C.E.J. Botha

Reduction in transport in wheat (Triticum aestivum) is caused by sustained phloem feeding by the Russian wheat aphid (Diuraphis noxia)

The Russian wheat aphid ( Diuraphis noxia ) feeds preferentially from the phloem of longitudinal veins of non-resistant wheat leaves. The xenobiotic, 5,6-CFDA was applied to exposed leaf blade mesophyll cells in control and aphid-infested plants. In control plants, the fluorophore moved approximately 5–6cm from the point of application of 5,6-CFDA within 3h of application. The fluorochrome was transported in the sieve tube companion cell complex, including those in the numerous interconnecting transverse veins. Leaf blades on which the Russian wheat aphid had been feeding demonstrated a marked decrease in 5,6-CF transport. Aphids feeding on the leaves formed local sinks and redirected the fluorophore (and presumably associated assimilate) to the aphids’ gut, with little longitudinal translocation of 5,6-CF below inserted stylets or aphid feeding areas. Aniline blue staining revealed massive deposits of wound and reaction callose caused by the aphids, with callose associated with the sieve plates, pore-plasmodesma between the companion cells and their associated sieve tubes, as well as with plasmodesmal aggregates in parenchymatous elements within the vascular bundles. Leaves that had been colonised by aphids but from which the aphids were removed, showed extensive wound callose deposits, which persisted for up to 48h after removal of aphid colonies, suggesting that the damage caused by aphid feeding is a long-term, non-transient event in non-resistant plants.

Stronger induction of callose deposition in barley by Russian wheat aphid than bird cherry-oat aphid is not associated with differences in callose synthase or beta-1,3-glucanase transcript abundance

The effects of infestation by the bird cherry-oat aphid (BCA), (Rhopalosiphum padi L) and the Russian wheat aphid (RWA) (Diuraphis noxia Mordvilko) on callose deposition and transcription of genes related to callose accumulation were investigated in barley (Hordeum vulgare L. cv. Clipper). The BCA, which gives no visible symptoms, induced very limited callose deposition, even after 14 days of infestation. In contrast, RWA, which causes chlorosis, white and yellow streaking and leaf rolling, induced callose accumulation already after 24 h in longitudinal leaf veins. The deposition was pronounced after 72 h, progressing during 7 and 14 days of infestation. In RWA-infested source leaves, callose was also induced in longitudinal veins basipetal to the aphid-infested tissue, whereas in sink leaves, more callose deposition was found above the feeding sites. Eight putative callose synthase genes were identified in a database search, of which seven were expressed in the leaves, but with similar transcript accumulation in control and aphid-infested tissue. Five out of 12 examined beta-1,3-glucanases were expressed in the leaves. All five were upregulated in RWA-infested tissue, but only two in BCA-infested tissue, and to a lesser extent than by RWA. The results suggest that callose accumulation may be partly responsible for the symptoms resulting from RWA infestation and that a callose-inducing signal may be transported in the phloem. Furthermore, it is concluded that the absence of callose deposition in BCA-infested leaves is not because of a stronger upregulation of callose-degrading beta-1,3-glucanases in this tissue, as compared to RWA-infested leaves.

Population growth rate and relative virulence of the two South African biotypes of Russian wheat aphid, Diuraphis noxia, and bird cherry‐oat aphid, Rhopalosiphum padi, on resistant and non‐resistant barley

In South Africa a new biotype of the Russian wheat aphid (RWA), Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), RWASA2, has appeared which exhibits an improved performance compared to the original biotype (RWASA1) on wheat containing the Dn1 resistance gene. We examined population growth rates as well as damage caused by RWASA1 and RWASA2, in addition to a different aphid species, the bird cherry‐oat aphid (BCA), Rhopalosiphum padi L. (Hemiptera: Aphididae), on three RWA‐resistant barley [Hordeum vulgare L. (Poaceae)] lines (STARS‐9577B, STARS‐0502B, and STARS‐9301B) and one susceptible control (PUMA). RWASA2 had a higher reproductive rate than RWASA1 on all barley lines tested, which is consistent with previous results on wheat. Two of the RWA‐resistant lines (STARS‐0502B and STARS‐9301B) also exhibited a similar resistance phenotype against BCA. In our experiments, severe chlorosis and leaf roll appeared earlier on the control PUMA barley variety as a result of RWASA2 feeding than was the case with RWASA1, probably due to the differences in reproductive rate. Although chlorosis appeared earlier on resistant plants after RWASA2 feeding, this symptom developed much faster during RWASA1 feeding on all three resistant lines tested. As chlorosis did not correlate well with aphid population numbers, we surmise that the differential chlorosis effects may be related to differences in the amount of saliva introduced by the two aphid clones during feeding. Our results indicate that the difference between RWASA2 and RWASA1 are broader than a ‘gene for gene’ interaction with the Dn1 resistance (R) gene in wheat, and that these biotypes also differ in important aspects of their biology.

Regulation within the supracellular highway — plasmodesma are the key

Plasmodesmal connections are unique, highly dynamic intercellular structures that are lined by the plasmamembrane. They are believed to be a vital intercellular communication channel between living cells, linking numbers of living cells into interconnected, highly specialised cellular domains, thus enabling the plant to act as an integrated organism. Their evolution in the higher plant was inevitable. It is accepted that cell heterogeneity rather than cell divergence pressurised developing plant systems along a route that led to the formation of intercellular passages and connections. With time, these connections have evolved to allow some degree of regulation and traffic control. This paper explores some of the structure/function relationships in plasmodesmata. Attention is focused on the potential role of the neck region of these remarkable structures and discusses models which may explain the processes involved in regulating the movement of substances from cell to cell.

A re-evaluation of plastochron index determination in peas — a case for using leaflet length

The plastochron index (PI) is a measure of plant growth and can be used to determine growth rate, based upon appearance of successive leaves on the axis of the plant. PI should under ideal growth conditions be a regular event and should be predictable with a relatively small error of a few hours. PI has been variously calculated in peas, and each method reported has had with it a number of problems that do not allow for reasonable prediction of PI. Internode length varies greatly and is dependent upon the variety, which may be short- or long-stemmed; thus this parameter is not ideal for determining growth rate or plant age. This paper reports our findings on PI using the average length of the first pair of leaflets on each node. Early leaflet growth in peas occurs exponentially and the early stages of growth of successive pairs of leaflets occur at the same relative growth rate. Given that growth of leaflets during early development can be measured successfully, we propose the use of leaflet growth as a measure of the plastochron index in peas. Our results suggest that plant age is best expressed using the plastochron index, which is a measure of the time interval between the initiations of successive events — in the case of peas, of successive pairs of leaflets.

Effect of water stress and diclofop-methyl on photosynthesis, carotenoid and abscisic acid content of leaves of Avena byzantina and Avena fatua

The effect of combined water stress and diclofop-methyl treatment on photosynthesis and carotenoid and abscisic acid (ABA) content of leaves of A. byzantina and A. fatua was investigated. Sublethal doses of diclofop-methyl caused a transient decline in net assimilation rate and a decrease in β-carotene and zeaxanthin in leaves of both species. The decline in carotenoid levels occurred concomitant with a substantial but transient increase in ABA. A similar but less dramatic trend was observed for water-stressed plants. Recovery of photosynthesis in seedlings exposed simultaneously to diclofop-methyl and water stress, was associated with an increase in β-carotene and zeaxanthin contentand a return to basal ABA levels in leaves of A. byzantina. By comparison, substantial accumulation of zeaxanthin was observed in leaves of A. fatua following combined water stress and herbicide treatment, apparently at the expense of ABA. Similar findings were made regarding levels of zeaxanthin when diclofop-methyl was applied to already water-stressed plants of A. byzantina and A. fatua. It is proposed that herbicide- and/or water-stress-induced alterations in acetyl-coenzyme A carboxylase activity coupled with reduced demand for fatty acid synthesis, facilitate channelling of photosynthetically fixed carbon into isoprenoids and that alterations in the capacity for terpenoid synthesis forms part of the mechanism by which drought stress antagonizes the activity of aryloxyphenoxypropionic acid herbicides.

Panicum ecklonii, a new record of a C4 photosynthetic variant

The anatomy of the leaf blade of P. ecklonii Nees collected in the eastern Cape was investigated with light and transmission electron microscopes. The leaf blades of this material contain three orders of closely spaced vascular bundles, each of which is surrounded by a radiating mesophyll, and an underlying bundle sheath, which consists of relatively thick-walled, chlorenchymatous cells. Bundle sheath cells contain large, centrifugally arranged, agranal chloroplasts. The longitudinal bundles are separated by one layer of achlorophyllous mesophyll. The outer tangential and radial walls of the bundle sheath cells contain a suberized compound middle lamella. Few mitochondria occur in bundle sheath cells and are usually closely associated with plasmodesmatal pits in the outer tangential walls of the bundle sheath cells. Large bundles are subtended by ad- as well as abaxial hypodermal sclerenchyma strands. Intermediate bundles are connected to the adaxial hypodermal sclerenchyma strands by a single-layered bundle sheath extension, and the bundle sheath is in direct contact with hypodermal sclerenchyma strands abaxially. Small bundles are embedded in mesophyll tissue. Specimens of P. ecklonii from the eastern Cape thus display typical C 4 NADP anatomy.

The interactive effects of light, temperature and CO2/O2 ratios on photosynthesis of Coix lachryma-jobi L.

A portable infra-red gas analyser was used to investigate the interactive effects of light, temperature, and CO 2 /O 2 ratios under controlled environmental conditions in an attempt to model gas exchange characteristics of Coix lachryma - jobi L. Plotting light response curves as a function of temperature (20, 25, 30 and 35°C) revealed no sign of light saturation even at a photosynthetic photon flux density (PPFD) close to 2000 μmol m -2 s -1 . High net assimilation rates ( A ) of approximately 24 μmol CO 2 m -2 s -1 were realized at 30 – 35°C. Assimilation ( A ) versus internal CO 2 partial pressure ( C i ) curves showed a steep rise in A with increase in C i but saturated at approximately 150 μl l -1 and the effect was similar in either the absence or presence of O 2 , under all temperature regimes. C . lachryma - jobi exhibited low CO 2 compensation points ( Γ *) between 0 and 10 μl l -1 at either 0 or 21% O 2 . The slopes of double reciprocal plots of 1/ A versus 1/ C i , were nearly identical and crossed the y-intercept at almost identical points under all O 2 concentrations. This data indicated that there was no apparent O 2 inhibition and that the apparent inhibitor constant ( K ) for O 2 at the site of carboxylation did not change with an increase in [O 2 ] from 0 to 21%. These observations were further confirmed by results obtained from the analysis of apparent carboxylation efficiency where no inhibition of A with increase of [O 2 ] occurred. These characteristics are thus consistent with typical features of C 4 photosynthesis.

An investigation of leaf-blade anatomy and photosynthetic characteristics of four Cyperaceae species from the Albany and Bathurst districts in the eastern Cape

The anatomy of the leaf-blades of four field-grown Cyperaceae, namely Cyperus albostriatus Schrad., Cyperus fastigiatus Rottb., Cyperus pulcher Thunb. and Mariscus congestus (Vahl.) C.B.CI., was investigated with light and scanning electron microscopes, to determine stomatal type and frequencies. Photosynthetic characteristics, including light and temperature response as well as post-illumination CO2 burst effect, were investigated using an ADC 225 MKIII infra-red gas analyser in open circuit. The leaf anatomy and gas exchange response suggest that C. pulcher is a C3 species, whilst C. albostriatus, C. fastigiatus and M. congestus are C4. The post-illumination CO2 burst studies suggest that C. albostriatus and M. congestus are C4-NADP-Me species type and that C. fastigiatus is either NAD-Me or PCK.