NUTRIENT AND WATER RELATIONS OF THE MISTLETOE PHORADENDRON LEUCARPUM (VISCACEAE): HOW TIGHTLY ARE THEY INTEGRATED?

Tissue mineral concentrations of calcium, copper, iron, magnesium, manganese, nitrogen, phosphorus, potassium, sodium, and zinc, and long-term water-use efficiency (as estimated by δ¹³C±) were measured in the mistletoe Phoradendron leucarpum and its hosts to investigate the relationship between nutrient concentrations and water economies of the parasite and hosts. The mistletoe had mineral concentrations 0.97 to 2.88 times greater than the hosts. Mean long-term mistletoe water-use efficiency (-27.89±) was comparable to mean long-term host water-use efficiency (-27.69±) and generally greater than the water-use efficiency reported for most mistletoe species. Differences between mistletoe and host water-use efficiency ranged from +0.65 to -0.75± and were more similar to mistletoes found growing on nitrogen-fixing host species as reported in other studies. Mistletoe nutrient concentrations, in particular nitrogen, were not related to changes in mistletoe water-use efficiencies. Nutrient: calcium ratios indicated that mistletoes acquire nutrients in excess of that which can be delivered by the host xylem. These data are discussed relative to the passive vs. active theories of nutrient uptake.     

氮肥处理对氮素高效吸收水稻根系性状及氮肥利用率的影响

2011—2012年在土培条件下,以氮素吸收效率差异较大的15个常规籼稻为供试材料,研究氮肥运筹对不同氮效率品种根系性状、成熟期吸氮量及氮肥利用率的影响,分析影响氮高效水稻氮素吸收的主要根系性状。结果表明:(1)各氮肥处理下,成熟期吸氮量均表现为氮高效品种氮中效品种氮低效品种。适量增施氮肥及基肥+促花肥处理有利于氮高效品种吸氮量的增加,氮素吸收受品种、氮肥处理的显著影响。(2)在施氮量处理下,氮高效品种单株不定根数、单株根干重、单株不定根总长大或较大,单株根活力在常氮(N2)、高氮(N3)处理下有一定的优势;在施氮时期处理下,氮高效品种单株不定根数、单株不定根总长、单株根干重、单株根系总吸收面积、单株根系活跃吸收面积、抽穗期冠根比多数处理有优势;增施氮肥有利于促进氮高效品种单株不定根总长和单株根活力的提高,适量施氮有利于单株不定根数、单株根干重增加,前期施氮可促进不定根的发生和伸长,后期施氮有利于不定根的充实和根系生理性状的提高。此外,增施氮肥可提高各类品种冠根比;(3)在常氮、高氮处理下,氮高效品种氮肥利用率大于氮中效、氮低效品种。(4)提高单株不定根数、单株不定根总长、单株根活力及抽穗期冠根比有利于各类品种吸氮量的提高,增加根干重对氮高效品种吸氮量的提高也有显著的促进作用。结合相关分析与通径分析结果,抽穗期冠根比及单株不定根数、单株根活力、单株不定根总长、单株根干重是影响氮高效品种吸氮能力的主要根系性状。     

The effect of nitrogen supply on growth and water-use efficiency of xylem-tapping mistletoes

Xylem-tapping mistletoes are known to have normally a higher rate of transpiration and lower water-use efficiency than their hosts. The relationships between water relations, nutrients and growth were investigated for Phoradendron juniperinum growing on Juniperus osteosperma (a non-nitrogen-fixing tree) and for Phoradendron californicum growing on Acacia greggii (a nitrogen-fixing tree). Xylem sap nitrogen contents were approximately 3.5 times higher in the nitrogen-fixing host than in the non-nitrogen-fixing host. The results of the present study show that mistletoe growth rates were sevenfold greater on a nitrogen-fixing host. At the same time, however, the differences in water-use efficiency between mistletoes and their hosts, which were observed on the non-nitrogen-fixing host did not exist when mistletoes were grown on hosts with higher nitrogen contents in their xylem sap. Growth rates and the accumulation of N, P, K, and Ca as well as values for carbon-isotope ratios of mistletoe tissues support the hypothesis that the higher transpiration rates of mistletoes represent a nitrogen-gathering mechanism.Abbreviation ¹³C carbon-isotope ratio Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday     

Breeding for high water-use efficiency

There is a pressing need to improve the water-use efficiency of rain-fed and irrigated crop production. Breeding crop varieties with higher water-use efficiency is seen as providing part of the solution. Three key processes can be exploited in breeding for high water-use efficiency: (i) moving more of the available water through the crop rather than it being wasted as evaporation from the soil surface or drainage beyond the root zone or being left behind in the root zone at harvest; (ii) acquiring more carbon (biomass) in exchange for the water transpired by the crop, i.e. improving crop transpiration efficiency; (iii) partitioning more of the achieved biomass into the harvested product. The relative importance of any one of these processes will vary depending on how water availability varies during the crop cycle. However, these three processes are not independent. Targeting specific traits to improve one process may have detrimental effects on the other two, but there may also be positive interactions. Progress in breeding for improved water-use efficiency of rain-fed wheat is reviewed to illustrate the nature of some of these interactions and to highlight opportunities that may be exploited in other crops as well as potential pitfalls. For C3 species, measuring carbon isotope discrimination provides a powerful means of improving water-use efficiency of leaf gas exchange, but experience has shown that improvements in leaf-level water-use efficiency may not always translate into higher crop water-use efficiency or yield. In fact, the reverse has frequently been observed. Reasons for this are explored in some detail. Crop simulation modelling can be used to assess the likely impact on water-use efficiency and yield of changing the expression of traits of interest. Results of such simulations indicate that greater progress may be achieved by pyramiding traits so that potential negative effects of individual traits are neutralized. DNA-based selection techniques may assist in such a strategy.     

Using multiple trait associations to define hydraulic functional types in plant communities of south-western Australia

Assessing the hydrological imbalance and associated land degradation issues facing much of southern Australia and other parts of the world requires a better understanding of the defining features of ecosystem water use and the design of sustainable agroecosystems. Thus, by grouping species with similar water-use strategies into 'hydraulic functional types' (HFTs), we investigated the characteristics of water use in species-rich plant communities of south-western Australia. HFTs were determined using multiple-trait associations between morphological and physiological traits relating to water transport, water-use efficiency and response to water deficit. Sixteen traits were assessed from a subset of 21 species from three plant communities located along a topographically determined soil- and water-availability gradient. Multivariate analyses showed that trait variation was least at sites with shallower soils and putatively lower water availability, suggesting a convergence of water-use strategies at sites where plants are exposed to large seasonal water deficits. Stem hydraulic parameters, including stem-specific hydraulic conductivity, conduit diameter and maximum percentage embolism, were positively correlated, indicating the generality that larger conduit diameter permits greater hydraulic efficiency and is associated with greater seasonal reductions in hydraulic conductivity in this ecosystem. Wood density was not correlated with these traits, but closely associated with species' ability to withstand more negative water potentials during summer. Long-term integrated water-use efficiency was lower in shallow-rooted species that exhibited more negative summer water potentials. Specific leaf area and minimum leaf water potential were correlated with a number of separate traits, and appear to represent key axes of trait variation that describe the water-use strategies of different HFTs along the topographic gradient. Five HFTs were classified using a resemblance analysis according to combinations of traits that pertain to different water-use strategies among species; year-round active tree, year-round active shrub, hemiparasite, drought-suppressed broad-leaved shrub and drought-suppressed narrow-leaved shrub.     

不同水稻品种对三化螟抗性差异的机理

研究了不同水稻品种对三化螟Scirpophaga incertulas抗性差异的机理。结果表明, 在汕优63、镇稻2号和92-109稻株中, 虫体的游离必需氨基酸含量极低（0.422～0.875 nmol/mg）, 幼虫生长发育缓慢;而在9-92、武育粳3号等稻株中, 虫体的游离必需氨基酸含量高（2.788～5.421 nmol/mg）, 幼虫生长快。虫体的游离必需氨基酸含量和生长发育的差异, 与不同水稻品种的游离必需氨基酸含量、总必需氨基酸含量、还原糖含量和叶鞘硅化细胞密度均无明显相关。叶鞘维管束间距、鞘脊宽度及小于蚁螟头宽的叶鞘维管束间距百分率的差异, 是不同水稻品种抗螟性差异的主要作用因子。     

Effects of environmental change, including drought, on water use by competing Calluna vulgaris (heather) and Pteridium aquilinum (bracken)

1. Competition for water between Calluna vulgaris (heather) and Pteridium aquilinum (bracken) was studied in conditions of increased temperature, drought and increased nitrogen supply. All these factors increased the intensity of competition for water, with the combination of drought and increased nitrogen having the greatest effect on water use.
2. Both species increased water-use efficiency in response to increased nitrogen and drought. The effects of temperature were however, equivocal. Calluna had a greater water demand than Pteridium but acclimated to water stress more readily.
3. Calluna was the superior competitor for water; its water-use efficiency was reduced as a consequence of its roots depleting water from the Pteridium rooting zone. Pteridium , the poorer competitor, increased water-use efficiency to cope with reduced water availability owing to competition.
4. There was a strong relationship between carbon isotope discrimination (Δ) and instantaneous water-use efficiency for both species, but discrimination provided a more sensitive measure of seasonal water-use efficiency. Reconstruction of the plant's history of water-use efficiency by retrospective measurement of Δ proved a useful technique for Calluna leaves but was inappropriate for Pteridium rhizome.     

Responses of Super Rice (Oryza sativa L.) to Different Planting Methods for Grain Yield and Nitrogen-Use Efficiency in the Single Cropping Season

To break the yield ceiling of rice production, a super rice project was developed in 1996 to breed rice varieties with super high yield. A two-year experiment was conducted to evaluate yield and nitrogen (N)-use response of super rice to different planting methods in the single cropping season. A total of 17 rice varieties, including 13 super rice and four non-super checks (CK), were grown under three N levels [0 (N0), 150 (N150), and 225 (N225) kg ha⁻¹] and two planting methods [transplanting (TP) and direct-seeding in wet conditions (WDS)]. Grain yield under WDS (7.69 t ha⁻¹) was generally lower than TP (8.58 t ha⁻¹). However, grain yield under different planting methods was affected by N rates as well as variety groups. In both years, there was no difference in grain yield between super and CK varieties at N150, irrespective of planting methods. However, grain yield difference was dramatic in japonica groups at N225, that is, there was an 11.3% and 14.1% average increase in super rice than in CK varieties in WDS and TP, respectively. This suggests that high N input contributes to narrowing the yield gap in super rice varieties, which also indicates that super rice was bred for high fertility conditions. In the japonica group, more N was accumulated in super rice than in CK at N225, but no difference was found between super and CK varieties at N0 and N150. Similar results were also found for N agronomic efficiency. The results suggest that super rice varieties have an advantage for N-use efficiency when high N is applied. The response of super rice was greater under TP than under WDS. The results suggest that the need to further improve agronomic and other management practices to achieve high yield and N-use efficiency for super rice varieties in WDS.     

Growth and water-use efficiency of 10 Triticum aestivum cultivars at different water availability in relation to allocation of biomass

In environments where the amount of water is limiting growth, water-use efficiency (biomass production per unit water use) is an important trait. We studied the relationships of plant growth and water use efficiency with the pattern of biomass allocation, using 10 wheat cultivars, grown at two soil moisture levels in a growth chamber. Allocation pattern and relative growth rate were not correlated, whereas allocation pattern and water use efficiency were. Variation in transpiration per plant resulted from variation in the rate of transpiration per unit leaf area or root weight, rather than from differences in leaf area or root weight per plant. Transpiration per unit leaf area or root weight was lower when the leaf area or root weight per unit plant weight was larger. Also, the efficiency of water use at the plant and leaf levels was higher for plants with a higher leaf area per unit plant weight, and it was not correlated with the plant's growth rate. Differences in water-use efficiency at the leaf level were related to variation in stomatal conductance, rather than in the rate of photosynthesis. A high photosynthetic water-use efficiency was associated with a low efficiency of nitrogen use for photosynthesis.     

DIFFERING SELECTION ON PLANT PHYSIOLOGICAL TRAITS IN RESPONSE TO ENVIRONMENTAL WATER AVAILABILITY: A TEST OF ADAPTIVE HYPOTHESES

I used phenotypic selection analysis to test the prediction from functional and comparative studies of plants that smaller leaves and more efficient water use are adaptive in drier environments. I measured selection gradients on leaf size and instantaneous water-use efficiency (a measure of carbon gain per unit water loss) in experimental populations of Cakile edentula var. lacustris placed into wet and dry environments in the field. Linear and nonlinear selection differed significantly between the two environments as predicted. Water-use efficiency was selected to be higher, and leaf area was selected toward a small intermediate optimum, in the dry environment. There was also significant positive correlational selection on water-use efficiency and leaf size, suggesting that the optimum leaf size in the dry environment is greater for plants with higher water-use efficiency. In contrast, neither leaf size nor water-use efficiency were selected in the wet environment, though larger leaves resulted in greater vegetative biomass. Path analysis of the linear selection gradients found that water-use efficiency affected plant fitness primarily because it increased vegetative biomass, as suggested by the hypotheses about the function of physiological traits. These results were not only consistent with the functional hypotheses but also with the observed genetic differentiation in water-use efficiency and leaf size between wet and dry site populations.     

Effects of plant size on photosynthesis and water relations in the desert shrub Prosopis glandulosa (Fabaceae)

The Jornada del Muerto basin of the Chihuahuan Desert of southern New Mexico, USA, has undergone a marked transition of plant communities. Shrubs such as mesquite (Prosopis glandulosa) have greatly increased or now dominate in areas that were previously dominated by perennial grasses. The replacement of grasses by shrubs requires an establishment phase where small shrubs must compete directly with similar-sized grass plants. This is followed by a phase in which large, established shrubs sequester nutrients and water within their biomass and alter soil resources directly under their canopy, creating “islands” of fertility. We hypothesized that these two phases were associated with shrubs having different physiological response capacities related to their age or size and the resource structure of the environment. As a corollary, we hypothesized that responses of small shrubs would be more tightly coupled to variation in soil moisture availability compared to large shrubs. To test these hypotheses, we studied gas exchange and water relations of small (establishing) and large (established) shrubs growing in the Jornada del Muerto as a function of varying soil moisture during the season. The small shrubs had greater net assimilation, stomatal conductance, transpiration, and xylem water potential than large shrubs following high summer rainfall in July, and highest seasonal soil moisture at 0.3 m. High rates of carbon assimilation and water use would be an advantage for small shrubs competing with grasses when shallow soil moisture was plentiful. Large shrubs had greater net assimilation and water-use efficiency, and lower xylem water potential than small shrubs following a dry period in September, when soil moisture at 0.3 m was lowest. Low xylem water potentials and high water-use efficiency would allow large shrubs to continue acquiring and conserving water as soil moisture is depleted. Although the study provides evidence of differences in physiological responses of different-sized shrubs, there was not support for the hypothesis that small shrubs are more closely coupled to variation in soil moisture availability than large shrubs. Small shrubs may actually be less coupled to soil moisture than large shrubs, and thus avoid conditions when continued transpiration could not be matched by equivalent water uptake.     

Photosynthetic traits in C3 and C4 grassland species in mesocosm and field environments

The North American tallgrass prairie is composed of a diverse mix of C₃ and C₄ plant species that are subject to multiple resource limitations. C₄ grasses dominate this ecosystem, purportedly due to greater photosynthetic capacity and resource-use efficiency associated with C₄ photosynthesis. We tested the hypothesis that intrinsic physiological differences between C₃ and C₄ species are consistent with C₄ grass dominance by comparing leaf gas exchange and chlorophyll fluorescence variables for seven C₄ and C₃ herbaceous species (legumes and non-legumes) in two different settings: experimental mesocosms and natural grassland sites. In the mesocosms, C₄ grasses had higher photosynthetic rates, water potentials and water-use efficiency than the C₃ species. These differences were absent in the field, where photosynthetic rates declined similarly among non-leguminous species. Thus, intrinsic photosynthetic advantages for C₄ species measured in resource-rich mesocosms could not explain the dominance of C₄ species in the field. Instead, C₄ dominance in this ecosystem may depend more on the ability of the grasses to grow rapidly when resources are plentiful and to tolerate multiple limitations when resources are scarce.     

The relationship between the soil water storage and water-use efficiency of seven energy crops

The aim of this work was to determine two types of photosynthetic water-use efficiency in order to examine their utility as selection criteria for tolerance of energy crops to soil water deficit. Furthermore, effects of crop cultivation on soil water content and storage were investigated. Seven energy crops were examined: miscanthus, prairie cordgrass, willow, thorn-free rose, Virginia mallow, Bohemian knotweed, and topinambour. The highest values of instantaneous (WUE) and intrinsic (WUE_i) water-use efficiencies were found for miscanthus and prairie cordgrass. The reduction of WUE and/or WUE_i was caused mainly by a rapid rise in the transpiration rate and a greater stomatal conductance, respectively. Principal component analysis showed that neither WUE nor WUE_i could be recommended as universal selection criteria for the drought tolerance in different energy crops. The proper localization of soil with a good supply of water is most the important condition for energy crop plantations.     

Linking drought-resistance mechanisms to drought avoidance in upland rice using a QTL approach: progress and new opportunities to integrate stomatal and mesophyll responses

The advent of saturated molecular maps promised rapid progress towards the improvement of crops for genetically complex traits like drought resistance via analysis of quantitative trait loci (QTL). Progress with the identification of QTLs for drought resistance-related traits in rice is summarized here with the emphasis on a mapping population of a cross between drought-resistant varieties Azucena and Bala. Data which have used root morphological traits and indicators of drought avoidance in field-grown plants are reviewed, highlighting problems and uncertainties with the QTL approach. The contribution of root-growth QTLs to drought avoidance appears small in the experiments so far conducted, and the limitations of screening methodologies and the involvement of shoot-related mechanisms of drought resistance are studied. When compared to Azucena, Bala has been observed to have highly sensitive stomata, does not roll its leaves readily, has a greater ability to adjust osmotically, slows growth more rapidly when droughted and has a lower water-use efficiency. It is also a semi-dwarf variety and hence has a different canopy structure. There is a need to clarify the contribution of the shoot to drought resistance from the level of the biochemistry of photosynthesis through stomatal behaviour and leaf anatomy to canopy architecture. Recent advances in studying the physical and biochemical processes related to water use and drought stress offer the opportunity to advance a more holistic understanding of drought resistance. These include the potential use of infrared thermal imaging to study energy balance, integrated and online stable isotope analysis to dissect processes involved in carbon dioxide fixation and water evaporation, and leaf fluorescence to monitor photosynthesis and photochemical quenching. Justification and a strategy for this integrated approach is described, which has relevance to the study of drought resistance in most crops.     

Water-use efficiency and whole-plant performance of nine tropical tree species at two sites with contrasting water availability in Panama

Across their natural distributions, tropical tree species are regularly exposed to seasonal droughts of varying intensities. Their ability to tolerate drought stress plays a vital role in determining growth and mortality rates, as well as shaping the functional composition of tropical forests. In order to assess the ability of species to acclimate to contrasting levels of drought stress, physiological and structural traits involved in drought adaptation—wood C isotope discrimination (δ¹³C), wood specific gravity, and wood C content—of 2-year-old saplings of nine tropical tree species were evaluated in common garden experiments at two study sites in Panama with contrasting seasonality. We assessed co-variation in wood traits with relative growth rates (RGR_BD), aboveground biomass, and basal diameter and the plasticity of wood traits across study sites. Overall, species responded to lower water availability by increasing intrinsic water-use efficiency, i.e., less negative wood δ¹³C, but did not exhibit a uniform, directional response for wood specific gravity or wood C content. Trait plasticity for all wood traits was independent of RGR_BD and tree size. We found that the adaptive value of intrinsic water-use efficiency varied with water availability. Intrinsic water-use efficiency increased with decreasing RGR_BD at the more seasonal site, facilitating higher survival of slower growing species. Conversely, intrinsic water-use efficiency increased with tree size at the less seasonal site, which conferred a competitive advantage to larger individuals at the cost of greater susceptibility to drought-induced mortality. Our results illustrate that acclimation to water availability has negligible impacts on tree growth over short periods, but eventually could favor slow-growing species with conservative water-use strategies in tropical regions experiencing increasingly frequent and severe droughts.     

Physiological and production responses of plant growth forms to increases in limiting resources in alpine tundra: implications for differential community response to environmental change

Physiological and growth measurements were made on forbs and graminoids following additions of water and N+water in a graminoid-dominated dry meadow and a forb-dominated moist meadow, to determine if the community-level response was related to differential responses between the growth forms. Graminoids had higher photosynthetic rates and lower transpiration rates and foliar N concentrations than forbs, and consequently maintained higher photosynthetic N- and water-use efficiencies. Photosynthetic rates, stomatal conductance, and transpiration rates increased significantly only in response to N fertilization and only in moist meadow species. The increase in photosynthetic rates was unrelated to variation in foliar N concentration, but instead correlated with variation in stomatal conductance. Growth based N-use efficiency was higher in moist meadow graminoids than in moist meadow forbs, but did not differ between the growth forms in the dry meadow. The moist meadow community had higher biomass and N standing crops, but the relative increase in these factors in response to N fertilization was greater in the dry meadow. Graminoids had a greater relative increase in biomass and N accumulation than forbs following N fertilization, but moist meadow graminoids exhibited a greater response than dry meadow graminoids. The difference in the growth response between the dry meadow and moist meadow graminoids to N fertilization was correlated with more conservative leaf gas exchange responses in dry meadow species, presumably related to a higher frequency of soil water deficits in this community. Community-level response to the resource additions was therefore mediated by the plant growth form response, corresponding with differences between the growth forms in physiological factors related to resource acquisition and use.     

Carbon isotope discrimination and the ratio of carbon gained to water lost in barley cultivars

Abstract A negative correlation between water-use efficiency (W), defined as the ratio of moles of carbon in the plant to moles of water transpired, and carbon isotope discrimination (Δ) was established for barley in pot experiments using 12 cultivars. The correlation was strong in two independent experiments in four different controlled environment where ambient temperature and vapour pressure deficit were varied and plants were either well-watered or given limited amounts of water. Variation among cultivars was found in both Δ and W and rankings of both parameters, according to cultivar, were similar in different environments. Limiting water usually increased water-use efficiency of plants. Total dry matter can be substituted for moles of carbon when calculating water-use efficiency but the correlation between W and Δ were calculated using the carbon content of dry matter. There were differences varied significantly among cultivars. Despite these differences, correlations were also large between whole plant W and Δ of any of the plant parts. The amount of dry matter partitioned into reproductive growth varied genetically, as did the effect of stress on the partitioning. Growth, W and Δ of barley were compared with theory derived from gas exchange properties and with other literature. The effect on W of variation in vapour pressure deficit in these experiments was removed by multiplying W by vapour pressure deficit to derive the parameter, k(Pa mol C/mol H₂O). This allowed comparisons among experiments with different vapour pressure deficits. The mean k for these barley cultivars was similar to that calculated by others for grasses. However, variation was found, and, in contrast with previous work which treats k as a species constant, we conclude that there is promise in selecting for increased k.     

共生菌在褐飞虱致害性变化中的作用

研究了不同虫源和致害性褐飞虱Nilaparvata lugens种群体内共生菌数量动态及其对褐飞虱在抗虫品种上的取食选择、生长发育、繁殖以及氨基酸转移酶活性的影响。结果表明,褐飞虱田间种群的致害性与其体内共生菌数量有关。广西南宁种群雌成虫体内的共生菌数量显著地高于浙江杭州和龙游两个虫源的雌成虫体内共生菌数量,而已纯化的3个不同致害性生物型体内的共生菌数量无显著差异。取食抗性品种能显著减少生物型Ⅰ雌成虫体内的共生菌数量。缺乏共生菌时,生物型Ⅰ、Ⅱ若虫对水稻品种TN1和ASD7的选择性增大,而对Mudgo的取食选择性下降。尽管缺共生菌的3个生物型在已适应的和不适应的感虫和抗虫品种上的若虫存活率和雌成虫产卵量均下降,若虫历期明显延长,但在已适应品种上的变化程度明显小于在不适应的抗虫品种上的变化程度。共生菌还明显影响成虫体内丙氨酸氨基转移酶和天门冬氨酸氨基转移酶的活性。这些结果证明体内共生菌的数量和质量在褐飞虱对水稻致害性的变化中发挥了重要作用。     

THE RESPONSE TO DIFFERING SELECTION ON PLANT PHYSIOLOGICAL TRAITS: EVIDENCE FOR LOCAL ADAPTATION

Understanding adaptive evolution to differing environments requires studies of genetic variances, of natural selection, and of the genetic differentiation between populations. Plant physiological traits such as leaf size and water-use efficiency (the ratio of carbon gained per water lost) have been suggested by physiological plant ecologists to be important in local adaptation to environments differing in water availability. In this study, I raised families of Cakile edentula var lacustris derived from a wet-site population and a dry-site population in a common greenhouse environment to determine the degree of genetic differentiation between the two populations and the genetic architecture of the traits. The dry-site population had significantly smaller leaf size and significantly greater water-use efficiency than the wet-site population. I used a retrospective selection analysis to compare long-term selection inferred from these results to measures of phenotypic selection from a field experiment. Both direct measures in the field and the retrospective selection gradients were consistent with the hypothesis that greater water-use efficiency and smaller leaves were adaptive in drier environments. Though the correlation between population means for water-use efficiency and leaf size was negative, the genetic correlation within populations between water-use efficiency and leaf size was positive and thus would be expected to constrain the evolutionary response to selection.     

Accumulation and localisation of sodium ions within the shoots of rice (Oryza sativa) varieties differing in salinity resistance

Oryza sativa L. (rice) is a salt-sensitive crop species which is relatively ineffective in controlling the influx of sodium and chloride ions to the shoot. Nonetheless, there is considerable varietal and individual variability in salinity resistance, much of which must derive, therefore, from differences in the fates and subsequent effects of saline ions after they have entered the plant. The destination of sodium ions within the plant has been investigated, in saline conditions, by examining the time-course of sodium ion concentrations in different leaves of four varieties and breeding lines of rice of differing salinity resistance. Radionuclide tracers were employed to study short term effects and the degree of retranslocation of these sodium ions. Sodium was not distributed uniformly but accumulated in the older leaves before the younger ones. At least some leaves were maintained at sub-lethal salt concentrations in at least the more salt resistant varieties. Radionuclide tracer studies showed that the discontinuous distribution of sodium (from leaf to leaf) is constitutive, and cannot be explained by time of exposure or differential leaf growth rates, and that significant quantities of sodium were not subsequently retranslocated, either within the plant or to the root medium.