A Morphometric Analysis of the Redistribution of Organelles in Columella Cells of Horizontally-oriented Roots of Zea mays

In order to determine what structural changes in graviperceptivecells are associated with the onset of root gravicurvature,the redistribution of organelles in columella cells of horizontally-oriented,graviresponding roots of Zea mays has been quantified. Rootgravicurvature began by 15 min after reorientation, and didnot involve significant changes in the (i) volume of individualcolumella cells or amyloplasts, (ii) relative volume of anycellular organelle, (iii) number of amyloplasts per columellacell, or (iv) surface area or cellular location of endoplasmicreticulum. Sedimentation of amyloplasts began within 1 to 2min after reorientation, and was characterized by an intenselystaining area of cytoplasm adjacent to the sedimenting amyloplasts.By 5 min after reorientation, amyloplasts were located in thelower distal corner of columella cells, and, by 15 min afterreorientation, overlaid the entire length of the lower cellwall. No consistent contact between amyloplasts and any cellularstructure was detected at any stage of gravicurvature. Centrally-locatednuclei initially migrated upward in columella cells of horizontally-orientedroots, after which they moved to the proximal ends of the cellsby 15 min after reorientation. No significant pattern of redistributionof vacuoles, mitochondra, dictyosomes, or hyaloplasm was detectedthat correlated with the onset of gravicurvature. These resultsindicate that amyloplasts and nuclei are the only organelieswhose movements correlate positively with the onset of gravicurvatureby primary roots of this cultivar of Zea mays. Zea mays, root gravitropism, ultrastructure, morphometry, graviperception     

A new genus of freshwater hoplonemertean from New Zealand

Potamonemertes percivali , a new freshwater hoplonemertean from the Selwyn River, New Zealand, is described. It is more closely related to the antipodean prosorhochmid supralittoral and terrestrial genera Acteonemertes and Geonemertes than to the only other freshwater hoplonemertean genus Prostoma,* but possesses a distinctive combination of characters which establish it as a new genus and species.     

SCAB, CANKER, AND BRANCH BLISTER OF APPLE AT EAST MALLING IN RELATION TO NUTRITIONAL TREATMENT OF THE HOST

On the sandy loam at East Mailing, moderate annual dressings of inorganic sources of nitrogen and potash exerted no consistent influence on the incidence of apple scab, even when the dressings were continued over an eleven-year period on established trees of four different varieties on M.IX rootstock. Likewise, no influence of such dressings continued over five seasons was shown on the incidence of apple canker on the rootstock M.VIII, which was grown under conditions favourable for severe infection.
There was evidence from early observations that grassing-down to excess can reduce the incidence of scab, but only at the expense of nitrogen starvation in the trees. Short-term mixed leys had no such effect.
The incidence of branch blister, a functional disease associated with adverse soil conditions and retarded vigour, was greatly reduced in the variety Cox's Orange Pippin by potash manuring, and was further influenced by the rootstock.     

Calcium and plant organelles

Abstract. The role of intracellular organelles in the regulation of cytosolic Ca²⁺ levels and whether changes in these levels affect organelle metabolism is considered. We have assessed the biochemical properties of the Ca²⁺ transporting systems in mitochondrial, chloroplast and microsomal fractions. It is proposed that although all of these organelles can transport Ca²⁺ to varying extents it would appear that in some tissues at least mitochondria do not play a significant role in the maintenance of cytosolic Ca²⁺. The most important Ca²⁺ transporting systems are probably the ATP dependent Ca²⁺ extrusion across the plasma membrane and Ca²⁺ uptake by endoplasmic reticulum, as well as light driven Ca²⁺ uptake by chloroplasts. Changes in cytoplasmic [Ca²⁺] do appear to regulate the activity of NAD kinase in chloroplasts, the mitochondrial external NADH dehydrogenase and intra-mitochondrial glutamate dehydrogenase, all of which play a key role in plant cell metabolism. Since some of these enzymes are affected by primary stimuli such as light or hormones, it is concluded that Ca²⁺ may act as a second messenger mediating some of the primary responses.     

The role of silica in protecting Italian ryegrass (Lolium multiflorum) from attack by dipterous stem-boring larvae (Oscinellafiit and other related species)

The importance of the silica content of the plant as a protection against dipterous stem-borers was studied. Levels of silica in the stubble were inversely related to the degree of infestation by stem-borers in 13 varieties of established swards of Italian ryegrass and also in newly-sown swards of four varieties. There were significant reductions in larval colonisation of Italian ryegrass (cv. RvP) grown on soil watered with sodium silicate solution. A small range of silica content in the stubble was related to large differences in the level of attack by stem-borers.     

On the significance of C3—C4 intermediate photosynthesis to the evolution of C4 photosynthesis

Abstract Evidence is drawn from previous studies to argue that C₃—C₄ intermediate plants are evolutionary intermediates, evolving from fully-expressed C₃ plants towards fully-expressed C₄ plants. On the basis of this conclusion, C₃—C₄ intermediates are examined to elucidate possible patterns that have been followed during the evolution of C₄ photosynthesis. An hypothesis is proposed that the initial step in C₄-evolution was the development of bundle-sheath metabolism that reduced apparent photorespiration by an efficient recycling of CO₂ using RuBP carboxylase. The CO₂-recycling mechanism appears to involve the differential compartmentation of glycine decarboxylase between mesophyll and bundle-sheath cells, such that most of the activity is in the bundlesheath cells. Subsequently, elevated phosphoenolpyruvate (PEP) carboxylase activities are proposed to have evolved as a means of enhancing the recycling of photorespired CO₂. As the activity of PEP carboxylase increased to higher values, other enzymes in the C₄-pathway are proposed to have increased in activity to facilitate the processing of the products of C₄-assimilation and provide PEP substrate to PEP carboxylase with greater efficiency. Initially, such a ‘C₄-cycle’ would not have been differentially compartmentalized between mesophyll and bundlesheath cells as is typical of fully-expressed C₄ plants. Such metabolism would have limited benefit in terms of concentrating CO₂ at RuBP carboxylase and, therefore, also be of little benefit for improving water- and nitrogen-use efficiencies. However, the development of such a limited C₄-cycle would have represented a preadaptation capable of evolving into the leaf biochemistry typical of fully-expressed C₄ plants. Thus, during the initial stages of C₄-evolution it is proposed that improvements in photorespiratory CO₂-loss and their influence on increasing the rate of net CO₂ assimilation per unit leaf area represented the evolutionary ‘driving-force’. Improved resourceuse efficiency resulting from an efficient CO₂-concentrating mechanism is proposed as the driving force during the later stages.