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Dendrobium officinale, endemic to China, is a rare and endangered medicinal herb. As a result of its high economic value, slow growth, and diminishing wild population, protected cultivation is preferred. However, little information is available on its growing environment and photosynthetic characteristics. In this study, the photosynthetic patterns of D. officinale were investigated under various environmental conditions by measuring the net CO2 exchange rates continuously for several days or weeks. Under non-stressed growth chamber conditions with 12-hour light and 12-hour dark periods, D. officinale had concomitance of C3 and crassulacean acid metabolism (CAM) photosynthesis patterns. Different degrees of CAM in D. officinale, expressed as the percentage of CO2 exchanges in the dark period to the daily amount of CO2 exchanges, were observed depending on environmental conditions. With decreasing substrate water content, a typical CAM pattern was found, and concomitance of C3 and CAM patterns was found again when plants were rewatered. The accumulation of leaf titratable acidity during a dark period increased as substrate dried out but decreased again as plants were rewatered. A shorter light–dark cycle (4-hour light and 4-hour dark periods) led to a C3 pattern alone. The substrate moisture and light–dark cycle were inducible factors for switching between C3 and CAM patterns in D. officinale. These results indicate that D. officinale is a facultative CAM plant and the C3 pathway can be induced by controlling the growing environment. Further studies are needed to identify the optimal environmental conditions to enhance the growth of D. officinale.

With randomized complete block design, long-term water use efficiency (WUEL), photosynthetic parameters (net photosynthetic rate Pn, transpiration rate Tr, stomatal conductance Gs, instantaneous water use efficiency WUEi, maximum photosynthetic rate P max, light saturation pointLSP, carboxylic efficiency Ce), stomatal properties (frequency, size, superior/inferior stoma amount ratio) and root/shoot ratio of twelve Populus deltoids clones were studied under water stress in a greenhouse. Four water treatments were designed, namely, well-watered condition, slight water stress, moderate water stress, and severe water stress. Although the volumes of irrigated water for different treatments were the same, the irrigation intervals were strictly controlled. The results showed that long-term water use efficiency (WUEL) differences among tested clones were obvious and became greater with the increase in water stress. J2, J6, J7, J8, and J9 were excellent clones with high WUEL. Stomatal frequency, stomatal size, Gs, Tr, Pn, and root/shoot ratio of J2 et al were moderate, whereas the superior/inferior ratio, Pmax, LSP, Ce of J2 et al were higher than other clones. The interrelations between WUEL and the physiological parameters were analyzed. The results revealed that the WUEL differences were induced by a series of physiological parameters, and clones with higher WUEL always had strong photosynthetic capacity, higher WUEi, and optimum root/shoot ratio.

The present study is expected to provide better understanding of how exo-β-glucanase acts on the biochemo-rheological property of the cell wall, and thus the regulation mechanism of pollen tube growth. A polymerase chain reaction (PCR)-based strategy and cDNA library screening were used to clone an exo-β-glucanase gene (LP-ExoI) from Lilium longiflorum pollen tubes. The LP-ExoI sequence showed a high level of homology to that of reported plant exo-β-glucan hydrolases. Phylogenetic analysis demonstrated that there was close relationship between LP-ExoI and other exo-glucan hydrolases in Poacea plants. Northern analysis indicated that LP-ExoI transcripts stored in pollen grains at low levels; LP-ExoI transcription level enhanced at 2 h after the onset of pollen incubation and remained relatively high during pollen tube elongation. Moreover, the expression of LP-ExoI was undetectable in petals, filaments, stigmas, styles, ovaries, leaves, and stems. These results suggest that LP-ExoI gene is a late pollen gene, specifically contributing to the regulation of lily pollen germination and pollen tube growth.

Recent studies have confirmed that cavitation in xylem is caused by air bubbles. We analyzed expansion of a preexistent bubble adhering to a crack in a conduit wall and a bubble formed by the passage of air through a pore of a pit membrane, a process known as air seeding. We consider that there are two equilibrium states for a very small air bubble in the xylem: one is temporarily stable with a bubble radius r1 at point s1 on the curve P(r) relating pressure within the bubble (P) with bubble radius (r); the other is unstable with a bubble radius r2 at point s2 on Pr (where r1 < r2). In each equilibrium state, the bubble collapse pressure (2sigma/r, where sigma is surface tension of water) is balanced by the pressure difference across its surface. In the case of a bubble from a crack in a conduit wall, which is initially at point s1, expansion will occur steadily as water potential decreases. The bubble will burst only if the xylem pressure drops below a threshold value. A formula giving the threshold pressure for bubble bursting is proposed. In the case of an air seed entering a xylem conduit through a pore in a pit membrane, its initial radius may be r2 (i.e., the radius of the pore by which the air seed entered the vessel) at point s2 on Pr. Because the bubble is in an unstable equilibrium when entering the conduit, it can either expand or contract to point s1. As water vaporizes into the air bubble at s2, P rises until it exceeds the gas pressure that keeps the bubble in equilibrium, at which point the bubble will burst and induce a cavitation event in accordance with the air-seeding hypothesis. However, other possible perturbations could make the air-seeded bubble contract to s1, in which case the bubble will burst at a threshold pressure proposed for a bubble expanding from a crack in a conduit wall. For this reason some cavitation events may take place at a xylem threshold pressure (Pl'*) other than that determined by the formula, Plp'* = -2sigma/rp, proposed by Sperry and Tyree (1988), which is applicable only to air-seeded bubbles at s2. The more general formula we propose for calculating the threshold pressure for bubble breaking is consistent with the results of published experiments.

Diurnal changes in net photosynthetic rate (PN), chlorophyll (Chl) fluorescence, and stomatal aperture of several hybrid poplar clones subjected to midday light stress were measured in July and August of 1996. Midday depression of PN, photosystem 2 (PS2) efficiency, stomatal conductance (gs), and stomatal aperture was observed in all clones, though at differing rates among them. Non-uniform stomatal closure occurred at noon and at other times, requiring a modification of intercellular CO2 concentration (C1). A linear relationship was found between gs and stomatal aperture. More than half of the photons absorbed by PS2 centre dissipated thermally when subjected to light stress at noon. There was a linear relationship between the rate of PS2 photochemical electron transport (PxPFD) and PN. There was a consensus for two fluorescence indicators (1 − qP/qN and (Fm' − F)/Fm') in assessment of susceptibility of photoinhibition in the clones. According to PN, Chl fluorescence, and stomatal aperture, we conclude that midday depression of photosynthesis can be attributed to both stomatal and non-stomatal limitations.

It is known that the redistribution of water and the formation of dispersed water units appears to be a prerequisite for deep supercooling. A concentration of the cell solute results from the migration of water during extracelullar freezing and lowers the temperature of homogeneous nucleation, but we are convinced that nucleation of ice within cells may be initiated by a heterogeneous mechanism, except we consider a small spherical cave, the water can freeze on the wall of this cave. We are also convinced that the solid walls of the capillary exert an external potential on the water molecules, causing the shift of the triple point of the confined fluids. Based on Fletcher's work for spherical particle, we have gotten the formula of critical free energy in the process of heterogeneous nucleation of water in a small spherical cave. This presentation introduces the theoretical background and counts the drop of temperature in heterogeneous nucleation. Then, putting two actions (depression of triple point and process of heterogeneous nucleation) together, we have calculated the freezing point. Sometimes it is lower than –38 °C. Some phenomena can be explained by using this theory: 1) Water is at the tension status, which means that it wets plant tissue, so the triple point (melting point) of tissue water can be lowered. 2) The redistribution of water, formation of dispersed water units, and dry region preventing ice from propagating, all allow heterogeneous nucleation, then the two actions can be synthesized and the water would lead to deep supercooling. If the barriers were destroyed, heterogeneous nucleation and deep supercooling would certainly be lost. 3) This theory is only suited to rigid wall of small cave, so we understand why cell wall rigidity has been shown to affect freezing characteristics. Project 39870234 supported by National Nature Science Foundation.

With lower-and higher heat-resistant varieties of eggplant (Solanum melongena L.) Heibei I and Heibei II as test materials, and by using Plant Efficiency Analyzer (PEA) from Hansatech, this paper measured the fast chlorophyll a fluorescence transient and its parameters. The results showed that PS II construction became more sensitive to heat stress when ambient temperature was higher than 40 degrees C. The F0 went up slowly, and Fv/Fm and deltaF/Fm' came down dramatically. Heibei II had a longer semi-attenuation temperature of Fv/Fm (T50) and deltaF/Fm' (t50) than Heibei I. Under strong heat stress (5 min at 48 degrees C or 20-30 min at 44 degrees C), the K-step in relation to the inactivation of oxygen-evolving complex appeared in fluorescence rise at about 700 micros, and the regular O-J-I-P transient was transformed to O-K-J-I-P one. The K-phase of Heibei I and Heibei II appeared when the treatment time was up to 20 and 30 minutes at 44 degrees C, respectively. In comparing with 35 degrees C heat treatment, the DI0/RC in the parameters of Strasser's specific energy fluxes model was increased by a great extent under 48 degrees C or more heat stress, reflecting a strong safeguard of energy dissipation to PS II. When the temperature of heat stress increased from 35 degrees C to 52 degrees C, the Fvi/Fv of PS II silent reaction centers of Heibei I and Heibei II increased remarkably.