Evidence of drought-sensitive periods from flowering to maturity on highbush blueberry

The effects of water deficit on highbush blueberry (Vaccinium corymbosum L.) have been little studied. This study, conducted on container-planted shrubs, aimed at determining the influence of water stress on growth, water relations and fruit production of the plant during two consecutive years. Drought periods of approximately 3 weeks, as monitored from the transpiration of control plants, were applied at various phenological stages of plant development, from blossoming to harvest, so as to evaluate their direct effects. Highbush blueberry reacted very quickly to drought by reducing transpiration and stopping stem diameter growth and shoot elongation. Its ability to recover depended on the stress level and the drought implementation dynamics. Under moderate stress (35% less transpiration than a well-watered shrub) the recovery potential was almost complete. In all cases water stress during fruit growth and ripening strongly influenced yield by reducing the mean fruit weight and size. Drought after-effects, as assessed from one season to the other, were small, except when stress occurred during flower induction. In that case the number of flowers was reduced in the following year as well as the number of fruits, although fruit size was greater resulting in only a slight reduction in yield. In all cases, photosynthetic performance during the following year appeared not to alter, but water stress imposed to obtain large fruit cannot replace pruning without jeopardising the shrub architecture.     

高丛、半高丛和矮丛越橘越冬冻害研究

试验在长春进行,2002年10月中旬温度骤降,造成越橘枝条成熟程度差。2003年4月撤除防寒物后发现部分品种不同程度受冻害,调查了高丛越橘品种斯巴坦、蓝丰、蓝乐,半高丛越橘品种圣云、北陆、北蓝和矮丛越橘品种北村、美登,比较了其基生枝、延长枝、2年生枝的受冻枝条数、受冻长度、花芽和叶芽受冻比例,韧皮部受冻级别。结果表明:越橘3种类型中,高丛越橘抗寒能力最低,半高丛越橘其次,矮丛越橘抗寒能力最强。各品种中,高丛越橘斯巴坦受害最重,其基生枝和延长枝受害枝条数和枝条受冻长度分别为82.5％、85.7％和88.0％、95.6％,远高于蓝丰和蓝乐2个品种;2年生枝受害数量和长度斯巴坦为85.2％和76.4％,而蓝丰仅为17.6％和5.5％,蓝乐仅为33.0％和0％;3个品种花芽和叶芽受害比例、韧皮部受害级别也表现出同样趋势。半高丛越橘按抗寒能力由强到弱顺序排列依次为:北蓝>北陆>圣云。矮丛越橘2个品种北村和美登基本无冻害发生。     

Root System Distribution of Highbush Blueberry Crops of Various Ages in Medium-Textured Soils

The paper deals with the spatial distribution of root density in highbush blueberry plantations of various ages for a proper planting distance recommendation and a better irrigation and fertilization management. The environmental conditions consist of sandy-loamy soils and a continental temperate climate. Two highbush blueberry cultivars were studied, ‘Bluecrop’ and ‘Blueray’. Root density (RD) and density of root cross-sectional area (RCSA) were investigated for various root diameters. It was found that the prevalent type of roots in highbush blueberry crops is the hair type, with root diameters less than 0.1?mm. Both RD and RCSA decreased with distance from plant and with soil depth; the 38-year-old crop showed a more developed root system versus the 8?year-old crop. In both cases the roots did not grow more than 0.6?m laterally and more than 0.7?m deep. According to these results, the planting distances in highbush blueberry could be reduced to substantially increase the number of plants/ha and fruit yield. Irrigation application should be carried out to wet a proper soil volume, about 0.6–0.7?m deep for older crops, and about 0.5?m deep for younger crops.     

淹水、干旱条件下北空越桔生理反应的研究

采用盆栽试验,研究了'北空'越桔在连续淹水、干旱条件下的生理反应。结果表明,在淹水和干旱条件下,越桔叶片的光合强度、叶绿素含量（Chla、Chlb、Chla＋b、Chla／Chlb）、光量子通量密度降低,而呼吸强度、气孔导度、蒸腾速率升高,随逆境时间加长,变化幅度增大。     

高丛蓝莓对干旱胁迫的生理响应及其抗旱性综合评价

 为研究高丛蓝莓对干旱胁迫的生理响应并筛选具有较强抗旱能力的品种,同时确定基于生理指标的隶属函数分析对于筛选蓝莓抗干旱品种的可行性,分别以北高丛蓝莓中的‘布里吉塔’、‘斯巴坦’以及南高丛蓝莓中的‘夏普蓝’、‘奥尼尔’4 个品种为材料,在人工控水模拟水分胁迫条件下,测定叶片相对含水量、电导率、丙二醛含量、H₂O₂ 含量与　产生速率、F_v/F_m 以及叶片气孔特性等生理生化指标的变化,并采用隶属函数法进行综合评价。结果表明,随着干旱胁迫程度的加深,4 个蓝莓品种叶片的相对含水量、Fv/Fm、气孔开放率和气孔大小均呈下降趋势,而相对电导率、丙二醛含量、H₂O₂ 含量与产生速率以及气孔密度则呈上升趋势。4 个高丛蓝莓品种的抗旱性强弱顺序为奥尼尔 > 夏普蓝 > 布里吉塔 > 斯巴坦,大田干旱试验验证了基于生理指标的隶属函数分析对于蓝莓抗旱品种的筛选的准确性及可靠性。     

二硫苏糖醇对海水胁迫下菠菜活性氧代谢及叶绿素荧光特性的影响

 以耐海水菠菜品种‘荷兰3号’为材料,采用水培方法,研究了二硫苏糖醇（DTT）对海水胁迫及甲基紫精（MV）诱导下菠菜活性氧代谢及叶绿素荧光特性的影响。结果表明,海水胁迫与MV处理一样,诱导菠菜叶片产生氧化胁迫,使超氧阴离子（）产生速率、过氧化氢（H₂O₂）含量和丙二醛（MDA）含量显著上升,叶绿素a（Chl.a）、叶绿素b（Chl.b）、总叶绿素[Chl.(a + b)]和类胡萝卜素（Car.）含量显著下降,最大光量子产量（F_v/F_m）、实际光量子产量（Yield）、电子传递速率（ETR）和光化学猝灭系数（q_P）显著降低,而非光化学猝灭系数（NPQ/4）显著上升;海水胁迫与MV处理下,由叶柄导入叶黄素循环活性抑制剂DTT,菠菜叶片活性氧（ROS）大量积累,导致光合色素降解加剧,F_v/F_m、Yield、ETR、NPQ、q_P进一步下降。上述结果表明,海水胁迫抑制了菠菜叶片叶黄素循环活性,降低了叶片非辐射能量耗散能力,加重了叶片ROS积累,从而导致光合色素含量降低,PSⅡ活性下降,电子传递速率降低,用于光化学反应的能量部分减少,光合作用受到严重影响,说明海水胁迫下叶黄素循环在保持菠菜叶片光合色素稳定和光合作用正常运转中发挥重要作用。     

Quality of Highbush Blueberry (Vaccinium corymbosum L.) and Bilberry (Vaccinium myrtillus L.) Jam

ABSTRACT

Jam from wild bilberries and from the blueberry culti vars ‘Bluecrop’ and ‘Berkeley’ were analyzed by means of sensory profiling and by instrumental measurement of anthocyanins, color, pH and soluble solids. The study shows that bilberry jam has more “bluish black color,” compared to a more “reddish-blue color” and “glossy” surface of the jam from cultivated blueberries. Bilberry jam was less “smooth” and higher in “viscosity” and “berry density” as well as less distinct in “flavor of flowers and fruits,” while the “blueberry flavor and odor” were more distinct in the bilberry jam than that made of highbush blueberries. Analysis of color by means of sensory hue and instrumental a Hunter lab corresponded well, as did saturation measured by the sensory method and chroma instrumentally measured.     

An Evaluation of Blueberry Cultivars Grown in Plastic Tunnels in Douglas County,Oregon

Abstract

Growing fruits and vegetables in plastic tunnels is known to accelerate maturity and protect quality. There are several studies on highbush blueberry (Vaccinium corymbosum) that demonstrate the ability of row covers to advance or delay crop maturity depending on how they are manipulated. Accelerating or delaying harvest time can prove to be very lucrative for producers. However, before producers start putting plastic tunnels over their crops it is important to know how blueberry cultivars will behave when grown under plastic. Six cultivars of southern highbush blueberry and eight cultivars of northern highbush blueberry were grown in a high hoop plastic tunnel for two seasons. Blueberry plants were grown under plastic from February 1 until their harvest was completed each year. After harvest, the plastic covering over the hoops was removed to help the blueberry plants develop fruit buds and meet winter chill requirements. On February 1 the plastic covering was returned to the high hoop tunnel after chilling requirements were met. As expected, fruit maturity dates were 1-3 weeks earlier for all the highbush blueberries grown inside the tunnel versus the same cultivars grown outside. The difference in maturity dates between cultivars grown under plastic and outside did change from one year to the next because of variation in the weather. Despite potential for poor pollination of blueberries in plastic tunnels, yields were significantly enhanced for four cultivars when grown under plastic tunnels. ‘Toro’, ‘Nui’, ‘Legacy’, and ‘Misty’ showed yield gains ranging from 1 to 4 times when grown under a plastic tunnel. Seed numbers per fruit were not significantly impacted by growing plants inside our tunnel.     

Root restriction-induced limitation to photosynthesis in tomato (Lycopersicon esculentum Mill.) leaves

Root restriction often depresses photosynthetic capacity and the mechanism for this reduction, however, remains unclear. To identify the mechanism by which root restriction affects the photosynthetic characteristics, tomato (Lycopersicon esculentum Mill.) seedlings were subjected to root restriction stress with or without supplemental aeration to the nutrient solution. With the development of the root restriction stress, CO₂ assimilation rate was decreased only in confined plants without supplemental aeration. There were also significant decreases in leaf water potential, stomatal conductance (g_s), intercellular CO₂ concentration (C_i), and increases in the stomatal limitation (l) and the xylem sap ABA concentration. Meanwhile, the maximum carboxylation rate of Rubisco (V_cmax) and the capacity for ribulose-1,5-bisphosphate regeneration (J_max) also decreased, followed by substantial reductions in the quantum yield of PSII electron transport (Φ_PSII). Additionally, root restriction resulted in accumulation of carbohydrates in various plant tissues irrespective of aeration conditions. It is likely that root restriction-induced depression of photosynthesis was mimicked by water stress.     

Growth and photosynthetic characteristics of blueberry (Vaccinium corymbosum cv. Bluecrop) under various shade levels

Blueberry can readily be shaded as a bush type plant, maybe affecting its growth and photosynthesis. Growth and photosynthetic characteristics of ‘Bluecrop’ blueberry grown under various shade levels were investigated to understand acclimation under shade conditions and to determine the optimal light conditions for agricultural purpose. Shade decreased the number of shoots per shrub, but increased shoot length. However, shade did not affect the number of leaves on the main axis. With increasing shade level, leaf length, width and area increased, but leaf thickness decreased. However, there was no obvious tendency in leaf length/width ratio with increasing shade level. Shade leaves had less dense stomata than sun leaves, but stoma was bigger in shade leaves than in sun leaves. With increasing shade level, non-photochemical quenching in blueberry leaves increased and the values were higher at low photosynthetic photon flux densities (PPFDs) in shade leaves than in sun leaves, resulting in the decreases in quantum yield, electron transport rate and net CO₂ assimilation rate (A_n). The maximum A_n at 31, 60, 73 and 83% shade levels was 11.8, 11.0, 8.4 and 7.5 μmol m⁻² s⁻¹, respectively. Following the slight decrease up to 100 μmol m⁻² s⁻¹ PPFD, stomatal conductance (g_s) linearly increased up to 600 μmol m⁻² s⁻¹ PPFD and became saturated at all shade levels. The leaves of the shrubs grown under the 83% shade level had a significantly lower g_s as compared to the leaves of the shrubs grown under the 31, 60 and 73% shade levels. Transpiration rate (E) linearly increased up to 600 μmol m⁻² s⁻¹ PPFD and was saturated at the 73 and 83% shade levels. However, E increased linearly at both 31 and 60% shade levels with increasing PPFD. The reproductive growth characteristics such as number of flowers, fruit set rate per flower bud and fruit yield also significantly decreased with increasing shade level. For agricultural purpose, therefore, shade level above approximately 60% of full sunlight must be avoided for optimal photosynthesis and growth of the ‘Bluecrop’ blueberry.     

Effect of cold and drought stress on blueberry dehydrin accumulation

Summary

Dehydrins are a group of plant proteins which respond to any type of stress that causes dehydration at the cellular level, such as cold and drought stress. Previously, three dehydrins of 65, 60, and 14.kDa were identified as the predominant proteins present in cold acclimated blueberry (Vaccinium corymbosumLinn.) floral buds. Levels were shown to increase with cold acclimation and decrease with deacclimation and resumption of growth. In the present study, to determine if dehydrins are induced in other organs in response to low temperature treatment (48C) and in response to drought, accumulation of dehydrins was examined in leaves, stems, and roots of two cultivars and one wild selection (a V. corymbosum cultivar, a V. ashei Reade cultivar, and a V. darrowi Camp selection) of blueberry by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) followed by immunoblotting. Cold treatment involved placing plants in a cold room maintained at 48C for five weeks; drought stress was imposed by withholding water from potted, greenhouse-grown plants for 34.d. Relative water content (RWC) of shoots was determined periodically throughout the drought treatment. Dehydrins accumulated with both cold and drought stress but their molecular masses varied depending upon blueberry species. Dehydrins accumulated to higher levels in stems and roots than in leaves with cold stress and to higher levels in stems than in either roots or leaves with drought stress. Furthermore, cold treatment combined with dark treatment induced higher levels of dehydrins than cold treatment combined with a 10.h light/14.h dark photoperiod, suggesting that dehydrins may be responsive to changes in photoperiod as well. In the cold-stress experiment, the level of dehydrin accumulation was correlated with expected level of plant cold hardiness in the three genotypes. In the drought stress experiment, dehydrins accumulated prior to significant changes in RWC, and dehydrin levels did not appear to be closely correlated with RWC either among or within genotypes.     

Einfluss der Bestäubung durch die Honigbiene (Apis mellifera) auf Ertragsparameter der Kulturheidelbeere (Vaccinium corymbosum)

The influence of honey-bees’ pollination on parameters of yield of the three highbush blueberry varieties ‘Bluecrop’, ‘Reka’ and ‘Brigitta Blue’ was investigated. Four pollination variants were realized: 1. without cage; 2. cage with bees; 3. cage without bees; 4. fine-meshed bag. Total yield, fruit weight, seed production and phenology of the three varieties were examined. An enormously positive influence of honey-bees on yields of ‘Bluecrop’ and ‘Reka’ was found: total yield was by 500–560% higher in cages with bees compared to cages without bees. The results of fruit weight and seed production were similarly clear. However, ‘Brigitta Blue’ hardly responded to different intensities of pollination: regardless of the variant, total yield was extremely low (in cage with bees only 35% of the ‘Bluecrop’-yield) and couldn’t be improved by intensive pollination. The same applied to number of seeds per fruit. However, the mean fruit weight resembled that of ‘Reka’, but wasn’t influenced by the intensity of pollination, either. ‘Bluecrop’ lived up to its reputation as most profitable standard grade with highest yields in all variants (3.7?kg/m³ bush in cage with bees), highest fruit weight (1.6?g) and maximum number of seeds per fruit (98). A direct correlation between fruit weight and number of seeds per fruit couldn’t be established.     

Temporary immersion and stationary bioreactors for mass propagation of true-to-type highbush,half-high,and hybrid blueberries (Vaccinium spp.)

True-to-type propagules in half-high, highbush, and hybrid blueberries (Vaccinium L. spp.) were produced using stationary (SB) and temporary immersion bioreactor (TIB) systems containing a liquid medium. Multiple shoots were produced in vitro from nodal segments of blueberry cultivars ‘St. Cloud’ and ‘Polaris’, and of six blueberry hybrids obtained from crossing between half-high/highbush and lowbush blueberries. Shoot proliferation was best in a liquid medium containing 4.6 µM zeatin in both TIB and SB systems, but the performance was genotype dependent. Shoot proliferation was better in hybrids than in cultivars. Although SB produced longer shoots with more leaves per shoot in most of the genotypes, TIB-derived shoots were more vigorous and rooted better under ex vitro condition. Liquid culture-derived elongated shoots were rooted ex vitro by treating with indole-3-butyric acid (39.4 mM) before planting on a 3 peat:2 perlite (v/v) medium. Micropropagules were acclimatized and maintained in a greenhouse with 80?90% survival rate of rooted plantlets. Expressed sequence tag (EST)-polymerase chain reaction (PCR) and EST- and genomic-simple sequence repeat (SSR) marker assay formed a homogenous monomorphic banding pattern in the in vitro-derived and donor control plants proving the clonal fidelity of liquid-culture-derived micropropagated plants.     

Acclimatisation to mannitol-induced water deficit in miniature rose (Rosa hybrida) plantlets cultivated in vitro under autotrophic conditions

Summary

Miniature rose plantlets at the flower development stage were grown photo-autotrophically on MS medium and subsequently exposed to water deficits of –0.23, –0.32, –0.40, or –0.67 MPa osmotic potential ( _s) for 14 d. The _s in the culture medium was raised by increasing the concentration of mannitol, which caused abnormal floral development in terms of the flowering percentage and the number of flowers per plantlet, as well as delayed flowering. In vitro flowering and the number of flowers per plantlet declined significantly when miniature rose plantlets were exposed to water deficit stress at –0.40 MPa or –0.67 MPa. Reductions in growth, pigment degradation, chlorophyll a fluorescence, and net photosynthetic rate (Pn) were greatest in plantlets exposed to a water deficit stress of –0.67 MPa. This was particularly evident in the case of Pn, with a decline of 73.7% compared to non-stressed control plantlets. In contrast, proline levels increased in plantlets under water deficit stress, as proline performs a key role as an osmoprotectant under such conditions. The flowering stage in miniature rose plantlets is particularly susceptible to water deficit stress, which suppresses the development of reproductive organs. Knowledge of the responses to water deficit stress at the reproductive stage may be applied to identify effective indices for the selection of genotypes with increased tolerance to water deficit in miniature rose breeding programmes.     

Photosynthetic and physiological responses to high temperature in grapevine (Vitis vinifera L.) leaves during the seedling stage

To study the effects of high temperature (HT) on grape growth, a controlled experiment with grapevine (Vitis vinifera L., cv. Hongti) was conducted from July to October, 2015. The HT treatments were 34, 36, 38, and 40°C, with 28°C as control. The changes to photosynthetic pigment characteristics, antioxidant enzyme activities, and relative water content (RWC) under different HTs were investigated. Severe chlorosis was observed during the late stages of HT treatment. Chlorophyll a (Chl a), chlorophyll b (Chl b), photosynthetic rate at irradiation saturation (P_max), light saturation point (LSP), apparent quantum efficiency (AQE), peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) activity in grape leaves increased at first, and then decreased under high-temperature stress (HTS), but the light compensation point acted contrary to the LSP. The carotenoid, malondialdehyde and relative electrical conductivity (REC) increased under HTS; and the Chl/carotenoid ratio and RWC were contrary to the REC. The results suggested that grapes subjected to 38°C for 4 d decreased their RWC, but Chl a, Chl b, P_max, AQE, LSP, SOD, POD, and CAT reached their maximums. Therefore, the high-temperature limit of Hongti was 38°C and the duration time was 4 d.     

Abscisic acid alleviates the deleterious effects of cold stress on ‘Sultana’ grapevine (Vitis vinifera L.) plants by improving the anti-oxidant activity and photosynthetic capacity of leaves

The effects of exogenous application of abscisic acid (ABA) on anti-oxidant enzyme activities and photosynthetic capacity in ‘Sultana’ grapevine (Vitis vinifera L.) were investigated under cold stress. When vines had an average of 15 leaves, 0 (control), 50, 100, or 200 µM ABA was sprayed to run-off on all leaves of each plant. Twenty-four hours after foliar spraying with ABA, half (n = 5) of the water-only control vines and half (n = 5) of each group of ABA-treated plants were subjected to 4°C for 12 h, followed by a recovery period of 3 d under greenhouse conditions (25°/18°C day/night). The remaining plants in each treatment group were kept at 24°C. Cold stress increased H₂O₂ and malondialdehyde (MDA) concentrations in vine leaves, whereas all foliar ABA treatments significantly reduced their levels. Chilled plants showed marked increases in their total soluble protein contents in response to each ABA treatment. ABA significantly increased the activities of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase in cold-stressed grapevine leaves. In contrast, cold stress markedly decreased the rates of leaf photosynthesis (A) and evaporation (E), stomatal conductance (g_s), and chlorophyll concentrations in leaves, but increased intercellular CO₂ concentrations (C_i) in leaves. Treatment with all concentrations of ABA resulted in lower leaf A, E, and g_s values, but higher C_i values at 24°C. However, following cold stress, ABA-treated vines showed higher leaf A, E, and g_s values, but lower C_i values compared to control vines without ABA treatment. The application of 50–200 µM ABA allowed chilled vines to recover more quickly when re-exposed to normal temperatures, enabling the vines to resume their photosynthetic capacity more efficiently following cold stress. These results showed that, by stimulating anti-oxidant enzyme systems and alleviating cold-induced stomatal limitations, ABA reduced the inhibitory effect of cold stress on the rate of CO₂ fixation in ‘Sultana’ grapevine plants.     

水分胁迫对‘赤霞珠’不同叶龄叶片光合特性的影响

【目的】探究水分胁迫对葡萄不同叶龄叶片光合及荧光变化规律的影响。【方法】以3 a(年)生‘赤霞珠’葡萄(Vitis vinifera‘Cabernet Sauvignon’)为试材,由黎明前叶片水势(ψb)反映胁迫程度,设置对照(无水分胁迫,CK)、中度水分胁迫(T1)和重度水分胁迫(T2)三个处理,测定‘赤霞珠’葡萄新梢不同叶龄叶面积、叶片光合参数、叶绿素荧光参数及Ru BP活化酶和果糖-1,6-二磷酸酯酶活性。【结果】葡萄叶片叶龄为80 d时,叶面积趋于稳定,T2显著降低了各叶龄叶片的面积,T1与CK无显著性差异;葡萄叶片SPAD值随叶龄的增加而增大,且T1增加了各叶龄叶片SPAD值,T2则降低;Pn、Gs、Tr、Ru BP活化酶和果糖-1,6-二磷酸酯酶活性随葡萄叶片叶龄的增加均呈先上升后下降的趋势,并且随水分胁迫程度的增加下降程度增大;水分胁迫不同程度地降低了各叶龄叶片Fv/Fm、Yield、ETR、q P,而Fo显著增加,T2增加了各叶龄叶片的NPQ,但随着叶龄的增加,T2叶片NPQ又显著低于其他两个处理。【结论】随‘赤霞珠’葡萄叶片叶龄的增加,光合能力呈先上升后下降的趋势,当葡萄叶片叶龄为60~90 d时,光合能力最强。水分胁迫降低了各叶龄叶片的光合效率,重度水分胁迫会阻碍葡萄幼龄叶片的正常发育,并加速老龄叶片的衰老。     

Capability of the ‘Ball-Berry’ model for predicting stomatal conductance and water use efficiency of potato leaves under different irrigation regimes

The capability of the ‘Ball-Berry’ model (BB-model) in predicting stomatal conductance (g_s) and water use efficiency (WUE) of potato (Solanum tuberosum L.) leaves under different irrigation regimes was tested using data from two independent pot experiments in 2004 and 2007. Data obtained from 2004 was used for model parameterization, where measurements of midday leaf gas exchange of potted potatoes were done during progressive soil drying for 2 weeks at tuber initiation and earlier bulking stages. The measured photosynthetic rate (A_n) was used as an input for the model. To account for the effects of soil water deficits on g_s, a simple equation modifying the slope (m) based on the mean soil water potential (Ψ_s) in the soil columns was incorporated into the original BB-model. Compared with the original BB-model, the modified BB-model showed better predictability for both g_s and WUE of potato leaves on the parameterization data set. The models were then tested using the data from 2007 where plants were subjected to four irrigation regimes: non-irrigation (NI), full irrigation (FI), partial root-zone drying (PRD), and deficit irrigation (DI) for 3 weeks during tuber initiation and earlier bulking stages. The simulation results showed that the modified BB-model better simulated g_s for the NI and DI treatments than the original BB-model, whilst the two models performed equally well for predicting g_s of the FI and PRD treatments. Although both models had poor predictability for WUE (0.47 < r² < 0.71) of potato leaves, the modified BB-model was able to distinguish the effects of the irrigation regimes on WUE being that the WUE was generally greater for PRD than for FI and DI plants. Conclusively, the modified BB-model is capable of predicting g_s and of accounting for the differential effects of irrigation regimes on WUE of potato leaves. This information is valuable for further simulating potato water use thereby optimizing WUE under field conditions.     

Floral induction in tropical fruit trees: Effects of temperature and water supply

Summary

Floral induction in tropical trees generally follows a check in vegetative growth. However, it is not easy to identify the environmental factors involved in flowering, which normally occurs during the dry season when temperatures are also often lower. The separate and combined effects of temperature and water supply on floral induction were investigated in ‘Hass’ avocado (Persea americana), ‘Lisbon’ lemon (Citrus limon). ‘Wai Chee’ litchi (Litchi chinensis) and ‘Sensation’ mango (Mangifera indica). Low temperatures (15°/10°C or 15°/10°C and 20°/15°C compared with 30°/25°C and 25°/20°C) generally decreased vegetative growth and induced flowering in well-watered avocado, litchi and mango. A pre-dawn leaf water potential (ψ_L) of ?1.7 to ?3.5 MPa compared with ?0.4 to ?0.7 MPa in control avocado and litchi, and a pre-dawn relative water content (R.W.C.) of 90-93% compared with 97% or above in control mango plants also reduced or eliminated vegetative growth, but did not induce flowering. Low temperatures (15°/10°C compared with 20°/5°C, 25°/20°C or 30°/25°C) and water stress (pre-dawn ψ_L of ?2.0 to ?3.5 MPa compared with ?0.7 to ?0.8 MPa in controls) reduced or eliminated vegetative growth in lemon. In contrast to the response in avocado, litchi and mango, flowering in lemon was very weak in the absence of water stress at 15°/10°C or outdoors in Brisbane in subtropical Australia (Lat. 28°S), and was greatest after a period of water stress. The number of flowers increased with the severity and duration of water stress (two, four or eight weeks) and was generally greater after constant rather than with cyclic water stress. In lemon and litchi, net photosynthesis declined with increasing water stress reaching zero with a midday ψ_L of ?3.5 to ?4.0 MPa. This decline in carbon assimilation appeared to be almost entirely due to stomatal closure. Despite the reduction in midday CO₂ assimilation, starch concentration increased during water stress, especially in the branches, trunk and roots of lemon. Leaf starch was uniformly low. The number of flowers per tree in lemon was strongly correlated with starch in the branches (r²=77%, P<0.01) and roots (r²=74%, P<0.001). In litchi, starch was lower than in lemon roots and was not related to flowering.

In separate experiments to test the interaction between temperature and water supply, low day/night temperatures (23°/18° and 18°/15°C compared with 29°/25°C) reduced vegetative growth and induced flowering in avocado, litchi and mango. None of these species flowered at 29°/25°C or as a result of water stress (ψ_L of ?1.5 MPa compared with ?0.3 MPa for avocado and ?2.0 MPa compared with ?0.5 MPa for litchi, and R.W.C, of 90-93% compared with 95-96% in mango). In contrast, in lemon, flowering was very weak (<10 flowers per tree) in the absence of water stress (pre-dawn ψ_L of ?2.0 MPa compared with ?0.5 MPa) and was only heavy (>35 flowers per tree) after stressed trees were rewatered. There were slightly more flowers at 18°/15°C than at 23°/18° and 29°/25°C in control plants, but no effect of temperature in stressed plants. Starch concentration in the roots of avocado, lemon, litchi and mango was generally higher at 18°/15°C and 23°/18°C than at 29°/25°C. Water stress increased the starch concentration in the roots of lemon and litchi and decreased it in avocado. There was no effect in mango. There was a weak relation (r²=57%, P<0.05) between the number of flowers per tree in lemon and the concentration of starch in the roots. In contrast, there was no significant relationship between flowering and starch levels under the various temperature and water regimes in the other species. In another experiment, only vegetative growth in litchi and mango occurred at 30°/25°C and only flowering at 15°/10°C. Six weeks of water stress (pre-dawn ψ_L of ?2.5 MPa compared with ?1.0 MPa or higher in litchi, and R.W.C, of 90-93% compared with 95% or higher in mango) in a heated glasshouse (30°C days/20°C night minimum) before these temperature treatments did not induce flowering.

Temperatures below 25°C for avocado and below 20°C for litchi and mango are essential for flowering and cannot be replaced by water stress. The control of flowering in lemon over the range of day temperatures from 18°C to 30°C differed from that of the other species in being mainly determined by water stress. Flowering was generally weak in well-watered plants even with days at 18°C. Starch did not appear to control flowering.