Rhizodeposition under ambient and elevated CO2 levels

As global CO₂ levels rise, can soils store more carbon and so buffer atmospheric CO₂ levels? Answering this question requires a knowledge of the rates of C inputs to soil and of CO₂ outputs via decomposition. Below-ground inputs from roots are a major component of the C flow into soils but are still poorly understood. In this article, new techniques for measuring rhizodeposition are reviewed and discussed and the need for cross-comparisons between methods is identified. One component of rhizodeposition, root exudation, is examined in more detail and evidence is presented which suggests that current estimates of exudate flow into soils are incorrect. A mechanistic mathematical model is used to explore how exudate flows might change under elevated CO₂.     

Alternate bearing influences annual nutrient consumption and the total nutrient content of mature pistachio trees

The influence of alternate bearing on nutrient utilization and total tree nutrient content was investigated in mature pistachio (Pistacia vera L. cv Kerman trees). Removal of N, P and Zn in fruit and abscised leaves of cropping (‘on’) trees averaged 5, 6, and 2 times, respectively, the removal in abscised leaflets of the non-fruiting, ‘off’ year trees. One hundred and thirty-five kg N, 131 kg K, 86 kg Ca, 39 kg Mg and 18 kg P per hectare were removed in fruits and abscised leaves in ‘on’ year trees. Tree nutrient contents and, presumably, the size of nutrient storage pools in dormant trees varied between ‘on’ and ‘off’ years. There was 22% and 14% more N and P, respectively, in dormant trees following ‘off’ than ‘on’ years. The greater N and P accumulation in ‘off’ year trees is depleted in support of the large fruit demand for N and P during ‘on’ years. In contrast to N and P, there was greater K and Ca accumulation in perennial tree parts during ‘on’ years than during ‘off’ years. The greater K accumulation in perennial tree parts and approximately 30% greater removal of K in annual organs during ‘on’ than ‘off’ years suggests that K uptake could be 4 times higher in ‘on’ year trees than in (non-cropping), ‘off’ year trees.     

Ex situ conservation of plant germplasm using biotechnology

Conservation of plant genetic resources attracts more and more public interest as the only way to guarantee adequate food supplies for future human generations. However, the conservation and subsequent use of such resources are complicated by cultural, economical, technical and political issues. Over the last 30 years, there have been significant increases in the number of plant collections and in accessions in ex situ storage centres throughout the World. The present review is of these ex situ collections and the contribution biotechnology has made and can make to conservation of plant germplasm. The applications and limitations of the new, molecular approaches to germplasm characterization are discussed. In vitro slow growth is used routinely for conserving germplasm of plants such as banana, plantain, cassava and potato. More recently, cryopreservation procedures have become more accessible for long-term storage. New cryopreservation techniques, such as encapsulation-dehydration, vitrification and desiccation, lengthen the list of plant species that can not only tolerate low temperatures but also give normal growth on recovery. Extensive research is still needed if these techniques are to be fully exploited.V.M. Villalobos is with the Food and Agriculture Organization of the United Nations, Viale delle Terme di Caracalia, 00100 Rome, Italy. F. Engelmann is with the International Plant Genetic Resources Institute (IPGRI), Via delle Sette Chiese 142, 00145 Rome, Italy.     

Two cDNA clones coding for the legumin protein of Pisum sativum L. contain sequence repeats

The sequence of two cDNA clones coding for the whole of the -subunit and most of the -subunit of legumin are presented together with a considerable amount of protein sequence data to confirm the predicted amino acid sequence. A unique feature shown by these cDNAs is the presence of three 56 base pair tandem repeats in the region encoding the C terminal of the polypeptide. The tandem repeats are also exhibited in the predicted polypeptide sequence as three 18 amino acid repeats which contain extremely high proportions of polar, mainly acidic, residues. The new sequences are compared to the previously published sequence of some shorter legumin cDNAs (Nature 295: 76–79). In the region where the sequences overlap, the previous cDNAs differ from the new ones by only a few base substitutions but most of the repeated region is not present though the sequences on either side are. The possibility that the absence of the repeats may reflect the difference between two types of legumin gene, rather than an artefact of the cloning of the cDNAs, is discussed.     

Changes in cell-wall polysaccharides in relation to seedling development and the mobilisation of reserves in the cotyledons of Lupinus angustifolius cv. Unicrop

Some 22% of the dry weight of the cotyledons of resting seeds of Lupinus angustifolius cv. Unicrop has been shown to be non-starch polysaccharide material comprising the massively thickened walls of the storage mesophyll cells. On hydrolysis this material released galactose (76%), arabinose (13%), xylose (4%), uronic acid (7%): only traces of glucose were detected indicating the virtual absence of cellulose from the walls. Changes in the amount and composition of this material following germination have been studied in relation to parameters of seedling development and the mobilisation of protein, lipid and oligosaccharide reserves. Starch, which was not present in the resting seed, appeared transitorily following germination: under conditions of continuous darkness starch levels were reduced. During the period of bulk-reserve mobilisation, 92% of the non-starch polysaccharide material disappeared from the cotyledons. The residual cell-wall material released galactose (14%), arabinose (19%), xylose (24%) and uronic acid (43%). The galactose and arabinose residues of the cotyledonary cell walls clearly constitute a major storage material, quantitatively as important as protein. The overall role of the wall polysaccharides in seedling development is discussed.     

Cloning of the natural gene for the sweet-tasting plant protein thaumatin

Five different clones, homologous to the structural gene for the sweet-tasting plant protein thaumatin, have been isolated from leaf DNA of Thaumatococcus daniellii Benth. Restriction maps, hybridization studies, S1-nuclease mapping and R-loop formation revealed that the thaumatin genes isolated belong to one multigene family, and have two very small introns situated at different positions in the various structural genes. A similar situation prevails in a number of seed storage genes. This suggests a similarity between the sweet-tasting protein thaumatin and seed storage proteins.     

METABOLIC SEGREGATION OF INTRACELLULAR FREE AMINO ACIDS IN PLATYMONAS (CHLOROPHYTA)1

¹⁴C-amino acids were supplied to Platymonas subcordiformis (Wille) Hazen and the incorporation of radioactivity into protein and other compounds was followed. Alanine was rapidly metabolized by both N-limited and N-sufficient cells. Arginine and lysine were metabolized rapidly by N-limited cells, but were sequestered from metabolism in N-sufficient cells. This suggests the existence of two functionally distinct pools; a “metabolic” pool that is rapidly metabolized and preferentially used for incorporation into protein, and a “storage” pool rich in basic amino acids that is sequestered from metabolism.     

Hormonal control of uric acid storage in the fat body during last-larval instar of Manduca sexta

In the last-larval instar of the tobacco hornworm, Manduca sexta, a switch from excretion of uric acid to storage in the fat body occurs during transition from the feeding to the wandering stage. Neuroendocrine control of this change from excretion to storage was demonstrated by neck-ligation experiments with synchronously reared larvae. Results indicate that a neurohormone is released from the head 24–30 hr before the initiation of wandering and coincident with the first release of ecdysone that initiates metamorphosis. Direct involvement of the moulting hormone was shown by the effects of multiple injections of 20-hydroxyecdysone into the abdomen of larvae that had been ligated before the release of hormone. Urate levels in the fat body were 20- to 100-fold higher from hormone-injected larvae as from saline inject controls. Topically applied juvenile hormone or methoprene reversed the 20-hydroxyecdysone-induced storage of urate. Increased levels of uric acid in the haemolymph during pupal development result from the presence of juvenile hormone, and the abrupt decrease in uric acid concentration in the haemolymph just prior to pupal ecdysis results from a decreased titre of juvenile hormone. Applications of methoprene prevented the decrease in uric acid levels in the haemolymph.     

Fixation and distribution of 14C in Populus deltoides during dormancy induction

Photosynthetically fixed ¹⁴C was analyzed in various chemical fractions from leaves and stems of cottonwood (Populus deltoides Bartr. ex. Marsh.) during dormancy induction. Dormancy was induced by 8-h photoperiods and 20/14°C temperature regimes. Within 4 weeks under short days, terminal buds were set and leaf expansion and stem elongation had stopped. ¹⁴C₂ was fed to a leaf at Leaf Plastochron Index 7 for 30 min. Either after this 30 min feeding period or after a 48-h translocation period the plants were sampled, freeze-dried, extracted and analyzed for¹⁴C. ¹⁴C-fixation decreased during dormancy induction from 60% to 17% of the 3.7 MBq ¹⁴C applied at 0 week and 8 weeks, respectively. Percentage distribution of ¹⁴C in chemical fractions of source leaves reflected leaf age and translocation inhibition. In rapidly growing plants, considerable ¹⁴C was incorporated into leaf protein while most of the soluble¹⁴C-sugars were either metabolized or translocated out of the leaf. After terminal bud set, the percentage of ¹⁴C in the protein and residue fractions decreased rapidly and that in the sugar fraction increased. Percent distribution in stems closely reflected changing metabolic pathways of carbon flow as influenced by dormancy induction. For example, the ¹⁴C in structural carbohydrates decreased in 5 weeks under short days from 65 to less than 10% of the ¹⁴C recovered in the chemical fractions, thus indicating cambium inhibition. At the same time the percentage of ¹⁴C in starch and sugar increased indicating storage. Short term (after 30 min) incorporation of ¹⁴C into the protein and starch fractions of leaves changed relatively little throughout the 8-week induction period. In contrast the turnover rates of these fractions (¹⁴C present after 48 h) increased considerably after active growth of the whole plant stopped.