ALBERT

Notes: Abstract Growth-chamber cultivated Raphanus plants accumulate nitrate during their vegetative growth. After 25 days of growth at a constant supply to the roots of 1 mol m−3 (NO−3) in a balanced nutrient solution, the oldest leaves (eight-leaf stage) accumulated 2.5% NO−3-nitrogen (NO3-N) in their lamina, and almost 5% NO3-N in their petioles on a dry weight basis. This is equivalent to approximately 190 and 400 mol−3 m−3 concentration of NO−3 in the lamina and the petiole, respectively, as calculated on a total tissue water content basis. Measurements were made of root NO−3 uptake, NO−3 fluxes in the xylem, nitrate uptake by the mesophyll cells, and nitrate reduction as measured by an in vivo test. NO−3 uptake by roots and mesophyll cells was greater in the light than in the dark. The NO−3 concentration in the xylem fluid was constant with leaf age, but showed a distinct daily variation as a result of the independent fluxes of root uptake, transpiration and mesophyll uptake. NO−3 was reduced in the leaf at a higher rate in the light than in the dark. The reduction was inhibited at the high concentrations calculated to exist in the mesophyll vacuoles, but reduction continued at a low rate, even when there was no supply from the incubation medium. Sixty-four per cent of the NO−3 influx was turned into organic nitrogen, with the remaining NO−3 accumulating in both the light and the dark.

Notes: Abstract. Wild radish plants deprived of, and continuously supplied with solution NO−3 for 7 d following 3 weeks growth at high NO−3 supply were compared in terms of changes in dry weight, leaf area, photosynthesis and the partitioning of carbon and nitrogen (NH2-N and NO−3-N) among individual organs. Initial levels of NO−3-N accounted for 25% of total plant N. Following termination of NO−3 supply, whole plant dry weight growth was not significantly reduced for 3 d, during which time plant NH2-N concentration declined by about 25% relative to NO−3-supplied plants, and endogenous NO−3-N content was reduced to nearly zero. Older leaves lost NO−3 and NH2-N, and roots and young leaves gained NH2-N in response to N stress. Relative growth rate declined due both to decreased net assimilation rate and a decrease in leaf area ratio. A rapid increase in specific leaf weight was indicative of a greater sensitivity to N stress of leaf expansion compared to carbon gain. In response to N stress, photosynthesis per unit leaf area was more severely inhibited in older leaves, whereas weight-based rates were equally inhibited among all leaf ages. Net photosynthesis was strongly correlated with leaf NH2-N concentration, and the relationship was not significantly different for leaves of NO3−-supplied compared to NO−3-deprived plants. Simulations of the time course of NO−3 depletion for plants of various NH2-N and NO−3 compositions and relative growth rates indicated that environmental conditions may influence the importance of NO−3 accumulation as a buffer against fluctuations in the N supply to demand ratio.

Notes: Summary The photosynthesizing branches of Hammada scoparia, one of the typical dwarf shrubs of the Negev desert, undergo a seasonal change from succulent to xeromorphic anatomy. This trend is accompanied by a marked decrease of water content and of total water Ψ plant and osmotic Ψ π plant potential. Irrigated plants do not show such transitions. The daily courses of Ψ plant and Ψ π plant showed minima around noon and a tendency for maxima before sunrise. Turgor pressure Ψ p plant reached minima around noon and became negative (until ca.-10 bars). Generally, Ψ plant decreases with increasing water vapour concentration difference between plant and air (WD) in the first half of the day, and in the second half the reversal of this trend occurs. Mostly smaller increments of Ψ plant were correlated with larger increases in WD which lead to the conclusion that stomates closed enough to maintain transpiration at a constant value. Non-irrigated and irrigated plants showed different hysteresis loops of relation between Ψ plant and WD. Regulatory reduction of transpiration appears largely independently of Ψ plant which is in spring and with irrigated plants on a high level, with non-irrigated plants in summer on a low level. In summer the continous but decreasing drop of Ψ plant with increasing WD was interpreted as caused by a change in soil or root resistance. Independent of the seasonal state and of the Ψ plant level, H. scoparia regulates its water status within limited ranges of Ψ p plant changes: the irrigated plants on a higher level, the non-irrigated on a lower level of Ψ p plant . The water contents of the tissues of H. scoparia are linearily related to Ψ plant as well as Ψ p plant . Steeper slopes with non-irrigated plants in summer than with spring palnts and with irrigated plants during the whole season signify that in the latter a certain increment in turgor pressure corresponds to a large gain in water content while in the non-irrigated summer plants it varies only little for an identical change in Ψ p plant . This behaviour of non-irrigated wild plants apparently is due to the change of the elastic properties of the tissues in the assimilating branches.

Notes: Summary An empirical model for describing daily courses of net photosynthesis in Hammada scoparia is being developed. The model is based on the functional relationships, by which various environmental factors affect the photosynthetic activity and which can be measured by experiment in the field. In a sequence of steady-states daily courses of net photosynthesis are predicted during a growing season considering the variability of the physiological states and the capacity for regulative adaptations. The rate of net photosynthesis at a certain date is calculated from the maximal rate of CO2 uptake being expected at that season and from the effects of light, temperature, and air humidity which are scaled from 0 to 1. All factors are connected multiplicatively. The light function accounts for the seasonal changes in the light curve, the temperature function is based on the seasonal shift of the temperature optimum, and the humidity function considers the increasing sensitivity of the stomatal humidity response at increasing water stress. The model is built to be a submodel of a general ecosystem model, where various other submodels (i.e. water stress model, phenology model) are supplied. The present model is tested by predicting daily courses at extreme climatic conditions during the year and by comparing the predicted values of gas exchange with values being measured in an independent experimental procedure. The result shows that the model is able to simulate the natural behaviour of Hammada scoparia during the growing and dry season of a desert habitat. The problems of incorporating the influence of water stress, the interaction of the various factors, and the phenological aspect of the photosynthetic activity is being discussed.

Notes: Abstract Maximal rates of CO2 assimilation of 8–11 μmol m-2 s-1 at ambient CO2 concentration were measured for Dendrosenecio keniodendron, D. brassica, Lobelia telekii and L. keniensis during the day in the natural habitat of these plants at 4,200 m elevation on Mt. Kenya. Even at these maximal rates, the CO2 uptake of all species was found to correspond to the linear portion of the CO2 response curve, with a calculated stomatal limitation for CO2 diffusion of 42%. Photosynthesis was strongly reduced at temperatures above 15° C. In contrast to this sensitivity to high temperatures, frozen leaves regained full photosynthetic capacity immediately after thawing. Stomata responded to dry air, but not to low leaf water potentials which occurred in cold leaves and at high transpiration rates. During the day reduced rates of CO2 uptake were associated with reduced light interception due to the erect posture of the rosette leaves and with high temperatures. Stomata closed at vapour pressure deficits which were comparable in magnitude to those characteristic of many lowland habitats (40 mPa Pa-1).

Notes: Abstract The responses of leaf water potential, leaf conductance, transpiration rate and net photosynthetic rate to vapour pressure deficits varying from 10 to 30 Pa kPa-1 were followed in Helianthus annuus as the extractable soil water decreased. With a vapour pressure deficit of 25 Pa kPa-1 around the entire plant as the soil water content decreased, the leaf conductance and transpiration rate showed a strong closing response to leaf water potential at a value of-0.65 MPa, whereas with a vapour pressure deficit of 10 Pa kPa-1 around the entire plant, the rate of transpiration and leaf conductance decreased almost linearly as the leaf water potential decreased from-0.4 to-1.0 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps decreased the leaf conductance by a similar proportion at all extractable soil water contents. At high soil water contents, the decrease in conductance with leaf water potential was greater when the vapour pressure deficit was increased than when it was not, indicating a direct influence of vapour pressure deficit on the stomata. The rate of net photosynthesis decreased to a smaller degree than the leaf conductance when the vapour pressure deficit around the leaf was varied. Overall, the net photosynthetic rate decreased almost linearly from 20 to 25 μmol m-2 s-1 at-0.3 MPa to 5 μmol m-2 s-1 at-1.2 MPa. As the soil water decreased, the internal carbon dioxide partial pressure was maintained between 14 and 25 Pa. No unique relationship between leaf conductance, transpiration rate or photosynthetic rate and leaf water potential was observed, but in all experiments leaf conductance and the rate of net photosynthesis decreased when about two-thirds of the extractable water in the solid had been utilized irrespective of the leaf water potential. We conclude that soil water status, not leaf water status, affects the stomatal behaviour and photosynthesis of H. annuus.