An automated greenhouse sand culture system suitable for studies of P nutrition

Abstract. Maintenance of realistically low solution P concentrations under controlled conditions is a major difficulty in studies of P nutrition. In this report, we describe a relatively simple and economical sand culture system capable of sustaining plant growth to maturity under controlled yet realistic P regimes. The system uses Al₂O₃ as a solid-phase P buffer, and modern process control technology to control irrigation and addition of other mineral nutrients. Aspects of the design, use and potential applications of automated solid-phase systems are discussed. The system was used to grow Phaseolus vulgaris to matarity at 0.4 mmol m³, 1.0 mmol m³ and 27 mmol m³ P with and without mycorrhizal inoculation. At flowering, low solution P concentrations were associated with reduced leaf concentrations of P in nonmycorrhizal plants, and reduced leaf concentrations of Ca in both mycorrhizal and nonmycorrhizal plants. Mycorrhizal inoculation increased leaf P, K, Mg and Mn concentrations, but reduced leaf N concentration. Low P regimes reduced final seed yield by diminishing both the number of pods per plant and the number of seeds per pod. Mycorrhizal inoculation significantly enhanced seed yield under low P regimes by increasing seed weight, the number of pods per plant, and the number of seeds per pod.     

Stimulation of root hair elongation in Arabidopsis thaliana by low phosphorus availability

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.     

The role of pigmentation in survival of the leaf spot fungus Cercospora beticola

Both germination and survival time of spores of a pigmented strain of Cercospora beticola were directly correlated with relative humidity. Pigmented cultures, under conditions of low water availability (15%) survived 242 days' continuous exposure to light (110 lx). The ability of various strains of the fungus to survive in soil was found to be influenced by pigmentation, rate of growth, inoculum ratio in the soil and the physiological state of the organism before exposure to the soil environment.     

The responses of wild-type and ABA mutant Arabidopsis thaliana plants to phosphorus starvation

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.