Leaf water maintains daytime transpiration in young Cryptomeria japonica trees.

Compared with stem water storage, leaf water storage is understudied although it may be important for alleviating water stress by contributing quickly and directly to transpiration demand. To quantify the relative contribution of stem versus leaf water storage to daily water deficit, we measured diurnal changes in transpiration rate, sap-flow velocity and stem radius of 10-year-old Cryptomeria japonica D. Don trees. We assumed that the duration of time lags between transpiration rate and sap-flow velocity reflected stored water in the stem and leaf, and that stem volume change represented water content of elastic tissue. The relationship between fresh mass and water potential of the whole tree indicated that the study trees had capacity to store, on average, 91.4 ml of water per kg fresh mass at turgor loss. Leaves, sapwood and elastic tissue contributed around 51%, 29% and 20% of stored water, respectively. During morning, transpiration rates were higher than sap-flow velocity suggesting depletion of stored water. During the first 2 h after onset of transpiration, stored water contributed more than 100% of whole-tree transpiration. Depletion of leaf water (PLeaf) and sapwood water (PSap) coincided with the onset of transpiration and became maximum around 15:00 h. Depletion of elastic tissue water (PElastic) lagged behind that of PLeaf and PSap by 1-2 h, indicating that replenishment of stored water occurs late in the day when low leaf water potentials resulting from daytime transpiration drive water uptake. Maximum depletion of PLeaf was about 1-3 times and 5-10 times that of PSap and PElastic, respectively. The contribution of PLeaf to total daily transpiration was 5-8%, while those of PSap and PElastic were 3-4% and 0.7-1%, respectively. Our results suggest the importance of leaf water storage in maintaining daily transpiration in young C. japonica trees.

Vertical and seasonal variations in temperature responses of leaf respiration in a Chamaecyparis obtusa canopy.

Leaf respiration (R) is a major component of carbon balance in forest ecosystems. Clarifying the variability of leaf R within a canopy is essential for predicting the impact of global warming on forest productivity and the potential future function of the forest ecosystem as a carbon sink. We examined vertical and seasonal variations in short-term temperature responses of leaf R as well as environmental factors (light and mean air temperature) and physiological factors [leaf nitrogen (N), leaf mass per area (LMA), and shoot growth] in the canopy of a 10-year-old stand of hinoki cypress [Chamaecyparis obtusa (Sieb. et Zucc.) Endl.] in Kyushu, Japan. Leaf respiration rate adjusted to 20 °C (R20) exhibited evident vertical gradients in each season and was correlated with light, LMA and leaf N. In contrast, the temperature sensitivity of leaf R (Q10) did not vary vertically throughout the seasons. Seasonally, Q10 was higher in winter than in summer and was strongly negatively correlated to mean air temperature. A negative correlation of R20 with mean air temperature was also observed for each of the three canopy layers. These results clearly indicate that leaf R was able to adjust to seasonal changes in ambient temperature under field conditions and down-regulate during warmer periods. We also found that the degree of thermal acclimation did not vary with canopy position. Overall, our results suggest that vertical and seasonal variations in temperature responses of leaf R within a hinoki cypress canopy could be predicted by relatively simple parameters (light and temperature). There was an exception of extremely high R20 values in April that may have been due to the onset of shoot growth in spring. Understanding thermal acclimation and variations in leaf R within forest canopies will improve global terrestrial carbon cycle models.

Overexpression of an isopentenyl diphosphate isomerase gene to enhance trans-polyisoprene production in Eucommia ulmoides Oliver.

BACKGROUND: Natural rubber produced by plants, known as polyisoprene, is the most widely used isoprenoid polymer. Plant polyisoprenes can be classified into two types; cis-polyisoprene and trans-polyisoprene, depending on the type of polymerization of the isoprene unit. More than 2000 species of higher plants produce latex consisting of cis-polyisoprene. Hevea brasiliensis (rubber tree) produces cis-polyisoprene, and is the key source of commercial rubber. In contrast, relatively few plant species produce trans-polyisoprene. Currently, trans-polyisoprene is mainly produced synthetically, and no plant species is used for its commercial production. RESULTS: To develop a plant-based system suitable for large-scale production of trans-polyisoprene, we selected a trans-polyisoprene-producing plant, Eucommia ulmoides Oliver, as the target for genetic transformation. A full-length cDNA (designated as EuIPI, Accession No. AB041629) encoding isopentenyl diphosphate isomerase (IPI) was isolated from E. ulmoides. EuIPI consisted of 1028 bp with a 675-bp open reading frame encoding a protein with 224 amino acid residues. EuIPI shared high identity with other plant IPIs, and the recombinant protein expressed in Escherichia coli showed IPI enzymatic activity in vitro. EuIPI was introduced into E. ulmoides via Agrobacterium-mediated transformation. Transgenic lines of E. ulmoides overexpressing EuIPI showed increased EuIPI expression (up to 19-fold that of the wild-type) and a 3- to 4-fold increase in the total content of trans-polyisoprenes, compared with the wild-type (non-transgenic root line) control. CONCLUSIONS: Increasing the expression level of EuIPI by overexpression increased accumulation of trans-polyisoprenes in transgenic E. ulmoides. IPI catalyzes the conversion of isopentenyl diphosphate to its highly electrophilic isomer, dimethylallyl diphosphate, which is the first step in the biosynthesis of all isoprenoids, including polyisoprene. Our results demonstrated that regulation of IPI expression is a key target for efficient production of trans-polyisoprene in E. ulmoides.

Periploca sepium Bunge as a model plant for rubber biosynthesis study.

Periploca sepium Bunge (Chinese silk vine) is a woody climbing vine belonging to the family Asclepiadaceae. It originally comes from Northwest China. Periploca resembles the Para-rubber tree, Hevea brasiliensis, regarding a similar body plan to produce a milky exudate containing rubber latex. The Periploca plant was assessed as a rubber-producing plant by rubber structure elucidation and its molecular weight distribution. A rubber fraction purified from the milky exudate was subjected to 1H NMR analysis, and a characteristic signal derived from cis-polyisoprene was observed. In addition, when the molecular weight distribution of rubber components in the exudate was measured (using size-exclusion chromatography), the number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity (Mw/Mn) were estimated to be Mn = 1.3 x 10(5), Mw = 4.1 x 10(5), and Mw/Mn = 3.1, respectively. Furthermore, the presence of polyisoprene, with Mn = 4.0 x 10(4), Mw = 7.6 x 10(4), and Mw/Mn = 2.5, was also confirmed in plantlets obtained from shoots as a result of tissue culture.