by Spring Wheat Cultivars Exposed to Sparingly Soluble P-sources

To estimate genetic variability in growth characteristics and phosphorus acquisition efficiency from sparingly soluble P-sources; four genetically diverse spring wheat cultivars, categorized as ‘P-efficient and low P-tolerant (class-I), and P-inefficient and low P-sensitive (class-II) cultivars’ based on P-efficiency characteristics in a hydroponic study, were grown in a sand culture in order to evaluate their relative performance in a glass house experiment. Pre-treated, cleaned uniform sized seeds of cultivars were sown in 4-kg capacity pots containing yamazuna sand with negligible available P. P-treatments were (i) (LP)-control treatment without any applied P (ii) (AP)-NH4H2PO4 @ 200 μM P kg-1 as an adequate P-supply, (iii) (TCP)-tri-calcium phosphate (Ca3(PO4)2 @ 0.5 g kg-1, and (iv) (RP)-rock phosphate @ 0.5 g kg-1 of sand. Tri-calcium phosphate and rock phosphate were sparingly soluble P-sources containing negligible P soluble in water. Biomass accumulation by plants, P-concentration and uptake in roots and shoots of plants, phosphorus stress factor and P-efficiency characteristics were differed in tested cultivars indicating sufficient genetic diversity in wheat cultivars. Biomass and growth parameters were significantly correlated with plant P-parameters indicating that P taken up by the plants from sparingly soluble P-sources was accumulated into the plant biomass. Class-I cultivars exhibited better performance than class-II cultivars indicating their better ability to scavenge P at all P levels.