D. J. Yates

The dynamics of photosynthetic acclimation to changes in light quanlity and quality in three Australian rainforest tree species

Photosynthetic acclimation was studied in seedlings of three subtropical rainforest species representing early (Omalanthus populifolius), middle (Duboisia myoporoides) and late (Acmena ingens) successional stages in forest development. Changes in the photosynthetic characteristics of pre-existing leaves were observed following the transfer of plants between deep shade (1–5% of photosynthetically active radiation (PAR), selectively filtered to produce a red/far-red (R/FR) ratio of 0.1) and open glasshouse (60% PAR and a R/FR ratio of 1.1–1.2), and vice versa. The extent and rate of response of the photosynthetic characteristics of each species to changes in light environment were recorded in this simulation of gap formation and canopy closure/overtopping. The light regimes to which plants were exposed produced significant levels of acclimation in all the photosynthetic parameters examined. Following transfer from high to low light, the light-saturated rate of photosynthesis was maintained near pre-transfer levels for 7 days, after which it decreased to levels which closely approximated those in leaves which had developed in low light. The decrease in photosynthetic capacity was associated with lower apparent quantum yields and stomatal conductances. Dark respiration was the parameter most sensitive to changes in light environment, and responded significantly during the first 4–7 days after transfer. Acclimation of photosynthetic capacity to increases in irradiance was significant in two of the three species studied, but was clearly limited in comparison with that of new leaves produced in the high light conditions. This limitation was most pronounced in the early-successional-stage species, O. populifolius. It is likely that structural characteristics of the leaves, imposed at the time of leaf expansion, are largely responsible for the limitations in photosynthetic acclimation to increases in irradiance.

Production and dispersal of Colletotrichum gloeosporioides spores on Stylosanthes scabra under elevated CO2.

This paper reports the effect of twice-ambient (700 ppm) atmospheric CO(2) concentration on infection, disease development, spore production and dispersal of the anthracnose pathogen Colletotrichum gloeosporioides in susceptible (Fitzroy) and partially resistant (Seca) cultivars of the tropical pasture legume Stylosanthes scabra under controlled environment and field conditions. Reduction in plant height due to anthracnose was partially compensated for by growth enhancement at elevated CO(2) in Fitzroy but not in Seca. Anthracnose severity was reduced under elevated CO(2) although the reduction was only significant in Fitzroy. Delayed and reduced germination, germtube growth and appressoria production were partly responsible for the reduced severity. Despite an extended incubation period, C. gloeosporioides developed sporulating lesions faster and produced more spores per day within the same latent period at high CO(2) and ambient CO(2). When Fitzroy seedlings grown at 700 ppm CO(2) were exposed to pathogen inoculum under field conditions, they consistently developed more severe anthracnose with more lesions than seedlings grown at ambient CO(2). The environmental variable, which correlated most strongly with the dispersal and infection of C. gloeosporioides spores in the field, was relative humidity in plant canopy. We have shown that an enlarged Stylosanthes canopy under elevated CO(2) can trap more spores, which can lead to more severe anthracnose under favorable weather. The implications of these findings for perennial Stylosanthes pastures are discussed.

Seasonal variation in the red/far-red ratio and photon flux density in an Australian sub-tropical rainforest

The temporal distribution of red light (R, in the range 655-665 nm) and far-red light (FR, in the range 725-735 nm) was measured at 10 min intervals for 1 year at three contrasting sites in a sub-tropical rainforest. It was measured in the understorey beneath a closed canopy, in the middle of a 450 m2 tree-fall gap, and at the transition between the gap and understorey. The red/far-red ratio (R/FR) was calculated from these data. Values of photon flux density (PFD, in the range 400-700 nm) were also determined from the output of the R sensing channel of each sensor. Although a higher overall R/FR ratio was measured in the gap site compared with the understorey and gap-edge sites, R/FR ratios in the gap were at levels which are indicative of canopy shade (between 0.4 and 0.8) for much of the day. No distinct seasonal variation in daily average R/FR ratio was observed in any of the three sites, although day to day variation was marked. Daily average R/FR ratio was generally higher in the gap-edge and gap sites than in the understorey. Daily average PFD in the understorey was below 20 μmol m-2 s-1 for all but 1 day during the year, and below 30 μmol m-2 s-1 for 7 months at the gap-edge site and 3 months in the gap. While daily average PFD also varied from day to day in the gap-edge and gap sites, observations showed considerable seasonal variation during the course of the year.

Photosynthesis in an Australian rainforest tree, Argyrodendron peralatum, during the rapid development and relief of water deficits in the dry season

SummaryRates of apparent photosynthesis were measured in situ at five positions between the upper crown and a lower branch of a 34 m tall Argyrodendron peralatum (F.M. Bailey) H.L. Edlin ex I.H. Boas tree, and on an understorey sapling of the same species growing in a northern Australian rainforest. At the end of the dry season, rapid reductions in photosynthetic rates occurred in the upper crown within three days after a rain event, but changes in the lower crown and the sapling were less marked. Complete recovery of photosynthesis followed a second rain event. At high photon flux densities, stomatal conductance to water vapour decreased in a curvilinear fashion as the vapour pressure difference between leaf and air increased. Apparent photosynthesis was linearly related to stomatal conductance on the first clear day after each rain event, but there was no relationship between these parameters at the end of a brief natural drying cycle. Under conditions of adequate water supply, stomatal conductances of both upper crown and understorey leaves increased linearly with increasing photon flux density up to about 300 μmol m-2 s-1. During water deficits, stomatal conductances in leaves from the understorey increased much more rapidly at very low photon flux densities than did conductances in leaves from the upper canopy.

Reflexion and absorption of solar radiation by flowering canopies of oil-seed rape (Brassica Napus L.)

Flowers were present over a period of approximately 6 weeks in 12 varieties of oil-seed rape. A linear relationship was found between flower cover (maximum 74%) and photon reflectivity (400–700 nm) which increased from around 0–047 during vegetative growth to 0.195 for a canopy in full flower. Reflexion and absorption of photons by flowers reduced light available to the leaf canopy and immature pods. Spectral measurements indicate that flowering canopies reflect substantially more radiation and absorb less than vegetative canopies between 500 and 700 nm but reflect less and absorb slightly more between 400 and 500 nm. Seed yield was not strongly correlated with estimates of the radiation absorbed by the leaf canopy under the flower layer. It appears that whilst increased flower density in a canopy might be expected to contribute to increased seed yield, increased reflectivity by petals may have a negative effect on yield. Breeding for reduced petal size or for apetalous lines may result in improved yields by producing more efficient light distribution within the canopy.

Identity and genetic diversity of the sorghum ergot pathogen in Australia

Sorghum ergot was first discovered in Australia in 1996. It affects seed production and grain usage in stock feed due to concerns of animal toxicity. Three species of Claviceps are known to cause ergot of sorghum with different epidemiological, animal toxicity, and management implications. Claviceps africana was identified as the causal agent but morphological differences between isolates raised the possibility of more than one species being involved. The major aim of this study was to identify the Claviceps species causing sorghum ergot and to determine the genetic diversity among isolates of the ergot pathogen from Australia and overseas. Symptom development, sequencing of the ITS1 region, and radiolabelled DNA amplification fingerprints (RAF) were used to confirm that ergot of sorghum in Australia is caused by C. africana. The morphology of sphacelia, microconidia, macroconidia, and secondary conidia of all 36 Australian isolates studied matched the description for C. africana and the DNA sequence of the ITS1 region of 2 selected Australian isolates was identical to that of C. africana. Based on RAF analysis of 110 Australian and overseas isolates of Claviceps spp., C. africana isolates could be clearly distinguished (<40% similarity) from C. pusilla, C. sorghicola, C. sorghi, and a Claviceps sp. isolated from Panicum maximum. The C. africana isolates formed 2 distinct clusters. Cluster 1 contained 72 Australian isolates and all 21 overseas isolates of C. africana. The 13 isolates in Cluster 2 were all from Australia and more diverse than those in Cluster 1. The high level of genetic diversity of C. africana isolates in Australia is unexpected given that ergot has only been reported recently. The most likely source of this diversity points to introductions from countries such as India.

Daily changes in plant water content in western Queensland: implications for red kangaroo foraging times

We measured daily changes in plant water content to see if red kangaroos (Macropus rufus), in foraging at night, may gain more water than foraging by day. If so, it could be influential in dictating their time of foraging. The study was stimulated by work in Africa which showed that, by choosing to feed at night, gazelles are able to take advantage of increased plant water content. Daily changes in water content of 15 known forage plants were examined at Idalia National Park in semi-arid western Queensland in February, April and July 2002, during progressive drying of the country in an extraordinarily dry year. Of 29 day-night comparisons of plant water content, thirteen showed no difference, two (both of red spinach, Trianthema triquetra) showed a substantial increase at night, eight showed small but statistically significant increases, and six showed small but statistically significant decreases. We conclude that the choice of red kangaroos to forage at night is unlikely to be explained by nightly increases in plant water content.

Short-Term Changes in Spectral Properties of Phyllodes of Brigalow (Acacia harpophylla F Muell Ex Benth) in Response to Wetting

The phyllodes of Acacia harpophylla (brigalow) possess a high reflectivity (0.20) in the visible wavelengths due to the presence of a dense covering of flattened trichomes. This results in a low value for phyllode absorptivity (0.79). While dry phyllodes appear grey-blue to the eye, wet phyllodes appear green and their reflection spectra are similar to those of ‘typical’ foliage elements. When wet, reflectivity in the visible wavelengths is reduced (0.09) and absorptivity is increased by 14%. Examination of the wetting and drying process, both visually and by measuring the time course of reflectivity, illustrates the importance of the wettable surfaces of both the epidermis and the individual trichomes. On drying, the reflectivity increase in the near infrared (NIR, 0.41-0.59) precedes that in the visible portion of the spectrum, although changes in the visible portion of the spectrum are larger than those in the NIR.

The involvement of noradrenergic nerves in the cardiovascular reflex responses to lower body negative pressure in the anaesthetised rabbit

Summary1.Resting cardiovascular status and the reflex responses to lower body negative pressure (LBNP) were examined in control rabbits and in rabbits sympathectomised with 6-hydroxydopamine (6-OHDA).2.Resting systemic arterial pressure and total peripheral resistance were significantly higher in control than in 6-OHDA treated animals.3.Control animals responded to LBNP at −25 mm Hg with a peripheral vasoconstriction and tachycardia. These responses were absent in 6-OHDA-treated animals.4.Control animals responded to LBNP at −70 mm Hg with a peripheral vasoconstriction and a bradycardia followed by a tachycardia. There was no significant change in heart rate in response to this stimulus in 6-OHDA-treated animals, although a slight vasoconstriction was seen.5.Bethanidine blocked the tachycardia and vasoconstriction seen in response to LBNP in control and 6-OHDA-treated rabbits. Under these conditions there was no significant difference between the total peripheral resistance of the control and treated animals.6.The results indicated that neither the maintenance of the resting systemic arterial pressure nor the reflexes elicited by LBNP in control or in chronically sympathectomised rabbits involved hormonal mechanisms, but LBNP was maintained for 1 min only. These findings are discussed in the context of previous observations.

Sky-canopy border length, exposure and thresholding influence accuracy of hemispherical photography for complex plant canopies

BackgroundHemispherical photography (HP) is a popular method to estimate canopy structure and understorey light environment, which analyses photographs acquired with wide view-angle lens (i.e. fisheye lens). To increase HP accuracy, the approaches of most previous studies were to increase the preciseness of exposure and thresholding of photographs, while ours quantified effects of canopy properties (gap fraction and length of sky-canopy border (SCB)) and errors of exposure and thresholding on the accuracy of HP.ResultsThrough analysing photographs of real and model canopies, it was showed that HP inaccuracy resulted from the mismatch between exposure and thresholding rather than exposure or thresholding errors alone. HP inaccuracy was a function of the SCB length and the extent of exposure and thresholding errors, but independent of gap fraction.DiscussionIn photographs, SCBs are recorded as grey pixels which greyness is in between that of sky and canopy pixels. When there are exposure and thresholding errors, grey pixels are those prone to be misclassified in image analysis. Longer (vegetation with taller canopies) and wider (lower image sharpness) SCBs in photographs can both result in a higher amount of grey pixels and ultimately higher HP inaccuracy for a given extent of exposure and threshold errors.ConclusionsUsing lenses with view angle narrower rather than that of fisheye lens can shorten the SCB length in photographs and in turn reduce HP estimation inaccuracy for canopy structure and understorey light environment. Since short SCBs and low levels of exposure and thresholding errors can both result in low HP inaccuracy, to identify the true performance of new exposure and thresholding methods for HP, photographs recording canopies with long SCBs and acquired with fisheye lenses should be used. Because HP inaccuracy in a function of the amount of grey pixels resulting from SCBs, the amount of these pixels in photographs can be used as a universal parameter to quantify canopy properties influential to HP estimation and in turn make cross-study comparisons feasible.