Effects of Timing of Drought Stress on Phenology, Yield and Yield Components of Short-duration Pigeonpea

Nine short-duration pigeonpea genotypes were given adequate soil moisture throughout growth or subjected to water stress during the late vegetative and flowering (stress 1), flowering and early pod development (stress 2), or podfill (stress 3) growth stages under field conditions. The stress 1 treatment had no significant effect on the time to flowering. No stress treatment affected maturity or inter-plant flowering synchronization. The interval from a newly opened flower to a mature pod was about 30 days for all genotypes, and was unchanged in plants that were recovenng from stress 1 or undergoing stress 2. Seed yield was reduced to the greatest extent by stress 2 (by 37 %) and not significantly affected by stress 3 for all genotypes. No consistent differences were found between determinate and indeterminate genotypes in the ability to maintain seed yield under both stress 1 and stress 2. The harvest index was significantly reduced (22 %) by stress 2 but not by stress 1. However, under each soil moisture treatment, genotypic differences for seed yield were associated largely with differences in total dry matter production (TDM). For all genotypes, the number of pods m^-2 was the only yield component significantly affected by the water stress treatments. The stability of other yield components should be fully exploited to improve the stability of seed yield under drought conditions (drought resistance). Possible characteristics which may improve the drought resistance of short-duration pigeonpea include the ability to maintain TDM, low flowering synchronization, small pod size with few seeds pod^-1, and large 100-seed mass.     

Root and shoot responses of upland New Rice for Africa varieties to fluctuating soil moisture conditions as affected by different levels of nitrogen fertilization

Drought cycling and soil re-watering trends due to intermittent rainfall patterns are key stress factors that influence rice growth and yield under upland cultivation conditions. However, upland rice adaptation responses to fluctuating soil moisture conditions remain poorly understood. This study investigated root and shoot responses of upland New Rice for Africa (NERICA) varieties to episodic drought and re-watering during growth. We examined root and shoot growth of NERICA 1 and NERICA 4 compared with those of IR72, an improved lowland variety, and Dular, a traditional drought-tolerant variety, in terms of soil moisture fluctuations with different levels of nitrogen fertilization under field conditions that impeded deep root development. During soil moisture fluctuation, all varieties reduced shoot dry weight compared with well-watered plants, regardless of nitrogen fertilization levels. However, total root length for the three upland varieties was enhanced by soil moisture fluctuations at moderate and high nitrogen fertilization, while that of the lowland variety was reduced. Comparing root development during water fluctuations revealed that NERICA 1 had a greater root system than NERICA 4, which was attributed to lateral root development. Furthermore, we found that NERICA varieties increase lateral root mass during soil desiccation under adequate nitrogen fertilization, while Dular and IR72 reduced their root growth rate during drought and increased it after re-watering. Both root growth patterns developed, from around maximum tillering to heading. The analysis of regression between root elongation and shoot growth with fluctuating soil moisture indicated that an enhanced root system during drought, on adequate nitrogen fertilization, can contribute to shoot growth when sufficient water becomes available, specifically around the maximum tillering to the heading growth stage of rice.     

Water Sensitive Periods during the Reproductive Growth Phase of Glycine max L. II Establishing Water Stress Sensitivity

The water sensitivity of soybeans was determined on a day-to-day basis from the onset of flowering to physiological maturity by correlating daily leaf water potential values of 50 different soil water regimes with corresponding grain, vegetative and biomass yields. Days, and consecutive days forming periods, during which significant negative correlations were obtained were regarded as water sensitive periods during the reproductive growth phase of the soybean crop.
Water deficits during flowering significantly inhibited vegetative and biomass production but inhibited grain yields only when occurring during early flowering. During pod elongation and seed development the sensitivity of the soybean plant to water stress was present but diminished in time with regard to vegetative and biomass yields. The effect on grain yields was restricted to pod elongation and to later seed development stages. Water stress affected grain yields to a far greater extent than vegetative growth during seed filling and physiological maturity.     

Response of Root Branching and Shoot Water Potentials of French Beans (Phaseolus vulgaris L.) to Soil Moisture and Fertilizer Potassium

Extensive branching patterns of roots and the maintenance of adequate water within shoots enables plants to overcome water stress. However, information on the relationship between fertilizer potassium, root branching patterns and shoot water potentials of food legumes grown under different soil moisture regimes is scarce. Thus, an experiment was conducted in a phytotron to ascertain the effect of fertilizer potassium on root branching patterns and shoot water potentials of a popular tropical food legumes (Frenchbeans Phaseolus vulgaris L). The plants were grown in a sand medium with 0.1, 0.8 or 3.0 mM of potassium under a suboptimal and optima) soil moisture regime.
Root lengths and dry weights were enhanced by potassium, especially under a suboptimal soil moisture regime. The branching patterns changed due to potassium, where the numbers of second and third order roots increased under both soil moisture regimes, although the impact was greater in plants grown with low soil moisture. Plant water contents measured in terms of shoot water potential, relative water contents, rurgid weight: dry weight ratio and water uptake capacity were also increased by potassium. A positive relationship was observed between root branching patterns and water potentials with increasing potassium levels especially in plants grown under suboptimal soil moisture conditions. Shoot growth and nodulation was also promoted by potassium. The ability of plants to develop a more extensive branching pattern of roots by inducing a greater number of second and third order roots and changing the root branching habit from a herringbone to a dichotomous type to maintain a greater shoot water potential especially under low soil moisture regimes is presented.     

Effect of Scheduling Irrigation at Different Phenophases of Sweet Corn (Zea mays saccharata) and Delineation of Critical Stages Based on Stress Day Index

A field experiment was conducted during the summer of 1996 at the Main Research Station, University of Agricultural Sciences, Hebbal, Bangalore to study the effect of phenophased irrigation schedules on green cob and fodder yield and to delineate critical stages based on Stress Day Index. There were seven irrigation schedules consisting of a combination of two IW/CPE ratios (irrigation water/cumulative pan evaporation), viz. 0.6 and 0.8 shuffled at three growth stages: 10–35 DAS (days after sowing), 36–65 DAS and 66 days to harvest in RCBD (randomized complete block design) with four replications. The investigations revealed that all yield attributing characters such as cobs per plant, cob weight, green cob and fodder yields were significantly higher under the irrigation schedule of 0.8 IW/CPE ratio throughout the crop growth. The reduction in green cob and fodder yields were 39 and 25 % respectively, when irrigations were scheduled at 0.6 IW/CPE ratio throughout as compared to 0.8 IW/CPE. Also, frequent irrigation at 0.8 IW/CPE ratio throughout maintained higher plant relative water content (86.90 %) and higher available soil moisture (59.95–81.51 %) before irrigation as compared to delayed irrigations at 0.6 IW/CPE ratio. The quality parameter viz. sucrose content (2900 μg g−1) in developed seeds was also higher under frequently irrigated treatment. The stress day index indicated that vegetative followed by silking and tasseling were the critical stages for moisture stress.     

开花期干旱胁迫对鲜食糯玉米产量和品质的影响

为探明开花期(抽雄吐丝期)干旱胁迫对鲜食糯玉米(吐丝后23 d采收)产量和品质的影响, 以苏玉糯5号和渝糯7号为试材, 采用负水头供水控水盆栽装置控制土壤含水量, 设置开花期正常供水(土壤相对含水量80%)和干旱胁迫(土壤相对含水量60%) 2个处理, 研究干旱胁迫对鲜食糯玉米产量(鲜果穗和鲜籽粒)、籽粒组分、糊化和热力学特性的影响。结果表明, 开花期干旱胁迫减少籽粒数量、降低籽粒重量、缩小籽粒体积, 导致鲜果穗和鲜籽粒产量损失。开花期干旱胁迫下鲜食期籽粒淀粉含量升高, 但对于蛋白质含量渝糯7号降低, 苏玉糯5号变化不显著。蛋白质组分中, 对球蛋白含量影响不显著, 清蛋白、谷蛋白和醇溶蛋白均显著降低。开花期干旱胁迫显著降低淀粉粒平均粒径。碘结合力2015年度显著下降, 2014年度受干旱影响不显著。开花期干旱胁迫下籽粒峰值黏度、谷值黏度和终值黏度在苏玉糯5号中降低, 在渝糯7号中升高。开花期干旱胁迫下两品种峰值温度降低, 回生热焓值和回生值升高, 而热焓值仅渝糯7号在2014年度升高。总之, 开花期干旱降低糯玉米鲜果穗和鲜籽粒产量, 增加籽粒淀粉含量, 降低籽粒蛋白质含量、淀粉粒径和支链淀粉中长链比例, 进而使籽粒回生增加, 但糊化黏度两品种表现不同(渝糯7号升高, 苏玉糯5号下降)。     

Cassava (Manihot esculenta Crantz) Growth Indices, Root Yield and its Components in Upland and Inland Valley Ecologies of Sierra Leone

Cassava is predominantly an upland crop that is also cultivated in inland valley swamps (IVS). Identifying physiological traits that can withstand excess moisture stress can aid in the selection and use of stable cassava cultivars in IVS. Three cassava cultivars were evaluated for growth and yield in the upland and IVS ecologies in the 1993–94 crop seasons using a randomized complete block design. In the upland, 80/40 outyielded 87/29 and 'coco', while in the IVS, 87/29 had the highest yield. Highest yielding cultivars in each ecology also had the highest tuberous root bulking rate (TBR), dry matter (DM) production, crop growth rate (CGR), relative growth rate, net assimilation rate (NAR), leaf area index, leaf production rate and leaf life. Positive correlations between these parameters and tuberous root yield were noted. High yield of 87/29 in IVS was partly due to its low tuberous root rotting. High root rotting in 80/40 was partly as a result of the greater depth of tuberous roots in the soil (0.3–0.6 m) as compared to the other cultivars (0.15–0.3 m) where roots were in contact with stressful water levels earlier than others. The yield and yield components, growth and leaf characters were all drastically reduced when the cultivars were grown in IVS as compared to upland. For example, root yields of 'coco', 87/29 and 80/40 were reduced by 53 %, 60 % and 92 %, respectively, in IVS. Selection of cassava cultivars with longer leaf life and leaf area maintenance leading to high CGR, TBR, NAR and harvest index (in IVS and upland), coupled with a shallow tuberous root formation zone (in IVS) can give higher root and leaf yields in the two ecologies.     

Growth and Metabolic Changes of Cowpea Plants as Affected by Water Deficiency and Indole-3-Yl-Acetic Acid

Cowpea plants ( Vigna sinensis Savi.) were grown in pots for two successive years. These plants were daily maintained water holding capacities (15, 25, 45, 65 %).
The foliage of plants grown at every level of soil moisture was sprayed till dripping with IAA solutions (0, 10, 50, 100 mg/l) three times after 3, 6 and 9 weeks from sowing. Samples were taken at the two physiological stages before flowering and during flowering for growth measurements and determinations of nitrogen fractions.
Water shortage led to retardation of stem length, number of internodes, number of leaves and dry weight shoots. IAA increasing the same parameters only when applied at concentrations of 10 and 50mg/1 at moisture level 65 %, 45 % and 25 % of W.H.C.
Water stress decreased total-N and protein-N contents of cowpea shoots. On the other hand total soluble- N content was increased under the same conditions. IAA increased total-N and protein-N content of cowpea shoots. Meanwhile, the same growth regulator decreased total soluble-N when applied at 45% 25 % and 15 % W.H.C. levels.     

Effects of Water Deficiency and Kinetin on Growth and Nitrogen Metabolism of Cowpea Plants

Cowpea plants ( Vigna sinensis Savi.) were grown in pots for two successive years. These plants were daily maintained water holding capacities (15, 25, 45, 65 %).
The foliage of plants grown at every level of soil moisture was sprayed till dripping with kinetin solutions (0, 10, 20, 40 mg/1) three times after, 3, 6 and 9 weeks from sowing. Samples were taken at the two physiological stages before flowering and during flowering for growth measurements and determination of nitrogen fractions.
Water shortage led to retardation of stem length, number of internodes, number of leaves and dry weight of shoots. Kinetin reduced stem length of cowpea shoots. On the other hand, it exhibited stimulative effect when applied at concentrations 10 and 20 mg/1 on number of internodes, and dry weight of shoots.
Water stress decreased total–N and protein–N contents of cowpea shoots. On the other hand total soluble–N content was increased under the same conditions. Kinetin increased total–N and protein–N contents of cowpea shoots. Meanwhile, the same growth regulator decreased total soluble–N when applied at 45 %, 25 % and 15 % W.H.C. levels.     

Effects of Timing of Drought Stress on Abscission and Dry Matter Partitioning of Short-duration Pigeonpea

Shoot dry mass partitioning and cumulative abscission of leaf, flowers and pods were determined for nine short-duration pigeonpea genotypes grown with adequate soil moisture throughout growth (control), or subjected to water stress during the late vegetative and flowering (stress 1), flowering and early pod development (stress 2), or pod fill (stress 3) growth stages. The total cumulative dry mass of abscised plant parts was lower for determinate genotypes, but it increased to a greater extent under water stress than that for indeterminate genotypes, with stress 2 having the greatest and stress 3 die least effects. The dry mass contribution of pods to total abscission was ≤ 5%, and not signiflcantiy affected by water stress, while the contribution of leaves increased and mat of flowers decreased. Stress 3 had no significant effects on abscission dry mass totals or components. Reduction in shoot dry mass under water stress was most pronounced for genotypes in die early pod development stage, and the dry mass contribution of leaves generally decreased and that of pods increased under stress 1 and stress 2. With similar abscission levels, the shoot dry mass of genotype ICPL 151, was similar to, or greater than, that of hybrid ICPH 9, under stress 1 and stress 2, and the contribution of pods to shoot dry mass was lower for hybrid ICPH 9 under both stress treatments. Genotypic differences in drought resistance were likely due to differences in the leaf area maintenance during, and in the recovery of dry mass and pod production following, water stress periods.     

Root and Shoot Development of Phaseolus vulgaris L. (French Beans) as Affected by Soil Moisture and Fertilizer Potassium

The development of an extensive root system enables plants to overcome water stress. However, there is little information on the response of food legumes to soil moisture, especially during early growth, which determines crop establishment. Thus, an experiment was conducted under controlled conditions to identify the effect of soil moisture and fertilizer potassium on root and shoot growth of french beans ( Phaseolus vulgaris L.) seedlings. The seedlings were grown in a sand medium under a high and low soil moisture regime and with 0.1, 0.8 or 3.0 mM potassium.
Root lengths, dry weights and numbers of root hairs were greater under low soil moisture conditions. Potassium increased root growth irrespective of soil moisture regimes. The impact of potassium on root length was more pronounced under a high soil moisture regime. In contrast, potassium increased root dry weights and root hairs to a greater extent when plants were grown under dry conditions. The lack of adequate soil moisture increased specific leaf weights, and this phenomenon was reduced by the application of potassium. Shoot:root ratios also showed a similar phenomenon. The development of an extensive root system by french bean seedlings under dry conditions to extract a greater quantity of available soil moisture fur establishment and plant growth and the ability of potassium to promote this phenomenon is presented in this study.     

Effect of drought stress on content and composition of seed alkaloids in narrow-leafed lupin, Lupinus angustifolius L.

The effect of drought stress at three growth stages on seed alkaloid content in two sweet (W26 and Polonez) and one bitter variety (Zubr) of spring sown Lupinus angustifolius L. was investigated in a lysimeter experiment on sandy soil in Denmark. Full irrigation resulted in medium alkaloid content. Drought stress during the vegetative phase increased alkaloid content to 0.094, 0.192 and 2.55% of seed dry matter, respectively. At the flowering stage alkaloid content decreased to 0.070, 0.152 and 1.99%, while drought stress during pod-filling increased alkaloid content for the determinate variety W26 to 0.097%, but decreased it to 0.161 and 1.98% for the two indeterminate varieties Polonez and Zubr, respectively. Drought during either flowering or pod-filling resulted in an increased mean seed weight and a reduced number of seed per square metre. Alkaloid content was negatively correlated to seed weight. The alkaloid composition differed for the three genotypes, and was affected by drought treatment. Compared to the bitter variety the sweet varieties were characterized by reduced concentrations of lupanin, 13-hydroxylupanin and angustifolin, whereas spartein, isolupanin and isoangustifolin were little affected.     

Effects of Types and Duration of Weeds on Yield Parameters of Cocoyam (Xanthosoma saghtifolium)

Weeds reduce yields of all tropical crops significantly, and these effects vary with different species. Thus, the influence of time of weed control and types of weeds on yield parameters of cocoyams ( Xanthosoma sagittifolium ), a tropical tuber crop, was determined by two experiments conducted simultaneously. In one experiment, weeds were removed at different times during crop growth. In the other, different weed types were removed selectively to leave either one or two of the identified categories, namely grasses, broadleaves and sedges. The presence of weeds throughout the growth cycle of the crop reduced yields of cocoyams by 60 %. Removal of weeds at early growth stages produced greater yields than weeds were present beyond 18–22 weeks after planting. The presence of weeds at the time of tuber initiation had the greatest adverse effect on yield parameters. Weeds had a delaying but not an inhibitory effect on cormel initiation. However, all other yield parameters were reduced by weeds. Broadleaved weeds, which have similar growth patterns as cocoyams reduced yields to a greater extent than grasses. Sedges had no significant impact on cocoyams. The presence of grasses and broadleaves together reduced growth and yields of this crop to a greater extent than any other combination.     

Growth and Yields of Cowpea (Vigna unguiculata (L.) Walp) as Influenced by Seed Characters, Soil Moisture and Season of Planting

Cowpea ( Vigna unguiculata L. Walp) is an important food legume grown under rainfed conditions in the tropics. However, the yield of this crop varies widely in most tropical ecosystems due to crop and environmental factors. Experiments were carried out to evaluate the effects of seed characteristics, soil moisture at planting and the season of crop establishment on growth and yield parameters of cowpea as these influence the productivity of the crop. Growth and yield of cowpea plants grown from small seeds with a high conductivity of their lea-chates were significantly lower than those grown from large seeds. Root growth of seedlings was most prominent under moderate soil moisture levels. Low soil moisture retarded root growth by reducing dry weights and suppressing the development of laterals. Shoot growth was more sensitive to increasing soil moisture stress than root growth. Planting cowpea in the wet season produced the highest yields. Within a season, growth and yield were higher when the crop was planted early. Establishment of the crop later in the season produced better quality seeds measured in terms of germination after controlled deterioration.     

Influence of Soil Moisture and Fertilizer Potassium on the Vegetative Growth of Mungbean (Vigna radiata L. Wilczek) and Cowpea (Vigna unguiculata L. Walp)

Tropical food legumes are grown in a wide range of environments, and water stress is considered the principal environmental factor limiting growth and yield. Potassium fertilizer mitigates the impact of water stress in plants. However, the benefits of potassium in overcoming stress in tropical food legumes have not been investigated in comparative studies. The purpose of this study was to determine the benefits of potassium in overcoming water stress in mungbean and cowpea, two important tropical food legumes with different adaptabilities to soil moisture regimes. The experiment carried out under controlled conditions placed emphasis on vegetative growth and selected physiological parameters. The impact of potassium was different in the two legumes grown at optimal and suboptimal soil moisture. Potassium increased shoot growth of mungbean to a greater extent than in cowpea under suboptimal moisture conditions. The roots of cowpea showed a greater response to potassium fertilizer than in mungbean under suboptimal soil moisture. The plant water relations and photosynthetic rates of mungbean were improved to a greater extent by potassium under suboptimal soil moisture than those of cowpea. Although differences were observed in the responses of the vegetative growth of these species to moisture and potassium, in overall terms potassium promoted growth of both species when subject to suboptimal soil moisture. While field studies are required to validate the results, the application of potassium fertilizer can be considered a significant factor in overcoming soil moisture stress in these legumes commonly grown in tropical cropping systems.     

Physiological Analysis of Growth and Yield of Watermelon (Citrullus ianatus Thunb Musf) in Relation to Irrigation and Nitrogen Fertilization

Analysis of growth and yield of watermelon in relation to irrigation and N fertilization carried out at the Indian Institute of Horticultural Research, Bangalore, during 1983–84 and 1984–85 indicated that frequent irrigations when the soil matric potential at 15 cm depth reached -25 kPa resulted in maximum dry matter accumulation and distribution, leaf area index (LAI), leaf area duration (LAD), net assimilation rate (NAR) and crop growth rate (CGR) leading to higher fruit yield as compared to irrigations at -50 and -75 kPa. Imposing a stress of -75 kPa either during pre-flowering or flowering or fruit development phase adversely affected various growth parameters resulting in reduced yield as compared to that irrigated uniformly at -25 kPa. Increasing levels of N fertilization increased the dry matter accumulation and distribution through higher LAI, LAD and CGR and contributed larger proportion of the dry matter to fruits resulting in higher fruit yield.     

Influence of Water Stress on Grain Yield and Vegetative Growth of Two Cultivars of Bean (Phaseolus vulgaris L.)

Lack of water during vegetative and/or reproductive growth stages is one of the most limiting factors for bean growth. The purpose of this study was to evaluate the effects of water stress applied during two phenological stages (flowering and pod filling) on growth, yield and yield components. Two genotypes of bean ( Phaseolus vulgaris L.) were used in this study, cv. Carioca, an indeterminate Brazilian landrace, and cv. Prince, a determinate cultivar grown in Europe. Carioca appears to be generally stress-tolerant while Prince is intolerant. Plants were grown in large plastic pots covered with a black plastic sheet to protect the soil from rain and evaporation. The water stress treatments were: control (well-watered plants), WSFS (water stress during flowering stage) and WSPFS (water stress during pod-filling stage). Water stress reduced yield and yield components at both flowering and pod-filling stages. The parameters affected were seed weight, number of seeds per plant and number of pods per plant. Number of seeds per pod and seed weight were not affected. No effects of water stress were detected on harvest index. Time to maturity was slightly prolonged, especially for WSFS. Water stress at both stages resulted in lower accumulated water loss compared to the control plants. Water stress during both phenological stages reduced other growth parameters, the number of trifoliate leaves, stem height, number of main branches and number of nodes on the main stem.     

Factors Associated with Reduced Yields of Delayed Plantings of Soybean1

Field experiments were conducted for four years (1981 to 1984) at Lexington, KY, USA (38° N latitude) to determine the causes of reduced yields associated with delayed plantings of soybean (Glycine max [L.] Merrill). Five cultivars were planted in mid-May and early July in row spacings ranging from 18 to 89 cm and the plots were irrigated to minimize moisture stress. Yields were significantly reduced by delayed planting because of reductions in the number of seed m^?2 and mass seed^?1. The yield reduction under irrigation indicates that moisture stress is not a major cause of reduced yields from delayed plantings. The reductions in seed number were associated with lower insolation interception during flowering and pod set and with smaller vegetative mass (g m^?2) at growth stage R5 (beginning seed fill). The data suggest that a vegetative mass at R5 greater than that required to intercept 90 % or more of incident insolation is required to maximize seed number. The length of the seed filling period was not affected by planting date. Mass seed^?1 was positively related to mean air temperature during seed filling, which suggests that the smaller seed from delayed plantings are a result of the lower temperatures associated with the later maturity.     

Pod, Seed and Germination Characteristics of Selected Cardamom (Elettaria cardamomum (Linn.) Maton) Varieties

Cardamom is a popular and important tropical spice, traditionally propagated by rhizomes, although seeds which are considered recalcitrant are being increasingly used in planting programmes. Thus a study evaluated the effects of pod maturity on pod, seed and germination characteristics of three popular varieties, along with the influence of drying on seed germinability.
Green and yellow mature pods and seeds of all varieties had a high moisture content. The variety Mysore, which is considered high yielding, had larger pods with a greater number of heavy seeds. Although the pods of Vazukka were heavier than the Malabar variety due to greater seed weights, seed numbers were similar in both varieties. Seed moisture contents declined with increasing pod maturity, while seed weights and germinability increased.
The germinability of Mysore seeds were greater, while that of the other varieties were lower. The larger seeds of all varieties germinated 15–20 days earlier, giving rise to a greater percentage (20–25 %) of seedlings. Desiccation at 20 °C maintained germinability for a longer period (5–9 days) in all varieties, than when dried at 30 °C. The seeds of the variety Mysore was more tolerant to desiccation than other varieties. The study illustrated the importance of selecting large seeds from yellow mature pods of cardamom for propagation and the value of establishment immediately after harvest to obtain a greater number of seedlings.     

不同时期水分调亏及不同调亏程度对冬小麦产量的影响

分盆栽对冬小麦在不同生长时期进行不同程度的调亏试验的结果表明,冬小麦从拔节至开花期间的轻度水分亏缺对其产量有显著影响,而灌浆和返青时间的轻度水分亏缺对产量无影响。各生育时期在不同调亏水平下的产量敏感指数（或敏感系数）不同。冬小麦经过一定的亏缺处理,复水后出现生长方面的补偿效应,因而产量降低幅度与耗水量减少幅度相比要小得多。气孔阻力和叶水势对土壤水分的变动有一阈值反应,只有当土壤含水量降至田间持水量