Mechanics of root growth

Summary A model is developed for the rate of elongation of a root tip in terms of the balance of pressures acting on the root. Differentials of this equation give expressions for the changes in root elongation rate with respect to soil water potential and soil mechanical resistance. The model predicts that root cells osmoregulate against both water stress and soil mechanical resistance with predicts that root cells osmoregulate against both water stress and soil mechanical resistance with similar efficiencies which are less than 100%. Analysis of published data leads to the conclusion that root tips of pea osmoregulate with 70% efficiency. A working equation is developed for the elongation rate of roots in conditions of combined water stress and mechanical resistance.     

Plant-water relations in wheat as influenced by root pruning

Summary A greenhouse study in which 24, 54 and 71 per cent roots of wheat (Triticum aestivum L.) were pruned on the 73rd day from the date of planting (anthesis stage) showed that during a 7-day period following root pruning, total transpiration and leaf water potential were significantly lower (P=0.05) and the stomatal resistance was significantly higher (P=0.05) where 54 and 71 per cent roots were pruned, as compared to no root pruning or 24 per cent root pruning. The leaf relative water content, however, showed no significant differences. Thus about one-fourth root sytem could be reduced without adversely affecting the plant-water status.     

Reversible effects of moderately elevated temperature on the distribution of excitation energy between the two photosystems of photosynthesis in intact avocado leaves

Initial (F_o), maximum (F_m) and steady-state (F_s) levels of modulated chlorophyll fluorescence were measured in intact avocado leaves (Persea americana Mill.) during state 1-state 2 transitions using a combination of modulated and non-modulated lights with synchronized detection. Under normal temperature conditions (20°C), transition from state 2 to state 1 was associated with a substantial increase (about 20%) in F_m and F_o whereas the F_m/F_o ratio remained constant, reflecting increased absorption cross-section of PS II. On the contrary, at moderately elevated temperature (35°C), these fluorescence changes were very limited, indicating marked inhibition of the state regulation. The fraction of light distributed to PS II () was calculated from the F_o, F_m and F_s levels for both types of leaves. In control leaves, varied from 48% (in state 2) to values as high as 58% (in state 1). In contrast, mild heat treatment resulted in values close to 50% in both states, indicating the inability of heated leaves to reach extreme state 1. The results suggested that avocado leaves under moderately elevated temperature conditions are blocked in a state close to state 2. This effect was shown to occur in a non-injurious temperature range (as shown by the preservation of the (photoacoustically monitored) oxygen evolution activity) and to be rapidly reversed upon lowering of the temperature. Thermally induced development of state 2 (independent on the light spectral quality) could possibly be a protective mechanism to avoid photodamage of the heat-labile PS II by high light intensities which usually accompany heat stress in the field.     

A low-temperature-sensitive intermediate state between S2 and S3 in photosynthetic water oxidation deduced by means of thermoluminescence measurements

The temperature dependence of S-state transitions in Photosystem II was measured by means of thermoluminescence using two different protocols for low-temperature flash excitation: protocol A, “last flash at low temperature”, and protocol B, “all flashes at low temperature”. Comparison of the temperature-dependence curves obtained by these two protocols revealed a marked difference particular for the three-flash experiments. The difference was attributed to the formation of a low-temperature sensitive precursor state between S₂ and S₃. The state is formed by two flash illumination given at −5 to −50°C, spontaneously transforms to normal S₃ on dark warming, and is not converted to S₀ by the 3rd flash. The precursor state was tentatively assigned to an S₃ in which H⁺ release is not completed.     

The duct system of the avian salt gland as a transporting epithelium: structure and morphometry in the duck Anas platyrhynchos

Summary The duct system of the nasal salt gland of the duck comprises central canals, secondary ducts and main ducts. The secondary and main ducts consist of a layer of columnar cells overlying a layer of small cuboidal cells. The columnar cells have complex intercellular spaces showing evidence of Na⁺ K⁺ -ATPase at the apical regions. Approximately 70% of surface area of the duct system is external to the gland. During adaptation to salt water the duct system increases in size as does the gland. Although the components of the gland of adapted ducks, including the duct system within the gland, increase in size compared with normal ducks, the percentage volume densities of the components remain similar in both categories of ducks, i.e. the duct system increases in size in proportion to the glandular tissue. The volume of the duct system external to the gland is six to seven times larger than the volume within the gland. Thus, if ductal modification of secreted fluid occurs, it will be most likely to take place in the ducts external to the gland.Total surface areas of the duct system were measured from serial sections of glands and ducts from one normal and one adapted duck. These were used to calculate possible flux rates of water and sodium across the duct epithelium, assuming the occurrence of either water reabsorption or sodium secretion. Although these flux rates are high it is shown that they are similar to calculated flux rates across the luminal surface of the secretory tubules.     

Changes in gas exchange characteristics and water use efficiency of mangroves in response to salinity and vapour pressure deficit

Summary Measurements were made of the photosynthetic gas exchange properties and water use efficiency of 19 species of mangrove in 9 estuaries with different salinity and climatic regimes in north eastern Australia and Papua New Guinea. Stomatal conductance and CO₂ assimilation rates differed significantly between species at the same locality, with the salt-secreting species, Avicennia marina, consistently having the highest CO₂ assimilation rates and stomatal conductances. Proportional changes in stomatal conductance and CO₂ assimilation rate resulted in constant and similar intercellular CO₂ concentrations for leaves exposed to photon flux densities above 800 mol·m^-2·s^-1 in all species at a particular locality. In consequence, all species at the same locality had similar water use efficiencies. There were, however, significant differences in gas exchange properties between different localities. Stomatal conductance and CO₂ assimilation rate both decreased with increasing salinity and with increasing leaf to air vapour pressure deficit (VPD). Furthermore, the slope of the relationship between assimilation rate and stomatal conductance increased, while intercellular CO₂ concentration decreased, with increasing salinity and with decreasing ambient relative humidity. It is concluded from these results that the water use efficiency of mangroves increases with increasing environmental stress, in this case aridity, thereby maximising photosynthetic carbon fixation while minimising water loss.Contribution No. 459 from the Australian Institute of Marine Science     

Water use efficiency and carbon isotope composition of plants in a cold desert environment

Summary The effects of the availabilities of water and nitrogen on water use efficiency (WUE) of plants were investigated in a sagebrush steppe. The four species studied wereArtemisia tridentata (shrub),Ceratoides lanata (suffrutescent shrub),Elymus lanceolatus (rhizomatous grass), andElymus elymoides (tussock grass). Water and nitrogen levels were manipulated in a two-by-two factorial design resulting in four treatments: control (no additions), added water, added nitrogen, and added water and nitrogen. One instantaneous and two long-term indicators of WUE were used to testa priori predictions of the ranking of WUE among treatments. The short-term indicator was the instantaneous ratio of assimilation to transpiration (A/E). The long-term measures were 1) the slope of the relationship between conductance to water vapor and maximum assimilation and 2) the carbon isotope composition (¹³C) of plant material. Additional water decreased WUE, whereas additional nitrogen increased WUE. For both A/E and ¹³C, the mean for added nitrogen alone was significantly greater than the mean for added water alone, and means for the control and added water and nitrogen fell in between. This ranking of WUE supported the hypothesis that both water and nitrogen limit plant gas exchange in this semiarid environment. The short- and long-term indicators were in agreement, providing evidence in support of theoretical models concerning the water cost of carbon assimilation.     

Photosynthesis and water relations and the role of anatomy in Umbilicariaceae (lichenes) from Central Spain

Summary The response of net photosynthesis and dark respiration in eight species of Umbilicariaceae (lichenes) to temperature (-5, 0, 5, 10, 15, 20, 25, 30°C) and irradiance (55, 110, 220, 400, 620 mol photons m^-2 s^-1 PAR) was studied. The samples were collected in montane and alpine localities of the Spanish Sistema Central. The species differed widely in their net photosynthetic rates. The optimal temperature for net photosynthesis in alpine species was significantly lower than in montane species. Montane species were more photophytic than alpine ones. Water saturation and water loss rate were dependent on morphology and particularly anatomy of the thallus. The physiological and structural data are useful in the interpretation of the ecology and altitudinal distribution of the Umbilicariaceae. No adaptation could be linked to particularities of the mediterranean climate.     

Water relations of the parasite: host relationship between the mistletoe Amyema linophyllum (Fenzl) Tieghem and Casuarina obesa Miq

Summary Seasonal and diurnal gas exchange and water relations of Amyema linophyllum and its host Casuarina obesa were studied at Gingin Western Australia. As recorded elsewhere for other species of mistletoe, stomatal conductances and transpiration rates were consistently higher in parasite than host, but assimilation rates did not differ significantly between partners, and water use efficiency was accordingly substantially lower in the parasite. Parallel responses of the species to environmental conditions suggested closely coordinated stomatal behaviour. However, sunlit and artifically shaded clumps of Amyema maintained high leaf conductances even when foliage fell below turgor loss point, yet their tissue capacitance values indicated maintenance of greater tissue water reserves during stress than in the host. Pressure-volume relationships indicated that differences in tissue water relations were unlikely to contribute significantly to the observed gradient in leaf water potential between partners. An experiment measuring changes in water potential of freshly detached host: parasite systems cut with the host shoot end immersed in water indicated that the haustorial junction was the principal site of resistance to transpiration-driven water flow into the parasite. A parallel experiment on intact attached shoots with mistletoe clumps enclosed and darkened just before dawn, demonstrated that, once the host commenced rapid transpiration, the water potential gradient between partners became reversed.     

Gas exchange and water balance of a mistletoe species and its mangrove hosts

Summary The gas exchange and water relations of the hemiparasite Pthirusa maritima and two its mangrove host species, Conocarpus erectus and Coccoloba uvifera, were studied in an intertidal zone of the Venezuelan coast. Carbon uptake and transpiration, leaf osmotic and total water potential, as well as nutrient content in the xylem sap and leaves of mistletoes and hosts were followed through the dry and wet season. In addition, carbon isotope ratios of leaf tissue were measured to further evaluate water use efficiency. Under similar light and humidity conditions, mistletoes had higher transpiration rates, lower leaf water potentials, and lower water use efficiencies than their hosts. Potassium content was much higher in mistletoes than in host leaves, but mineral nutrient content in the xylem sap of mistletoes was relatively low. The resistance of the liquid pathway from the soil to the leaf surface of mistletoes was larger than the total liquid flow resistance of host plants. Differences in the daily cycles of osmotic potential of the xylem sap also indicate the existence of a high resistance pathway along the vascular connection between the parasite pathway along the vascular connection between the parasite and its host. P. maritima mistletoes adjust to the different physiological characteristics of the host species which it parasitizes, thus ensuring an adequate water and carbon balance.