This review focuses on physiological responses in higher plants to B deficiency at the whole plant and organ level. Plants respond to decreasing B supply in soil solutions by slowing down or ceasing growth. Boron deficiency inhibits root elongation through limiting cell enlargement and cell division in the growing zone of root tips. In the case of severe B deficiency, the root cap, quiescent centre and protoderm of root tips disappear and root growth ceases, leading to the death of root tips. Although vascular bundles are weakly developed in B-deficient roots, early effects of B deficiency on their initiation and differentiation is poorly understood. Inhibited leaf expansion by low B indirectly decreases the photosynthetic capacity of plants, though exact roles of B in photosynthesis remain to be explored. The early inhibition of root growth, compared to shoot growth, increases the shoot:root ratio. It is hypothesised that this may enhance the susceptibility of plants to environmental stresses such as marginally deficient supplies of other nutrients and water deficit in soil. In the field, sexual reproduction is often more sensitive to low soil B than vegetative growth, and marked seed yield reductions can occur without symptoms being expressed during prior vegetative growth. In flowers, low B reduces male fertility primarily by impairing microsporogenesis and pollen tube growth. Post-fertilisation effects include impaired embryogenesis, resulting in seed abortion or the formation of incomplete or damaged embryos, and malformed fruit. However, there is a great diversity of effects of low B on reproductive growth among species, and within the same species between sites and seasons. Much of this diversity is not explained by the current literature. Key processes in reproductive development which may be impaired under B deficiency are proposed and discussed. These include the formation of a diverse array of cell wall types, the supply of carbohydrates for growth and storage reserves, and the production of flavonols. Inflorescence architecture, floral morphology, canopy structure and prevailing weather conditions are suggested as being important for xylem B delivery into flowers because of their impact on transpiration. The extent of phloem translocation of B into reproductive organs has yet to be fully assessed. The timing of B sensitive stages in reproduction of most crop plants need defining in order to facilitate appropriate timing of corrective B treatments. As most container studies have imposed B deficiency by withholding B, much of the data on severely B-deficient plants requires re-evaluation. Further studies are warranted to understand the effects of realistically low levels of B in solution on the growth of meristematic tissues and floral organs. A B-buffered solution culture system is recommended for some of this work.

Physiological response of plants to low boron

Zinc in Soils and Plants

Function of Micronutrients in Plants

Seed nutrient reserves may be important for an early establishment of crops on low-fertility soils. This glasshouse pot study evaluated effects of seed Zn content on vegetative growth of two wheat (Triticum aestivum L.) genotypes differing in Zn efficiency. Low-Zn (around 250 ng Zn per seed) and high-Zn seed (around 700 ng Zn per seed on average) of Excalibur (Zn efficient) and Gatcher (Zn inefficient) wheats were sown in a Zn-deficient siliceous sand fertilised with 0, 0.05, 0.2, 0.8 or 3.2 mg Zn kg -1 soil. After 3 weeks, plants derived from the high-Zn seed had better root and shoot growth; the cv. Excalibur accumulated more shoot dry matter than the cv. Gatcher. After 6 weeks, greater root and shoot growth of plants grown from the high-Zn seed compared to those from the low-Zn seed was obvious only at nil Zn fertilisation. A fertilisation rate of 0.2 mg Zn kg -1 soil was required for achieving 90% of the maximum yield for plants grown from the high-Zn seed compared to 0.8 mg Zn kg -1 soil for plants derived from the low-Zn seed. The critical Zn level in youngest expanded leaves for 90% maximum yield was 16 mg Zn kg -1 dry matter for both genotypes. Zn-efficient Excalibur had greater net Zn uptake rates compared to Zn-inefficient Gatcher after 3 weeks but they were not different at the 6-week harvest. Zinc-deficient plants had greater net uptake rates of Cu, Mn, B, P, and K but a reduced uptake rate of Fe. It is concluded that higher seed Zn content acted similar to a starter-fertiliser effect by improving vegetative growth and dissipating differences in Zn efficiency of wheat genotypes.

Importance of seed Zn content for wheat growth on Zn-deficient soil. I: Vegetative growth

This book is written for practitioners and stakeholders in the fertiliser industry and for policy makers whose decisions may impact on the use of micronutrients in agriculture, horticulture and forestry. The aim of the book is to explain the growing importance of micronutrients in balanced fertilisation, to consider the micronutrient fertiliser types that are currently available and how to best use them, to assess the current market and prospects for micronutrient fertilisers, and to discuss the policy, regulatory and quality control framework needed to maximize the benefits from using micronutrient fertilisers.

/pdf/micronutrients-for-sustainable-food-feed-fibre-and-bioenergy-4pfelc4r2u.pdf

Micronutrients for Sustainable Food, Feed, Fibre and Bioenergy Production

Effects of zinc supply on the distribution of zinc and dry weight among plant parts were examined during the first 55 days of vegetative development of Seaton Park subterranean clover grown in a zinc-deficient soil in a glasshouse. Symptoms of zinc deficiency first appeared in young trifoliate leaves. Zinc deficiency decreased the expansion of blades and petioles, delayed the development of leaves and lateral branches, depressed dry weights of roots and shoots, and increased the proportion of plant dry weight in roots and leaf blades. In each treatment and at each harvest, zinc concentrations varied widely amongst plant parts and with their physiological age. Plant parts also differed widely in the response of their dry matter and zinc concentrations to both zinc treatment and harvest time. It is suggested that these complex relationships explain why plant samples consisting of composite plant parts are not suitable for diagnosis of zinc deficiency. In the present experiment, zinc concentration in whole shoots was unsatisfactory for diagnosing zinc deficiency since concentrations were higher in young, zinc-deficient plants than in older, zinc-adequate plants. In young leaf blades of the same physiological age, zinc concentrations showed reasonably constant relationships with plant growth throughout the entire experiment. However, they varied two- to three-fold in leaves of different ages from the same plants. The results show the importance for diagnosis of zinc deficiency of selecting as a sample a single organ of defined physiological age. The youngest open leaf blade is recommended for diagnosis of zinc deficiency in subterranean clover.

Zinc in subterranean clover (Trifolium subterraneum L. cv. Seaton Park). I. Effects of zinc supply on distribution of zinc and dry weight among plant parts

Black gram (Vigna mungo (L.) Hepper) cv. Regur was grown to maturity in three potted soils [Badgingarra (Sl), Lancelin (S2) and Quailing (S3)] with (+B) or without (-B) added H3BO3. The subsequent germination and vigour of seeds from treated plants was tested by germinating them for 7 days with 2000 8M CaSO4 and 98M H3BO3and, for S2 and S3 only, by staining with tetrazolium. No symptoms of B deficiency were observed in plants on any soil:leaf B concentrations at early vegetative and flowering stages were above values regarded as critical for this species (7-8 mg kg-1). On S1, B depressed pod and seed dry matter (DM), weight per seed and % hard seed (14 v. 58); it enhanced % non-viable imbibed seed (18 v. 5) and abnormal seedlings (37 v. 2). On S2, -B had no effect on reproductive yield or % hard seed but increased the % abnormal seedlings (32 v. 0) and depressed the tetrazolium rating for seed vigour, and weight per seed. No effects of B were observed on seeds from plants grown on S3 soil. When considered over all replicates of all treatments, a B concentration of 6 mg kg-1 DM seed was critical for viability of imbibed seed and production of normal seedlings. The results suggest that the subsequent germination and morphology of seeds and seedlings are more sensitive to low B supply during seed production than are growth of the plant and the amount of seed produced. It is thus likely that black gram crops low in B but with no symptoms of B deficiency may produce seed with low % germination and a high % abnormality in their seedlings.

Germination and vigour of black gram (Vigna mungo (L.) Hepper) seed from plants grown with and without boron

The effect of phosphorus on the relationship of zinc concentrations in various plant parts to yield of Seaton Park subterranean clover was examined. Plants were grown in a glasshouse at three levels of phosphorus (39, 65 and 130 mg phosphorus/pot; denoted P1, P2 and P3 respectively) and six levels of zinc added in factorial combination to a sand deficient in both phosphorus and zinc. At the lowest three levels of zinc supply, plants were severely to moderately deficient in zinc: in them, increasing levels of phosphorus depressed growth and induced high concentrations of phosphorus (> 1% DM) in several plant parts and symptoms of phosphorus toxicity in leaves. At the highest three levels of zinc supply, plants at P1 were phosphorus deficient: application of P2 and P3 increased growth and induced zinc deficiency primarily by diluting the available zinc. In addition, P3 appeared to depress slightly the zinc content of plant tops by another mechanism. In severely zinc-deficient plants, phosphorus supply changed the relationships between zinc concentrations in various plant parts and yield of whole tops, probably as the result of phosphorus toxicity. In the youngest open leaf blades, an asymptotic relationship at P1 changed at P2 and P3 to sigmoidal and to ‘Piper-Steenbjerg’ relationships respectively. These changes would not have invalidated the use of plant analysis for diagnosing zinc deficiency. In moderately zinc-deficient plants, phosphorus supply had little or no effect on the relationships of zinc concentration in plant parts to yield of shoots. As a result, critical concentrations in plant parts generally remained constant over the whole range of phosphorus supply. The data refute suggestions that high levels of phosphorus in plant parts inactivate the zinc within them, thus removing a potential problem in the use of plant analysis for diagnosing zinc deficiency in subterranean clover. The results confirm the previous suggestion that a concentration range of 12-14 µg zinc/g in the youngest, open leaf blade is critical for diagnosis of zinc deficiency in subterranean clover.

Zinc in subterranean clover (Trifolium subterraneum L. cv. Seaton Park). II. Effects of phosphorus supply on the relationship between zinc concentrations in plant parts and yield

The responses of five Eucalyptus species (E. calophylla, E. globulus, E. maculata, E. marginata and E. resinifera) to four levels of calcium phosphate and three levels of lime (CaCO3) on a granitic pallid zone clay of pH 4.0 from a jarrah forest soil were examined in a glasshouse trial. In the presence of a complete fertilizer, except for lime and phosphate, seedlings made poor growth and had weak root systems with few fine roots. Application of calcium phosphate promoted shoot and root growth in all species. Application of lime had no effect on plant growth except at very high levels (7.1 g per kg soil), where it induced iron deficiency chlorosis in some species. All species achieved maximum yield with an application of calcium phosphate of between 30 and 300 mg per kg soil. Phosphorus concentrations in individual leaves and shoots were low (0.1-0.3%) and responded weakly to phosphate treatments. Phosphorus concentrations in stems responded strongly to increasing phosphate applications over the whole range of phosphate supply. We suggest that stem tissues may be used to indicate the phosphorus status of eucalypts.

The phosphorus response of Eucalyptus Seedlings grown in a pallid zone clay treated with three levels of lime

Critical K concentrations for diagnosis of K deficiency were determined in various leaf, petiole and stem tissues at growth stages R3–4 (37 days after sowing‐D37) and R5–6 (D55) of soybean (Glycine max (L.) Merr.) grown at eight levels of K supply on a K deficient soil. Critical concentrations declined substantially between D37 and D55 in all tissues examined except the basal stem internode and the petiole of the leaf below the youngest fully expanded leaf (YFEL). Calculation of K concentrations on the basis of tissue water content did not arrest the decline in critical concentrations. It is suggested that the main factors causing the decline in critical K concentrations were decreasing K supply from the soil with time and the remobilization of K out of leaves, petioles and stems into developing seeds. Until reasons for the decline in critical concentrations for deficiency diagnosis in soybean are clarified, critical values for YFEL must be related to growth stages of the plant. In the YFEL, crit...

Diagnosis of potassium deficiency in soybean

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