We present a conceptual model linking dry‐mass allocational allometry, hydraulic limitation, and vertical stratification of environmental conditions to patterns in vertical tree growth and tree height. Maximum tree height should increase with relative moisture supply and both should drive variation in apparent stomatal limitation. Carbon isotope discrimination (Δ) shouldnotvary with maximum tree height across a moisture gradient when only hydraulic limitation or allocational allometry limit height, but increase with moisture when both hydraulic limitationandallocational allometry limit maximum tree height. We quantified tree height and Δ along a gradient in annual precipitation from 300 to 1600 mm from mallee to temperate rain forest in southeastern Australia;Eucalyptuson this gradient span almost the entire range of tree heights found in angiosperms worldwide. Maximum tree height showed a strong, nearly proportional relationship to the ratio of precipitation to pan evaporation. Δ increased with ln P/Ep, suggesting that both hydraulic limitation and allocational allometry set maximum tree height. Coordinated shifts in several plant traits should result in different species having an advantage in vertical growth rate at different points along a rainfall gradient, and in maximum tree height increasing with relative moisture supply, photosynthetic rate, nutrient supply, and xylem diameter.

Inoculation with Rhizobium strain IC3342 induces in pigeonpea (Cajanus cajan (L) Millsp.) a leaf curl syndrome and elevated cytokinin levels in the xylem sap. High nitrogen (N) nutrition was found to inhibit onset of the syndrome which could then be induced by N-free nutrient after development of seven trifoliate leaves. This provided a new system to study the role of xylem cytokinin in shoot development and yielded plants suitable for determining the rate of delivery of xylem cytokinin to the shoot which for IC3342-inoculated plants was found to be three times that of control plants. Relative to leaves of control plants, the non-curled leaves of these IC3342 plants exhibited higher nitrogen and chlorophyll content and greater photosynthetic rate and stomatal conductance. Induction of the syndrome increased leaf thickness in developing leaves but not in expanded leaves already formed. Diameter of stems and number of laterals were also increased markedly by IC3342 inoculation which in addition induced leaf hyponasty. Exogenous cytokinins when applied directly to control leaves induced leaf curl and increased leaf thickness. The present studies are discussed in relation to the role of xylem cytokinins in plant development and especially the release of lateral buds from apical dominance.

Root restriction experiments on cotton plants were conducted under conditions where water and nitrogen supplies were not limiting. Xylem sap and leaf tissues were obtained before significant biomass reduction as a result of root restriction. Xylem-derived cytokinins (CTKs) entering a leaf and the CTK levels in the same excised leaf were measured, thus, enabling xylem sap and foliar CTKs to be compared concurrently under root-restrictive and control conditions. Abscisic acid (ABA) was also measured simultaneously in the same xylem sap samples. Root restriction reduced photosynthesis and δ18O of leaf matter showed that leaves of root-restricted plants generally had lower stomatal conductances. Root-restriction significantly decreased (37%) the delivery rate per unit leaf area of CTK bases (dihydrozeatin (DZ) and N6-(2-isopentenyl)adenine (iP)) to the leaf but did not alter the export of CTK O-glucosides and ABA from the roots to leaf 4. Root-restriction also did not alter significantly the levels of total CTK bases in the leaves but increased (54–57% higher) the CTK O-glucosides (especially zeatin O-glucosides). Root restriction also slowed down the apparent CTK turnover rates in the leaves. The slow down in CTK turnover rate can be interpreted as a reduction in leaf CTK metabolism during root restriction.

The bacterial cytokinin biosynthesis gene ipt under control of a chalcone synthase promoter (PCHS) was introduced into potato (Solanum tuberosum L.). Two transgenic lines were selected for detailed study, because in these, root development was reduced only moderately, thus, enabling the plants to be grown in pots. Expression of the PCHS-ipt gene elevated the level of zeatin cytokinins markedly in the apical bud, subapical stems and leaves. The transgenic (IPT) plants exhibited a lower and denser leaf canopy relative to wild-type (WT) plants owing to reduction in main stem length, increase in node number per stem and promotion of lateral shoot development. Main stem diameter was increased markedly due to promotion of cell division associated with activation of cyclin-dependent kinase in the subapical stem. Expression of the PCHS-ipt gene induced aerial stolons, promoted growth of underground stolons and increased tuber number but reduced tuber weight and nitrogen content. The gene expression also increased pinnae and pinnule number per leaf, increased thickness of pinnae and promoted transpiration, photosynthesis and stomatal conductance – effects monitored by gas exchange and 18O and 13C analysis. The elevation of [CO2] to 900 μmol mol–1 promoted growth of both WT and IPT plants, ameliorated the negative effect of high cytokinin on tuber weight and interacted additively with ipt gene expression to promote stem growth.

The most widely used method for pyrolysing samples for hydrogen or oxygen isotopic analysis involves heating them to greater than 1300 degrees C in a helium stream passed through a glassy carbon tube in an alumina casing. There are a number of difficulties with this. Glassy carbon tubes are expensive and interaction between the carbon tube and the outer casing produces unwanted carbon monoxide by reduction of the alumina at high temperatures. The latter effect is overwhelming if temperatures of 1400 degrees C or greater are used for pyrolysis. We experimented with lining alumina casings with pure molybdenum sheet. It is relatively cheap, conforms well to the interior of the reactor tube (to avoid carrier and sample bypassing of the carbon pack), resists high temperatures and neither oxidises excessively nor absorbs the gases. The main disadvantages are that silver sample cups must be used and that the molybdenum degrades over time by formation of the carbide. We can maintain sharp peaks, high precision and good accuracy over more than 700 solid samples for both hydrogen and oxygen. The reactors last longer for water injections. The molybdenum in the columns does not contribute greatly to memory effects. The precision of analysis is dependent on other factors as well as the pyrolysis column, but for oxygen we typically achieve approximately <0.2 per thousand (sucrose), <0.25 per thousand (water) and <0.25 per thousand (leaf), sometimes using only a linear correction of drift, after dividing the run into 1 to 3 segments.

Cuticular properties affect the gas exchange of leaves, but little is known about how much CO2 and water vapor cross the cuticular barrier or whether low water potentials affect the process. Therefore, we measured the cuticular conductances for CO2 and water vapor in grape (Vitis vinifera L.) leaves having various water potentials. The lower leaf surface was sealed to force all gas exchange through the upper surface, which was stoma-free. In this condition both gases passed through the cuticle, and the CO2 conductance could be directly determined from the internal mole fraction of CO2 near the compensation point, the external mole fraction of CO2, and the CO2 flux. The cuticle allowed small amounts of CO2 and water vapor to pass through, indicating that gas exchange occurs in grape leaves no matter how tightly the stomata are closed. However, the CO2 conductance was only 5.7% of that for water vapor. This discrimination against CO2 markedly affected calculations of the mole fraction of CO2 in leaves as stomatal apertures decreased. When the leaf dehydrated, the cuticular conductance to water vapor decreased, and transpiration and assimilation diminished. This dehydration effect was largest when turgor decreased, which suggests that cuticular gas exchange may have been influenced by epidermal stretching.