ALBERT

Notes: We characterized the response of root hair density to phosphorus (P) availability in Arabidopsis thaliana. Arabidopsis plants were grown aseptically in growth media with varied phosphorus concentrations, ranging from 1 mmol m−3 to 2000 mmol m−3 phosphorus. Root hair density (number of root hairs per mm of root length) was analysed starting at 7 d of growth. Root hair density was highly regulated by phosphorus availability, increasing significantly in roots exposed to low-phosphorus availability. The initial root hairs produced by the radicle were not sensitive to phosphorus availability, but began to respond after 9 d of growth. Root hair density was about five times greater in low phosphorus (1 mmol m−3) than in high phosphorus (1000 mmol m−3) media. Root hair density decreased logarithmically in response to increasing phosphorus concentrations within that range. Root hair density also increased in response to deficiencies of several other nutrients, but not as strongly as to low phosphorus. Indoleacetic acid (IAA), the auxin transport inhibitor 2-(p-chlorophenoxy)-2-methylpropionic acid (CMPA), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), and the ethylene synthesis inhibitor amino-oxyacetic acid (AOA) all increased root hair density under high phosphorus but had very little effect under low phosphorus. Low phosphorus significantly changed root anatomy, causing a 9% increase in root diameter, a 31% decrease in the cross-sectional area of individual trichoblasts, a 40% decrease in the cross-sectional area of individual atrichoblasts, and 45% more cortical cells in cross-section. The larger number of cortical cells and smaller epidermal cell size in low phosphorus roots increased the number of trichoblast files from eight to 12. Two-thirds of increased root hair density in low phosphorus roots was caused by increased likelihood of trichoblasts to form hairs, and 33% of the increase was accounted for by changes in low phosphorus root anatomy resulting in an increased number of trichoblast files. These results show that phosphorus availability can fundamentally alter root anatomy, leading to changes in root hair density, which are presumably important for phosphorus acquisition.

Notes: The involvement of ethylene in root architectural responses to phosphorus availability was investigated in common bean (Phaseolus vulgaris L.) plants grown with sufficient and deficient phosphorus. Although phosphorus deficiency reduced root mass and lateral root number, main root length was unchanged by phosphorus treatment. This resulted in decreased lateral root density in phosphorus-deficient plants. The possible involvement of ethylene in growth responses to phosphorus deficiency was investigated by inhibiting endogenous ethylene production with amino-ethoxyvinylglycine (AVG) and aerating the root system with various concentrations of ethylene. Phosphorus deficiency doubled the root-to-shoot ratio, an effect which was suppressed by AVG and partially restored by exogenous ethylene. AVG increased lateral root density in phosphorus- deficient plants but reduced it in phosphorus-sufficient plants. These responses could be reversed by exogenous ethylene, suggesting ethylene involvement in the regulation of main root extension and lateral root spacing. Phosphorus-deficient roots produced twice as much ethylene per g dry matter as phosphorus-sufficient roots. Enhanced ethylene production and altered ethylene sensitivity in phosphorus-deficient plants may be responsible for root responses to phosphorus deficiency.

Notes: The growth patterns of plants subjected to phosphorus starvation resemble those caused by treatment with ABA, suggesting that ABA could mediate the response of the plant to phosphorus starvation. We examined the role of ABA in phosphorus stress by comparing growth and biochemical responses of Arabidopsis thaliana ABA mutants aba-1 and abi2-1 to those of wild-type plants. We first characterized acid phosphatase production of wild-type Arabidopsis in response to phosphorus starvation. We found that several acid phosphatase isozymes are present in roots and shoots, but only a subset of these isozymes are induced by phosphorus stress, and they are induced in both organs. Production of acid phosphatase in response to phosphorus stress was not affected by the aba-1 or abi2-1 mutations. Low phosphorus also resulted in decreased growth of both wild-type and ABA mutant plants, and the root-to-shoot ratio was increased in both wild type and mutants. Anthocyanins accumulated in response to phosphorus stress in both wild-type and mutant plants, but the increase was reduced in the aba-1 mutant. Thus, two different ABA mutants responded normally in most respects to phosphorus stress. Our data do not support a major role for ABA in coordinating the phosphorus-stress response.

Notes: Low phosphorus availability stimulates root hair elongation in many plants, which may have adaptive significance in soil phosphorus acquisition. We investigated the effect of low phosphorus on the elongation of Arabidopsis thaliana root hairs. Arabidopsis thaliana plants were grown in plant culture containing high (1000 mmol m−3) or low (1 mmol m−3) phosphorus concentrations, and root hair elongation was analysed by image analysis. After 15d of growth, low-phosphorus plants developed root hairs averaging 0.9 mm in length while high-phosphorus plants of the same age developed root hairs averaging 0.3 mm in length. Increased root hair length in low-phosphorus plants was a result of both increased growth duration and increased growth rate. Root hair length decreased logarithmically in response to increasing phosphorus concentration. Local changes in phosphorus availability influenced root hair growth regardless of the phosphorus status of the plant. Low phosphorus stimulated root hair elongation in the hairless axr2 mutant, exogenously applied IAA stimulated root hair elongation in wild-type high-phosphorus plants and the auxin antagonist CM PA inhibited root hair elongation in low-phosphorus plants. These results indicate that auxin may be involved in the low-phosphorus response in root hairs.

Notes: Experiments were conducted to determine the fate of nitrogen (N) remobilized as a result of ozone (O3)-induced accelerated senescence in hybrid poplar subjected to declining N availability concurrent with O3 stress. Cuttings were grown in sand culture where the supply of N to the plant could be controlled on a daily basis and reduced in half of the plants when desired. Plants all initially received 3·57 mm N daily until approximately the 20 leaf stage after which daily supply of N was reduced to 0·71 mm. Plants were grown in open-top chambers in the field (Rock Springs, PA, USA) and received charcoal-filtered air, half also received supplemental O3 to a level of 0·08 µL L−1. Allocation of newly acquired N was determined with 15N. The specific allocation (mg labelled N mg−1 total N) of labelled N to upper, expanding leaf N was not affected by O3, but was strongly affected by N treatment. However, O3 increased the relative partitioning of labelled N to the expanding leaves and the roots. The balance between partitioning of newly acquired N to the upper leaves and roots was not affected by O3, but was reduced by N withdrawal. Calculated net N flux was strongly negative in the lower leaves of O3-exposed, N withdrawal plants. Nitrogen uptake was not reduced by O3. The allometric relationships between the roots and shoots were not affected by O3 or N availability. The relative contribution of newly acquired versus remobilized N to new growth appears to be determined by N supply. Ozone exposure alters the allocation of newly acquired N via alterations in plant size, whereas N availability exerts a strong effect upon both plant size and N allocation.

Notes: Contrasting effects of soil CO2 concentration on root respiration rates during short-term CO2 exposure, and on plant growth during long-term CO2 exposure, have been reported. Here we examine the effects of both short- and long-term exposure to soil CO2 on the root respiration of intact plants and on plant growth for bean (Phaseolus vulgaris L.) and citrus (Citrus volkameriana Tan. & Pasq.). For rapidly growing bean plants, the growth and maintenance components of root respiration were separated to determine whether they differ in sensitivity to soil CO2. Respiration rates of citrus roots were unaffected by the CO2 concentration used during the respiration measurements (200 and 2000 μmol mol−1), regardless of the soil CO2, concentration during the previous month (600 and 20 000 μmol mol−1). Bean plants were grown with their roots exposed to either a natural CO2 diffusion gradient, or to an artificially maintained CO2 concentration of 600 or 20 000 μmol mol−1. These treatments had no effect on shoot and root growth. Growth respiration and maintenance respiration of bean roots were also unaffected by CO2 pretreatment and the CO2 concentration used during the respiration measurements (200–2000 μmol mol−1). We conclude that soil CO2 concentrations in the range likely to be encountered in natural soils do not affect root respiration in citrus or bean.