Light-Sensing in Roots

Light gradients in the soil have largely been overlooked in understanding plant responses to the environment. However, roots contain photoreceptors that may receive ambient light through the soil or piped light through the vascular cylinder. In recent experiments we demonstrated linkages between phototropin-1 photoreceptor production, root growth efficiency, and drought tolerance, suggesting that root plasticity in response to light signals contributes to the ecological niche of A. thaliana. However, the availability of light cues in natural soil environments is poorly understood, raising questions about the relevance of light-mediated root growth for fitness in nature. Additionally, photoreceptor expression is characterized by pleiotropy so unique functions cannot be clearly ascribed to root vs. shoot sensory mechanisms. These considerations show that challenges exist for resolving the contribution of light-sensing by roots to plant adaptation. We suggest that blue-light sensing in roots of A. thaliana provides a model system for addressing these challenges. By calibrating blue light gradients in soils of diverse A. thaliana habitats and comparing fitness of phot1 mutant and wild-type controls when grown in presence or absence of soil light cues, it should be possible to elucidate the ecological significance of light-mediated plasticity in roots.Key Words: phototropin, roots, drought-tolerance, photoreceptors, Arabidopsis thalianaIn plants, the capacity to sense and respond to variation in light quality is exploited in ecological interactions with neighbors,¹ in optimizing light interception for photosynthesis,² and even in collecting heat as a reward for insect pollinators in cold environments.³ Perhaps because the physiological and developmental functions modified by these light responses are readily observed in above ground organs (leaves, stems, flowers, etc.) light sensing and its adaptive significance belowground have largely been ignored. Light gradients underground are commonly considered redundant in information content to gravity, based on the similar directional responses of root growth to the two stimuli. This premise assumes that roots do not respond to light gradients established by vegetative canopies, or to light mosaics created in heterogeneous soils. However, such assumptions are problematic on several grounds—light piping by vascular elements makes the air-soil interface less of a barrier than a filter for light signals,⁴ even in uniform soil, the attenuation of light with depth informs the root of its position relative to the surface in a way that gravity cannot; and natural selection has favored a role for photo-sensory systems in other underground processes (e.g., phytochrome-mediated seed germination, ref. ¹) suggesting that light signals in the soil can provide valuable indicators of environmental conditions for growth and development.Because root growth responds to hormonal and ionic gradients established by signal reception in leaves,⁵ one might argue that photoreceptors in the roots themselves are redundant or at best, relatively unimportant. Yet photoreceptors provide unique information when deployed in different locations. For example, the quality of light intercepted at the leaf or stem allows for rapid reallocation of resources from root to shoot system in relation to crowding by vertically oriented foliage (e.g., during shade avoidance, ref. ¹), but may be less effective at directing responses to soil disturbance, desiccation or rosette density.In new research on the blue light photoreceptor phototropin-1 we show that the abundance of the photoreceptor in roots correlates with enhanced root growth efficiency. Mutants lacking the phot1 protein exhibit comparably random root growth and lower desiccation tolerance, suggesting that natural selection may have acted on root-mediated light sensing to improve drought tolerance in A. thaliana. Demonstrating that plastic responses of roots to soil light stimuli contribute to drought tolerance in the wild will require new research that characterizes underground light environments in natural habitats and measures selection on light-sensing in roots independent of pleiotropic effects on above-ground (leaf and stem) functions. We review our findings on blue-light mediated plasticity in root growth of A. thaliana, and propose that genetic polymorphism in Arabidopsis phototropin-1 provides a model system for addressing the adaptive significance of root photo-sensory systems in nature.Arabidopsis thaliana mutant plants lacking the blue light photoreceptor, phototropin-1, exhibit significantly reduced drought tolerance compared to the wild type background COL- O genotype for the phot1 mutation. Under dry (but not wet) soil conditions, wild type plants grow twice as large as phot1 mutants and plant size is highly correlated with root growth efficiency, the capacity of roots to grow directly away from the soil surface toward a belowground water supply. Using a translationally-fused phot1-gfp (green fluorescent protein) gene-construct to localize protein expression in roots, we found that high root growth efficiency is primarily limited to shallow rooting zones where soil drying is most rapid and phot1 protein most concentrated. This pattern suggests a role of phot1 in promoting efficient root growth by cueing roots to their proximity to the soil surface. However, if this conclusion is correct then blue light must attain sufficient intensity in natural soils to activate root-localized phot1 and pleiotropic effects in the shoot system cannot solely explain the impact of phot1 on drought tolerance.     

Application of silicon enhanced drought tolerance in Sorghum bicolor

The effects of silicon application on the drought tolerance of sorghum ( Sorghum bicolor (L.) Moench) were investigated for two cultivars differing in drought susceptibility. Silicon application ameliorated the decrease in dry weight under drought stress conditions, but had no effect on dry matter production under wet conditions. Under dry conditions, silicon-applied sorghum had a lower shoot to root (S/R) ratio, indicating the facilitation of root growth and the maintenance of the photosynthetic rate and stomatal conductance at a higher level compared with plants grown without silicon application. The diurnal determination of the transpiration rate indicated that the silicon-applied sorghum could extract a larger amount of water from drier soil and maintain a higher stomatal conductance. Very similar effects of silicon application were observed for both cultivars regardless of their drought susceptibility. These results suggest that silicon application may be useful to improve the drought tolerance of sorghum via the enhancement of water uptake ability.     

Importance of internal hydraulic redistribution for prolonging the lifespan of roots in dry soil

Redistribution of water within plants could mitigate drought stress of roots in zones of low soil moisture. Plant internal redistribution of water from regions of high soil moisture to roots in dry soil occurs during periods of low evaporative demand. Using minirhizotrons, we observed similar lifespans of roots in wet and dry soil for the grapevine 'Merlot' (Vitis vinifera) on the rootstock 101-14 Millardet de Gramanet (Vitis riparia x Vitis rupestris) in a Napa County, California vineyard. We hypothesized that hydraulic redistribution would prevent an appreciable reduction in root water potential and would contribute to prolonged root survivorship in dry soil zones. In a greenhouse study that tested this hypothesis, grapevine root systems were divided using split pots and were grown for 6 months. With thermocouple psychrometers, we measured water potentials of roots of the same plant in both wet and dry soil under three treatments: control (C), 24 h light + supplemental water (LW) and 24 h light only (L). Similar to the field results, roots in the dry side of split pots had similar survivorship as roots in the wet side of the split pots (P = 0.136) in the C treatment. In contrast, reduced root survivorship was directly associated with plants in which hydraulic redistribution was experimentally reduced by 24 h light. Dry-side roots of plants in the LW treatment lived half as long as the roots in the wet soil despite being provided with supplemental water (P < 0.0004). Additionally, pre-dawn water potentials of roots in dry soil under 24 h of illumination (L and LW) exhibited values nearly twice as negative as those of C plants (P = 0.034). Estimates of root membrane integrity using electrolyte leakage were consistent with patterns of root survivorship. Plants in which nocturnal hydraulic redistribution was reduced exhibited more than twice the amount of electrolyte leakage in dry roots compared to those in wet soil of the same plant. Our study demonstrates that besides a number of ecological advantages to protecting tissues against desiccation, internal hydraulic redistribution of water is a mechanism consistent with extended root survivorship in dry soils.     

Growth,abscisic acid content,and carbon isotope composition in wheat cultivars grown under different soil moisture

Changes in dry matter accumulation and allocation, abscisic acid content and carbon isotope composition of three wheat cultivars from dry, middle and wet climate regions were recorded at full maturity after exposure to different watering regimes (100, 50 and 25 % field capacity). Compared with the wet climate cultivar, the dry climate cultivar showed lower stem height, total leaf area, total dry biomass and total grain dry mass, and higher root/shoot ratio, abscisic acid content and carbon isotope composition under all watering regimes. Both water-limited treatments significantly reduced leaf growth and increased dry matter allocation into the roots leading to a significant raise of root/shoot ratio in all cultivars tested. In addition, drought affected morphological and physiological properties more in the dry climate cultivar than in the wet climate cultivar.     

EFFECTS OF ROOT PRUNING AT THE REGREENING STAGE ON DROUGHT TOLERANCE AND WATER USE EFFICIENCY OF WINTER WHEAT IN LATE GROWING STAGE

 通过盆栽试验研究了返青期根修剪对冬小麦(Triticum aestivum)后期耐旱性及水分利用效率的影响。在返青期设置了两个根修剪处理: 1)小剪根, 在植株一侧切去部分侧生根; 2)大剪根, 在主茎四周切去部分侧生根。不剪根者设为对照(CK)。研究结果显示, 两个根修剪处理均显著减少了小麦的根系, 但对根冠比没有显著影响。在花期, 两个根修剪处理的小麦旗叶的叶绿素荧光参数最大光化学效率(The maximum photochemical efficiency of PSⅡ, F_v/F_m)、 PSⅡ潜在活性 (PSⅡ potential activity, F_v/F_o)、实际光化学量子产量(Effective PSⅡ quantum yield, Φ_PSⅡ)、表观光合电子传递速率(Apparent rate of photosynthetic electron transport, ETR)、光化学淬灭系数( Coefficient of photochemical quenching, q_P)和非光化学淬灭系数(Coefficient of non-photochemical quenching, NPQ)值, 在停止供水7 d后, 均显著高于对照, 这表明根修剪小麦的耐旱性强于对照, 因此在干旱胁迫下有较高的光化学活性。小剪根处理在高水条件下对小麦产量无显著影响, 而在中度干旱条件下显著提高了小麦的产量, 因此, 小剪根处理显著提高了小麦的抗旱系数; 小剪根处理在高水分处理(土壤水分含量为田间持水量的85%)和中度干旱胁迫处理(土壤水分含量为田间持水量的55%)条件下, 均显著提高了小麦的水分利用效率。但大剪根处理由于严重影响了群体数量和产量, 水分利用效率和抗旱系数均没有提高。可见, 适当地减少根系有助于小麦的耐旱性和水分利用效率的提高。     

Effect of soil salinity, soil drought, and their combined action on the biochemical characteristics of cotton roots

Stress caused by soil salinity and soil drought limits cotton productivity in China. To determine the tolerance levels of cotton, we assessed the effects of soil salinity and soil drought on the biochemical characteristics of the roots of two cotton cultivars (CCRI-44, salt-tolerant; Sumian 12, salt-sensitive). Specifically, we analyzed root biomass, fatty acid composition, antioxidative enzyme activity, lipid peroxidation, H⁺-ATPase and Ca²⁺-ATPase activities. The cotton root biomass of the two cultivars declined significantly under conditions of soil salinity, soil drought, and the two stressors combined. The antioxidant enzyme activity of the roots also decreased markedly, which caused lipid peroxidation to increase, and changed the composition of the fatty acid membrane. H⁺-ATPase, Ca²⁺-ATPase and antioxidant enzyme activity decreased more under the two stressors combined. However, H₂O₂ content and O₂ ^? generation increased under the two stressors combined, compared to each stressor separately. Overall, the combination of soil salinity and drought has a greater inhibitory effect and more harmful impact on root growth than each stressor separately. The higher tolerance of CCRI-44 to soil salinity and drought stress than Sumian 12 might be explained by differences in cotton root antioxidative enzyme activity. The lipid peroxidation levels of cotton roots might represent an important biochemical trait for stress tolerance.     

The impact of limited soil moisture and waterlogging stress conditions on morphological and anatomical root traits in maize (Zea mays L.) hybrids of different drought tolerance

Effects of soil drought or waterlogging on the morphological traits of the root system and internal root anatomy were studied in maize hybrids of different drought tolerance. The investigations comprised quantitative and qualitative analyses of a developed plant root system through determining the number, length and dry matter of the particular components of the root system and some traits of the anatomical structure of the seminal root. Obtained results have demonstrated a relatively broad variation in the habit of the root system. This mainly refers, to the number, length and dry matter of lateral roots, developed by seminal root, seminal adventitious and nodal roots as well as to some anatomical properties of the stele, cortex and metaxylem elements. Plants grown under waterlogging or drought conditions showed a smaller number and less dry matter of lateral branching than plants grown in control conditions. The harmful effect of waterlogging conditions on the growth of roots was greater when compared with that of plants exposed to drought. In the measurements of the root morphological traits, the effect of soil drought on the internal root anatomical characteristic was weaker than the effect of soil waterlogging. The observed effects of both treatments were more distinct in a drought sensitive hybrid Pioneer D than in drought resistant Pioneer C one. The drought resistant hybrid Pioneer C distinguished by a more extensive rooting and by smaller alterations in the root morphology caused by the stress conditions than drought sensitive hybrid Pioneer D one. Also the differences between the resistant and the sensitive maize hybrids were apparent for examined root anatomical traits. Results confirm that the hybrid Pioneer D of a high drought susceptibility was found to be also more sensitive to periodieal soil water excess. A more efficient water use and a lower shoot to root (S:R) ratio were found to be major reasons for a higher stress resistance of the hybrid Pioneer C. The reasons for a different response of the examined hybrids to the conditions of drought or waterlogging may be a more economical water balance and more favourable relations between the shoot and root dimensions in the drought resistant genotype. The observed modifications of the internal root structure caused by water deficit in plant tissues may partly influence on water conductivity and transport within roots. The results suggest that the morphological and anatomical traits of the maize root system may be used in practice as direct or indirect selection criteria in maize breeding.     

Root System Dynamics of Miscanthus × giganteus and Panicum virgatum in Response to Rainfed and Irrigated Conditions in California

Miscanthus (Miscanthus × giganteus) and switchgrass (Panicum virgatum) are large perennial grass bioenergy crops in the USA and Europe. Despite much research into their agronomic potential, few studies have examined in situ root growth dynamics under irrigation and soil water deficits, particularly as they relate to shoot performance. We grew miscanthus and switchgrass in outdoor mesocosms under irrigated and rainfed conditions and assessed the spatial distribution and abundance of roots using minirhizotron images and whole root system sampling. Despite surviving an extended period of drought, shoot and root biomass, root length density, numbers of culms, and culm height were reduced in both species under rainfed (dry) conditions. However, rainfed switchgrass far outperformed rainfed miscanthus in all shoot and root growth metrics. The rainfed (drought) treatment reduced switchgrass and miscanthus whole plant biomass by 83 and 98 %, culm production by 67 and 90 %, and root length density by 67 and 94 % compared to irrigated plants, respectively. Root nitrogen concentration was higher for miscanthus (3-fold) and switchgrass (4-fold) in the rainfed treatment compared to irrigated plants and did not significantly differ between species. Unlike miscanthus, switchgrass grew roots continuously into regions of available soil moisture as surface soil layers grew increasingly dry, indicating a drought avoidance strategy. Our study suggests that switchgrass is more likely to tolerate drought by mining deep wet soils, while miscanthus relies on shallow rhizome production to tolerate dry soils.     

The influence of soil drought and partial waterlogging on water relations of Gmelina arborea seedlings

Summary Stomatal conductance of unstrossed, soil drought, and previously drought (predrought) Gmelina arborea seedlings increased in the morning and decreased before or immediately after midday. In the unstressed and predrought seedlings, leaf water potential decreased with increases in transpiration. In soil drought seedlings, there was some evidence of decreased hydraulic conductivity from soil to the plant, as indicated by the shape in the slope of the water potential/transpiration relationship. Root growth of drought plants was greater than in their unstressed counterparts at the lowest soil segment of a pot. The partial recovery of predrought seedlings was attributed to this subtantial root growth in the lowest soil segment.In the second experiment, Gmelina arborea seedlings were partially waterlogged, by flooding the polyethylene bag to half its length, for a period of 23 days. Waterlogging induced stomatal closure and reduction in leaf water potential but there was some evidence of tolerance to waterlogging towards the end of treatment. Root growth, shoot and root dry weights were slightly reduced below those of controls. After 9 days of waterlogging, adventitious roots began to form which correlated with depletion of soluble sugars in the shoot but with an increase in the roots.It is suggested that the tolerance of Gmelina plants to either soil drought or waterlogging may partly be due to partitioning of the soluble sugars from shoot to roots for production of roots and formation of adventitious roots respectively which are likely to enhance the flow of water from the soils to the plant. Therefore the plant response is very similar under conditions of increased deficits and surplus of soil water.     

Effects of varied water regimes on root length,dry matter partitioning and endogenous plant growth regulators in sunflower (Helianthus annuus L.)

Abstract

A field experiment was conducted to quantify the effect of varied water regimes on root length, partitioning of dry matter and plant growth regulators by using sunflower genotypes differing in maturity and drought tolerance. Significant depressing effect of drought stress was evident on traits (i.e., reproductive dry matter, leaf area index and cytokinin concentrations in leaves). However, root/shoot, reproductive/vegetative ratios and Abscisic acid (ABA) concentration were found to increase under drought stress. Drought stress also changed the dry matter accumulation pattern of genotypes. In most cases it reduced the days to reach the maximum peak showing early senescence.

ABA was identified as a multi-functional plant growth regulator under drought stress, causing early senescence of plants and translocation of assimilates to the roots and reproductive part while root growth under drought stress was explained by the indole-acetic acid (IAA) concentrations. Maintaining higher cytokinin contents were involved in accumulation of higher reproductive dry matter under drought stress. Although ABA and IAA were both involved in the development of defense responses during the adaptation and survival to drought stress but higher productivity under drought stress was only realized through maintaining higher cytokinin contents.     

Sex-Related Responses of Populus cathayana Shoots and Roots to AM Fungi and Drought Stress

We investigated the impact of drought and arbuscular mycorrhizal (AM) fungi on the morphological structure and physiological function of shoots and roots of male and female seedlings of the dioecious plant Populus cathayana Rehder. Pot-grown seedlings were subjected to well watered or water-limiting conditions (drought) and were grown in soil that was either inoculated or not inoculated with the AM fungus Rhizophagus intraradices. No significant differences were found in the infection rates between the two sexes. Drought decreased root and shoot growth, biomass and root morphological characteristics, whereas superoxide radical (O₂–) and hydrogen peroxide content, peroxidase (POD) activity, malondialdehyde (MDA) concentration and proline content were significantly enhanced in both sexes. Male plants that formed an AM fungal symbiosis showed a significant increase in shoot and root morphological growth, increased proline content of leaves and roots, and increased POD activity in roots under both watering regimes; however, MDA concentration in the roots decreased. By contrast, AM fungi either had no effect or a slight negative effect on the shoot and root growth of female plants, with lower root biomass, total biomass and root/shoot ration under drought. In females, MDA concentration increased in leaves and roots under both watering regimes, and the proline content and POD activity of roots increased under drought conditions; however, POD activity significantly decreased under well-watered conditions. These findings suggest that AM fungi enhanced the tolerance of male plants to drought by improving shoot and root growth, biomass and the antioxidant system. Further investigation is needed to unravel the complex effects of AM fungi on the growth and antioxidant system of female plants.     

CO2浓度倍增和土壤干旱对两种幼龄沙生灌木碳分配的影响

利用大型环境生长箱研究了两种幼龄沙地优势灌木柠条 (Caraganaintermedia) 和羊柴 (Hedysarummon golicum) 对CO2 浓度倍增和土壤干旱交互作用的响应。CO2 浓度倍增并没有改善两种沙生灌木叶片的水分状况, 而土壤干旱使叶片的相对含水量 (RWC) 显著降低。在土壤水分充足条件下, CO2 浓度倍增促进两种沙生灌木植株生长, 在干旱条件下则主要促进根的生长, 提高根冠比。土壤干旱显著减少了植株生物量, 但相对促进了根的生长, 特别是显著提高了羊柴的根冠比。CO2 倍增使稳定性碳同位素组分 (δ13 C) 降低, 但土壤干旱使之增加。两种沙生灌木叶片与根部的δ13 C值呈极显著线性关系, 羊柴的斜率大于柠条的, 表明前者叶片与根部在光合产物分配上具有较高的生态可塑性, 这和干旱条件下羊柴的根冠比增加相关联。羊柴的“源库”调节特性反映了对土壤水分胁迫具有较高的耐性。     

干旱条件下接种AM真菌对小马鞍羊蹄甲幼苗根系的影响

为了探讨岷江干旱河谷丛枝菌根真菌(AMF)对寄主植物幼苗根系的影响,通过接种购买的AMF摩西球囊霉菌(Funneliformis mosseae)到优势乡土灌木小马鞍羊蹄甲(Bauhinia faberi var.microphylla)幼苗,在重度、中度和轻度干旱条件下培养3个月,研究不同干旱条件下AMF对幼苗根系形态特征、结构特征、功能性状的影响。方差分析结果表明:(1)3种干旱胁迫条件下,接菌均显著增加了幼苗的根总长、根表面积、根分枝数、根尖数(P0.001),在中度胁迫和轻度胁迫下,接菌显著促进根鲜重、根体积的增加(P0.001),轻度胁迫条件下接菌幼苗的根鲜重、根总长、根表面积、根体积、根尖数最高并显著高于其它处理,但接菌与未接菌的根平均直径之间没有显著差异;(2)接菌幼苗根系趋向于叉状分支结构,在重度胁迫时,叉状分支趋势更显著(P0.001);(3)接菌幼苗的根比例都显著小于未接菌的,但幼苗比根长不存在显著差异。相关分析结果表明:菌根侵染率与根鲜重、根总长、根表面积、根体积、根分枝数、根尖数呈极显著正相关(P0.001),与拓扑指数、根比例呈极显著负相关(P0.001)。研究表明,在干旱条件下,AMF虽然没有提高生长初期的根系的吸收效率,但接种AMF显著影响幼苗根系形态特征和结构特征,更利于植物适应干旱环境,并且AMF对幼苗根系的促生作用随着干旱胁迫程度减轻而提高。     

Drought adaptability of Agrobacterium rhizogenes-induced roots in oilseed rape (Brassica napus var. oleifera)

Abstract. Soil grown oilseed rape ( Brassica napus L. var. oleifera M., cv. Darmor) seedlings at the cotyledon stage (one week old), were inoculated in vivo at the base of the hypocotyl with Agrobacterium rhizogenes harbouring the pRi 15834 plasmid. Resulting adventitious root formation was observable about 2 or 3 weeks after infection. Differential Ri-induced root emergence and subsequent development occurred depending on water conditions and closeness of the wounding site to the soil surface: either thin, hairy roots growing rapidly and plagiotropically at the soil level under humid atmosphere, or hairless and fleshy, slowly growing aerial roots developed. The hairy roots were highly drought susceptible, whereas aerial roots revealed some potential for drought tolerance. Unlike normal roots, none of these Ri-induced roots appeared able to give rise to drought rhizogenesis in plants subjected to progressive drought stress. However, under hardening, achieved through successive and moderate drought stress-rehydration cycles, both types of Ri-induced roots improved drought tolerance and could express the morphogenetic differentiation programme leading to the formation of short, tuberized, drought-adapted, roots. These results, discussed in terms of hormonal imbalance and drought tolerance regulation, suggest that the Ri T-DNA gene expression, responsible for adventitious root induction and growth behaviour, is further regulated through the host plant.     

土壤水分梯度对白三叶(Trifolium repens)光合作用和根系分布的影响

通过对两个品种白三叶Trifolium repens cv.Haifa(海发)和Trifolium repens cv.Rivendel(瑞文德)盆栽试验,模拟3种不同的土壤水分状况(无水分胁迫:保持植株良好的水分供应;轻度胁迫:表层0～20cm土壤处于干旱状态;重度胁迫:表层0～20cm土壤处于极干旱状态,20～40cm土壤处于干旱状态)对白三叶光合作用和根系生长的影响.结果表明,当植株未遭受水分胁迫时,两个品种白三叶的光合作用和根系生长状况没有明显差异;当表层0～20cm处于干旱状态时,'海发'在处理后期的净光合速率和水分利用效率升高,根系生长量增大,表现出促进作用,'瑞文德'受到的影响不显著;当表层0～20cm处于极干旱、20～40cm处于干旱状态时,'海发'在处理前期受到轻微影响,随后恢复正常状态,'瑞文德'则受到较严重的影响.随着干旱程度的加深和时间的延长,白三叶的根冠比逐渐增大.与'瑞文德'相比,在相同时期相同胁迫程度下,'海发'的根冠比没有显著差异,但深根数量大大超过'瑞文德',因而,'海发'的耐旱能力强于'瑞文德'.     

干旱胁迫对宁夏枸杞生长及果实糖分积累的影响

文章研究不同干旱胁迫下宁夏枸杞生长及果实糖分积累的变化规律,为宁夏枸杞在干旱地区高产栽培提供参考依据。采用盆栽控水试验,设置正常灌水、轻度干旱、中度干旱和重度干旱处理,研究了干旱胁迫对宁夏枸杞植株生长、生物量分配以及果实糖分积累的影响。结果表明：干旱抑制宁夏枸杞新稍、果实、株高和地径的生长：随着干旱程度加剧,根和茎中干物质分配率逐渐升高,而枝条、叶和果实中干物质分配率大幅降低;轻度干旱有利于果实发育过程中果糖的积累,中度和重度干旱胁迫则不利于成熟期果糖和蔗糖积累;干旱胁迫明显降低成熟期转化酶、蔗糖磷酸合成酶（SPS）和蔗糖合成酶（SS）的活性;果实发育过程中果糖的含量与SPS和转化酶活性存在极显著相关。可见,在果实发育期,土壤含水量为田间持水量55％以上,能促进宁夏枸杞果实中糖分积累,有效提高果实品质。     

The Effects of Drought and Shade on the Performance,Morphology and Physiology of Ghanaian Tree Species

In tropical forests light and water availability are the most important factors for seedling growth and survival but an increasing frequency of drought may affect tree regeneration. One central question is whether drought and shade have interactive effects on seedling growth and survival. Here, we present results of a greenhouse experiment, in which seedlings of 10 Ghanaian tree species were exposed to combinations of strong seasonal drought (continuous watering versus withholding water for nine weeks) and shade (5% irradiance versus 20% irradiance). We evaluated the effects of drought and shade on seedling survival and growth and plasticity of 11 underlying traits related to biomass allocation, morphology and physiology. Seedling survival under dry conditions was higher in shade than in high light, thus providing support for the “facilitation hypothesis” that shade enhances plant performance through improved microclimatic conditions, and rejecting the trade-off hypothesis that drought should have stronger impact in shade because of reduced root investment. Shaded plants had low biomass fraction in roots, in line with the trade-off hypothesis, but they compensated for this with a higher specific root length (i.e., root length per unit root mass), resulting in a similar root length per plant mass and, hence, similar water uptake capacity as high-light plants. The majority (60%) of traits studied responded independently to drought and shade, indicating that within species shade- and drought tolerances are not in trade-off, but largely uncoupled. When individual species responses were analysed, then for most of the traits only one to three species showed significant interactive effects between drought and shade. The uncoupled response of most species to drought and shade should provide ample opportunity for niche differentiation and species coexistence under a range of water and light conditions. Overall our greenhouse results suggest that, in the absence of root competition shaded tropical forest tree seedlings may be able to survive prolonged drought.     

Field phenotyping of potato to assess root and shoot characteristics associated with drought tolerance

Aims

Potatoes are a globally important source of food whose production requires large inputs of fertiliser and water. Recent research has highlighted the importance of the root system in acquiring resources. Here measurements, previously generated by field phenotyping, tested the effect of root size on maintenance of yield under drought (drought tolerance).

Methods

Twelve potato genotypes, including genotypes with extremes of root size, were grown to maturity in the field under a rain shelter and either irrigated or subjected to drought. Soil moisture, canopy growth, carbon isotope discrimination and final yields were measured. Destructively harvested field phenotype data were used as explanatory variables in a general linear model (GLM) to investigate yield under conditions of drought or irrigation.

Results

Drought severely affected the small rooted genotype Pentland Dell but not the large rooted genotype Cara. More plantlets, longer and more numerous stolons and stolon roots were associated with drought tolerance. Previously measured carbon isotope discrimination did not correlate with the effect of drought.

Conclusions

These data suggest that in-field phenotyping can be used to identify useful characteristics when known genotypes are subjected to an environmental stress. Stolon root traits were associated with drought tolerance in potato and could be used to select genotypes with resilience to drought.     

Root plasticity maintains growth of temperate grassland species under pulsed water supply

Background and aims

The frequency of rain is predicted to change in high latitude areas with more precipitation in heavy, intense events interspersed by longer dry periods. These changes will modify soil drying cycles with unknown consequences for plant performance of temperate species.

Methods

We studied plant growth and root traits of juveniles of four grasses and four dicots growing in a greenhouse, when supplying the same total amount of water given either regular every other day or pulsed once a week.

Results

Pulsed water supply replenished soil moisture immediately after watering, but caused substantial drought stress at the end of the watering cycle, whereas regular watering caused more moderate but consistent drought. Grasses had lower water use efficiency in the pulsed watering compared to regular watering, whereas dicots showed no difference. Both grasses and dicots developed thinner roots, thus higher specific root length, and greater root length in the pulsed watering. Growth of dicots was slightly increased under pulsed watering.

Conclusions

Temperate species coped with pulsed water supply by eliciting two responses: i) persistent shoot growth, most likely by maximizing growth at peaks of soil moisture, thus compensating for slower growth during drought periods; ii) plasticity of root traits related to increased resource uptake. Both responses likely account for subtle improvement of growth under changed water supply conditions.     

Heterogeneity in Water Availability Alters Cellular Development and Hydraulic Conductivity along Roots of a Desert Succulent

Plants of the desert succulent Agave deserti were grown in partitionedcontainers to determine whether heterogeneity in soil moistureleads to differences in cellular development and hydraulic conductivityalong individual roots. Roots from containers with a dry distalcompartment (furthest from the shoot), a wet middle compartment,and a dry proximal compartment had distal regions (includingthe root tips) that were more suberized and lignified in theendodermis and adjacent cell layers than were root regions fromthe wet middle compartment. Proximal root regions about 40 mmfrom the succulent shoot base were also relatively unsuberized,suggesting that both external and internal supplies of waterdelayed tissue maturation. Root segments from wet middle compartmentsand from dry proximal compartments had higher hydraulic conductivitythan did the more suberized root segments from dry distal compartments.Unlike distal root segments from wet compartments, segmentsfrom dry compartments suffered no decrease in hydraulic conductivityafter immersion in mercuric chloride, suggesting that aquaporinactivity diminished for roots during drought. The possible closureof water channels could help limit root water loss to a dryingsoil. The delayed development of suberized cell layers may allowroot regions to maximize water uptake from wet soil patches(such as under rocks), and the relatively immature, absorptiveroot region near the base of the shoot may help A. deserti capturewater from a briefly wetted surface soil. Copyright 2000 Annalsof Botany Company Agave deserti, root plasticity, water uptake, aquaporins, suberization, endodermis, divided pots.