Use of maximum trunk diameter measurements to detect water stress in mature ‘Arbequina’ olive trees under deficit irrigation

Olive is one of the fruit tree species for which measurements of the trunk diameter variation (TDV) has shown a lower potential both for monitoring water stress and for scheduling irrigation. This is particularly true in the case of old, big olive trees with heavy fruit load. Fernández et al. (2011, J. Environ. Exp. Bot. 72, 330-338) observed that the daily difference for maximum trunk diameter between deficit irrigated ‘Manzanilla’ olive trees and control trees growing under non-limiting soil water conditions, termed D_MXTD, is more sensitive and reliable for detecting the water stress of the trees than other widely used TDV-derived indices. However, they considered their results as preliminary because of the lack of replications. The aim of this work was to evaluate the usefulness of the D_MXTD index for detecting plant water stress in an orchard with 12-year-old ‘Arbequina’ olive trees with heavy fruit load. The performance of D_MXTD for detecting water stress of the trees was compared to that of the signal intensity for the maximum daily shrinkage (S_I-MDS) derived from the same TDV records. Results showed that S_I-MDS was not useful for indicating the water stress of the trees. On the other hand, the dynamics of D_MXTD mimicked those of the soil and plant water status. Four instrumented trees per treatment (replicates) were enough to reduce the uncertainty of the TDV measurements to a reasonable level. Our results show that D_MXTD is a useful index to detect the onset, and severity, of water stress in mature ‘Arbequina’ olive trees with heavy fruit load. They also suggest a potential of D_MXTD for scheduling low frequency deficit irrigation strategies.     

基于茎直径变化监测番茄水分状况的机理与方法

该研究以春季温室番茄为试验材料,以筒栽和小区相结合的方法探索了番茄茎直径变化的机理与规律、外界环境因素对茎直径变化的影响以及如何消除气象因子对实测日最大收缩量（MDS）数值干扰等问题,目的在于为基于茎直径变化监测作物水分状况、实现自动灌溉的技术提供理论和实践依据。试验结果表明,番茄茎直径变化落后于叶水势变化,二者存在很好的相关性。番茄茎直径收缩过程是由韧皮部及木质部收缩同步构成,而恢复过程则是不同步的,木质部恢复较快。番茄盛果期蒸腾强度大于花果期蒸腾强度,蒸腾强度越大一天中最小茎直径出现的时间越晚,而其茎直径开始恢复的临界气孔导度值越小。番茄MDS的变化趋势和日均辐射的变化趋势一致,但变化幅度由土壤水分决定。当土壤相对含水率由田间持水率降至50%时,MDS随土壤水分的下降而变大,天气晴好时MDS能够很好的反映出土壤水分的差异;而当土壤相对含水率小于50%后,MDS随土壤水分的下降而变小。通过统计分析,由实测MDS与参照MDS相比的相对日最大收缩量（RMDS）指标基本上可以消除气象因子对监测结果的影响,稳定的反映土壤水分状况。     

Evaluation of grapevine water status from trunk diameter variations

We evaluated the usefulness of short-term trunk diameter variations (TDV) as water stress indicator in field-grown grapevines cv. Tempranillo. Two indices were calculated from TDV, maximum daily trunk shrinkage (MDS), and trunk growth rate (TGR). The seasonal evolution of both indicators was compared with occasional determinations of pre-dawn leaf water potential and stem water potential, measured at early morning (Ψ_s^em) and at midday (Ψ_s^md) in irrigated and non-irrigated vines. In the second season, the effect of crop load on the vine water status indicators was also studied. Crop load did not affect either the vine water relations or the TDV. All water potential determinations had much lower variability and were more sensitive than both MDS and TGR to water restrictions. The ability of both indices to detect plant water stress varied largely depending upon the phenological period. In fact, MDS and TGR were only able to detect vine water stress during a short period of time before veraison. During this period, TGR was linearly related to both Ψ_s^em and Ψ_s^md, while for MDS a curvilinear, quadratic equation, better described the relationship with plant water status. After veraison no apparent relationship existed between plant water status and MDS or TGR. Hence, our results question the practical use of both MDS and TGR as variables to automate irrigation scheduling for grapevine.     

Usefulness of trunk diameter variations as continuous water stress indicators of pomegranate (Punica granatum) trees

Pomegranate trees (Punica granatum L.) is a deciduous fruit tree included in the so-called group of minor fruit tree species, not widely grown but of some importance in the south east of Spain. Pomegranate trees are considered as a culture tolerant to soil water deficit. However, very little is known about pomegranate orchard water management. The objective of this research was to asses the feasibility of using trunk diameter variation (TDV) indexes, obtained by means of LVDT sensors, as a plant water stress indicators for pomegranate trees. The experiment was carried out with mature trees grown in the field under three irrigation regimes: control well watered trees; trees continuously deficit irrigated at 50% of the control regime (SDI); and trees that had a summer water stress cycle being irrigated at 25% of the control rates only in July and August (RDI). The seasonal variations of maximum diurnal trunk shrinkage (MDS) and trunk growth rates (TGR) were compared with midday stem water potential (Ψstem) measurements. During the course of the entire season, control trees maintained lower MDS values than the SDI ones. In the RDI treatment, as water restrictions began, there was a slow increase in MDS, in correspondence with a decrease in Ψstem. When water was returned at full dosage, the RDI quickly recovered to MDS and Ψstem values similar to the control. However, lower MDS for a given Ψstem values were observed as the season advanced. The magnitude of differences between well watered and deficit irrigated trees was much larger in the case of MDS than for Ψstem. However, the tree-to-tree variability of the MDS readings was more than four times higher than for Ψstem; average coefficient of variation of 7.5 and 36% for Ψstem and MDS, respectively. On the other hand, TGR did not clearly reflect differences in tree water status. Overall, results reported indicated that MDS is a good indicator of pomegranate tree water status and it can be further used for managing irrigation. However, the seasonal changes in the MDS-Ψstem relationship should be taken into account when attempting to use threshold MDS values for scheduling irrigation.     

Early morning fluctuations in trunk diameter are highly sensitive to water stress in nectarine trees

The sensitivity to water stress of different plant water status indicators was evaluated during two consecutive years in early nectarine trees grown in a semi-arid region. Measurements were made post-harvest and two irrigation treatments were applied: a control treatment (CTL), irrigated at 120 % of crop evapotranspiration demand to achieve non-limiting water conditions, and a deficit irrigation treatment, that applied around 37 % less water than CTL during late postharvest. The plant water status indicators evaluated were midday stem water potential (Ψ _stem) and indices derived from trunk diameter fluctuations: maximum daily shrinkage (MDS), trunk daily growth rate, early daily shrinkage measured between 0900 and 1200 hours solar time (EDS), and late daily shrinkage that occurred between 1200 hours solar time and the moment that minimum trunk diameter was reached (typically 1600 hours solar time). The most sensitive [highest ratio of signal intensity (SI) to noise] indices to water stress were Ψ _stem and EDS. The SI of EDS was greater than that of Ψ _stem, although with greater variability. EDS was a better index than MDS, with higher SI and similar variability. Although MDS was linearly related to Ψ _stem down to ?1.5 MPa, it decreased thereafter with increasing water stress. In contrast, EDS was linearly related to Ψ _stem, although the slope of the regression decreased as the season progressed, as in the case of MDS. Further studies are needed to determine whether EDS is a sensitive index of water stress in a range of species.     

水分胁迫对番茄幼苗生理特性的影响

为了探讨番茄苗期生理特性在水分胁迫胁迫下的响应,试验设置充分灌溉的对照(T)和轻度(T1)、中度(T2)、重度(T3)水分胁迫处理.在试验开始的第5,10,15和20 d,测定各处理番茄幼苗的叶绿素荧光参数、气孔特征及抗氧化酶等指标.与对照相比,不同程度水分胁迫条件下,随胁迫程度的增大,各处理番茄幼苗的PSⅡ光化学效率(Fv/Fm)、光化学淬灭(qP)及电子传递速率(ETR)均有所下降,且由大到小基本表现为T,T1,T2,T3;处理T1,T2和T3的气孔密度、长度及宽度均有所减小,重度水分胁迫条件下,气孔关闭;叶片中的超氧化物歧化酶、过氧化物酶及过氧化氢酶活性随着水分胁迫的加重而增加.轻度水分胁迫番茄幼苗各生理指标与对照差异较小,故可在番茄苗期进行50%~60%田间持水量的参考灌溉制度.     

Irrigated guayule — Evapotranspiration and plant water stress

Natural rubber is a critical agricultural material, and guayule (Parthenium argentatum Gray) is the most promising domestic, agricultural source. A study on guayule was initiated to provide information on the water requirements and plant water stress behavior for this new perennial crop. Measured evapotranspiration averaged 3000, 2410, 2040, 1720, 1470 and 1520 mm, from May 1981 to December 1982, in decreasing order of water applications on the six irrigation treatments from the wettest to the driest. These evapotranspiration values indicated that the water use by guayule can be higher than many former estimates for an arid-type crop. Plant canopy temperatures also showed a progressive increase in plant water stress as irrigation water amounts decreased. The two-fold decrease in evapotranspiration from the wet to dry treatments corresponded to a four-fold increase in plant water stress indices based on remote infrared thermometer measurements. Both the concepts of stress degree days and crop water stress index, evaluated over the entire growing cycle, correlated well with the measured evapotranspiration. Even though the guayule plant can withstand long periods of drought, moisture stress will occur within a relatively short period of time after an irrigation for the guayule crop.     

Assessment of trunk diameter variation derived indices as water stress indicators in mature olive trees

Plant age and size, seasonal growth patters and crop load, among other factors, have been reported to decrease the usefulness of trunk diameter variation (TDV) derived indices as water stress indicators in olive trees. Our hypothesis, however, is that indices derived from TDV records in old, big olive trees are sensitive enough to detect levels of water stress in trees of orchards under deficit irrigation that, although severe, are below the threshold for fruit shrivelling. This is of importance for the production of good quality oils, since fruit shrivelling may affect oil quality. The aim of this work was to assess different TDV-derived indices as water stress indicators in 40-year-old ‘Manzanilla’ olive trees with heavy crop load. We derived the maximum daily shrinkage (MDS), daily growth (DG) and daily recovery (DR) from TDV records taken during the 2008 dry season both in well-irrigated FAO trees and in deficit-irrigated RI trees. Measurements of volumetric soil water content (θ_v), leaf water potential (Ψ_l), stomatal conductance (g_s), net CO₂ assimilation rate (A), water and oil accumulation in the fruits and yield parameters were made for both treatments. The trunks did not grow during the experimental season, either in the FAO or RI trees, likely because of the heavy crop load. Therefore, DG was useless as water stress indicator. For MDS and DR, which were responsive to the increase of the trees’ water stress, we calculated the variability, quantified by the coefficient of variation (CV), the signal intensity (S_I) and the sensitivity (S_I/CV) values. In addition, we derived reference equations for irrigation scheduling from the relationships between MDS values in the FAO trees and main meteorological variables. Values both of S_I-MDS and S_I-DR were steady until September 9, despite of increasing differences in θ_v between treatments from early in the dry season. The Ψ_l vs θ_v values showed an outstanding capacity of the RI trees to take up water from the drying soil, and the Ψ_l vs g_s values showed a near-isohydric behaviour of those deficit-irrigated trees. These results explain, at least in part, the lack of response of MDS and DR on that period. Both S_I-MDS and S_I-DR peaked for the first time on September 9, 16 days before the appearance of fruit shrivelling. Our results suggest that using TDV-derived indices as water stress indicators for irrigation scheduling in old olive orchards with medium to low plant densities, i.e. with large root zones, may be useless in case the irrigation strategy is aimed at keeping the soil close to field capacity. Nevertheless, the MDS and DR indices may be useful indicators for the avoidance of fruit shrivelling in deficit irrigated olive orchards for the production of good quality oil. Reliable reference equations for scheduling irrigation with the signal intensity approach were obtained from the regression of MDS values vs the daily maximum values of both the air temperature and the vapour pressure deficit of the air.     

Assessing water stress in a hedgerow olive orchard from sap flow and trunk diameter measurements

We used sap flow and trunk diameter measurements for assessing water stress in a high-density ‘Arbequina’ olive orchard with control trees irrigated to replace 100 % of the crop water needs, and 60RDI and 30RDI trees, in which irrigation replaced ca. 60 and 30 % of the control, respectively. We calculated the daily difference for both tree water consumption ( $ D_{{E_{\text{p}} }} $ ) and maximum trunk diameter (D _MXTD) between RDI trees and control trees. The seasonal dynamics of $ D_{{E_{\text{p}} }} $ agreed reasonably well with that of the stem water potential. We identified peculiarities on the response $ D_{{E_{\text{p}} }} $ to changes in water stressing conditions, which must be taken into account when using the index. An analysis of the water stress variability in the orchard is required for choosing the instrumented trees. The reliability of the D _MXTD index was poorer than that of $ D_{{E_{\text{p}} }} $ . The maximum daily shrinkage (MDS) was not a reliable water stress indicator.     

Comparison of corn yield response to plant water stress caused by salinity and by drought

The effect of water stress on corn yield was studied in a salinity experiment and in a drought experiment. The plant water status was determined by measuring the pre-dawn leaf water potential regularly during the whole growing season and expressed by the water stress day index (WSDI). The yield response of corn did not differ under salinity and drought conditions. The WSDI is a useful indicator for determining crop-response to salinity and drought.     

Determining reference values for stem water potential and maximum daily trunk shrinkage in young apple trees based on plant responses to water deficit

The use of plant water status indicators such as midday stem water potential (Ψ_stem) and maximum daily trunk shrinkage (MDS) in irrigation scheduling requires the definition of a reference or threshold value, beyond which irrigation is necessary. These reference values are generally obtained by comparing the seasonal variation of plant water status with the environmental conditions under non-limiting soil water availability. In the present study an alternative approach is presented based on the plant’s response to water deficit. A drought experiment was carried out on two apple cultivars (Malus domestica Borkh. ‘Mutsu’ and ‘Cox Orange’) in which both indicators (Ψ_stem and MDS) were related to several plant physiological responses. Sap flow rates, maximum net photosynthesis rates and daily radial stem growth (DRSG) (derived from continuous stem diameter variation measurements) were considered in the assessment of the approach. Depending on the chosen plant response in relationship with Ψ_stem or MDS, the obtained reference values varied between −1.04 and −1.46 MPa for Ψ_stem and between 0.17 and 0.28 mm for MDS. In both cultivars, the approach based on maximum photosynthesis rates resulted in less negative Ψ_stem values and smaller MDS values, compared to the approaches with sap flow and daily radial stem growth. In the well-irrigated apple trees, day-to-day variations in midday Ψ_stem and MDS were related to the evaporative demand. These variations were more substantial for MDS than for midday Ψ_stem.     

The relative sensitivity to plant water stress during the reproductive phase of upland cotton (Gossypium hirsutum L.)

Summary The relative sensitivity of the cotton plant to water stress throughout the growing season was determined to identify when irrigation will have the greatest beneficial effect. Daily plant water stress for 72 different data sets of water applications was correlated to corresponding yield criteria. The magnitude of significant correlation was interpreted as the degree of sensitivity. Plant water stress during square formation and early flowering resulted in fewer bolls to reach maturity, but this detrimental effect was cancelled by the development of bigger bolls due to greater lint growth. This resulted in better seedcotton and lint yields. Flower and boll senescence which resulted from water stress during flowering peak, however, inhibited seedcotton and lint yields. The most pronounced inhibiting effect stress had on yields, was during boll development well after the end of effective flowering, when it inhibited boll development. Stress during ripening of the bolls increased lint and boll development and consequently enhanced yields. When limited quantities of water is available, preference should be given to irrigation during boll development, then by irrigation when the first flowers appear, followed by irrigation during peak flowering. Water should be withheld from opening of the first bolls.     

Regulated deficit irrigation in almonds: effects of variations in applied water and stress timing on yield and yield components

The impacts of three different water stress-timing patterns for three levels of seasonal applied water on production were evaluated in mature almond trees [Prunus dulcis (Mill.) Webb cv. Nonpareil] grown under high-evaporative demand conditions in the southern San Joaquin Valley of California. The stress timing patterns involved biasing water deficits to the pre-harvest or post-harvest periods in addition to uniform deficit irrigation for the entire season, referred to as A–C patterns. The three levels of water availability were 55, 70, and 85% of potential seasonal evapotranspiration (ET_c) equivalent to 580, 720, and 860 mm of applied water per season, respectively. Treatments were imposed over four seasons. Predawn leaf water potential was used as the stress indicator and approached −4.0 MPa with the A pattern at the lowest applied water level and −3.5 MPa with the B pattern at the same irrigation level. For every level of applied water, kernel weight at harvest was significantly reduced in the A pattern relative to the B and C patterns. At harvest, the most severe reduction in kernel dry weight relative to the control (17%) occurred in 580A, while there were 11% reductions in 580B and 580C. At the 860 mm level, only the A pattern dry kernel weight was less than the control. Moreover, the A patterns for all irrigation levels had lower kernel percentages than for the B and C patterns, indicating the greater sensitivity of kernel growth relative to shell growth in the regulated deficit irrigation (RDI) scenarios that biased the stress toward pre-harvest. The B stress patterns had a strong negative impact on fruit load relative to the A patterns at the 580 and 720 mm levels of applied water. No differences in crop load relative to the control were observed among the A and C regimes for all three levels of applied water. Nut load tended to increase during the experiment with 580A and 720A while it decreased with time with the B patterns for the same irrigation levels. We believe that the lower fruit loads involve stress during flower bud differentiation, which occurs mid-August–September in this cultivar and location, quite late in the season relative to other fruit and nut crops. The most successful stress timing pattern in terms of yield (the integrator of fruit size and load) was C, which avoided the large swings in tree stress observed with A and B. The onset of hull splitting was delayed by the severe pre-harvest stress in 580A while being accelerated by the milder stress of 720A. Spider mite levels were unaffected by the RDI. Canopy size was reduced with the A patterns at all irrigation levels. This occurred without any concomitant reduction in fruit load, resulting in higher fruiting densities (305 and 283 nuts/m² of orchard floor shaded area in 580A and 720A, respectively, vs. 214 nuts/m² in the control). Coupling the higher fruiting densities and smaller canopy sizes with higher tree planting densities offers growers the possibility of increasing yields while consuming less water. Maintaining more compact canopies with RDI rather than pruning would also lessen the amount of wood requiring disposal, thereby moderating air quality degradation resulting from burning. It must be emphasized that the scenario we outline—increasing kernel yields while using less water due to stress-related higher fruiting densities—requires that the smaller canopies be maintained by RDI, not pruning.     

Effects of different emitter space and water stress on yield and quality of processing tomato under semi-arid climate conditions

The objective of the study was to determine the effects of different emitter spaces and water stress on crop yield, such that the tomatoes would be suitable for processing and paste output (Lycopersicon esculentum Mill cv. Shasta). Such variables were also analyzed with respect to crop quality characteristics (e.g., mean fruit weight - MFW, fruit diameter - FD, penetration value of fruit - PV, pH, total soluble solids - TSS, and ascorbic acid contents - AA). The experiment was conducted under ecological conditions typical of the Konya Plain, a semi-arid climate, in 2004 and 2005. Drip irrigation laterals were arranged in such a way that every row had one lateral. Emitters were spaced at 25, 50, and 75 cm intervals in the main plots, while four levels of water supply, irrigation at 7-day intervals with enough water to fill the soil depth of 0-60 cm until capacity was reached (I₁), and 25, 50, and 75% decreased water supply levels were applied as subplots of the experiment. Results of the field experiments showed that yield suitable for processing (68.7-72.7 t ha⁻¹) and paste output (12.2-12.9 t ha⁻¹) were obtainable under conditions of I₁ application (p < 0.01). MFW, FD, PV, and TSS were significantly affected from treatments (p < 0.05). High stress resulted in the highest soluble solids. The total irrigation water amount and water consumptive use of the mentioned application (I₁) were determined as 426 and 525 mm in 2004. In 2005, the total irrigation water amount and water consumptive use of the same treatment were 587 and 619 mm, respectively.     

Seasonal sensitivity of stem water potential to vapour pressure deficit in grapevine

Boundary lines of stem water potential (Ψ_stem) responses to vapour pressure deficit (VPD) have been reported for several species and are generally referred to as VPD reference lines (RL). In order to study the response of Ψ_stem to VPD, RLs were determined in plants that received full (Control) and deficit (SSDI) irrigation during three consecutive years. The Control plants received irrigation equivalent to full crop water evapotranspiration minus effective rainfall and the SSDI plants were irrigated at 50% of the Control level. Ψ_stem values for the Control treatment during crop development tended to decrease, and those corresponding to SSDI plants were always lower than those for Control plants. Considering the 3-year data set, no relationship was found between Ψ_stem and VPD. However, there was a differential seasonal response between Ψ_stem and VPD, which depended on the stage of fruit development. A separate phenological analysis enabled the detection of RL for stage II (from fruit-set to veraison) and for stage III (post-veraison), whereas during stage I (from bud-break to fruit set) RL was not apparent. RL slopes increased as the season progressed and were significantly correlated to average values of Ψ_stem. The seasonal decrease in midday Ψ_stem for Control plants was interpreted as being a result of a progressive increase in canopy size and water consumption, which led to increased water depletion before each afternoon’s daily irrigation event. The apparent lack of RL during stage I was related to lower levels of water demand and high Ψ_stem.     

Crop water stress index is a sensitive water stress indicator in pistachio trees

Regulated deficit irrigation (RDI) strategies, often applied in tree crops, require precise monitoring methods of water stress. Crop water stress index (CWSI), based on canopy temperature measurements, has shown to be a good indicator of water deficits in field crops but has seldom been used in trees. CWSI was measured on a continuous basis in a Central California mature pistachio orchard, under full and deficit irrigation. Two treatments—control, returning the full evapotranspiration (ET_c) and RDI—irrigated with 40% ET_c during stage 2 of fruit grow (shell hardening). During stage 2, the canopy temperature—measured continuously with infrared thermometers (IRT)—of the RDI treatment was consistently higher than the control during the hours of active transpiration; the difference decreasing after irrigation. The non-water-stressed baseline (NWSB), obtained from clear-sky days canopy–air temperature differential and vapour pressure deficit (VPD) in the control treatment, showed a marked diurnal variation in the intercept, mainly explained by the variation in solar radiation. In contrast, the NWSB slope remained practically constant along the day. Diurnal evolution of calculated CWSI was stable and near zero in the control, but showed a clear rising diurnal trend in the RDI treatment, increasing as water stress increased around midday. The seasonal evolution of the CWSI detected large treatment differences throughout the RDI stress period. While the CWSI in the well-irrigated treatment rarely exceeded 0.2 throughout the season, RDI reached values of 0.8–0.9 near the end of the stress period. The CWSI responded to irrigation events along the whole season, and clearly detected mild water stress, suggesting extreme sensitivity to variations in tree water status. It correlated well with midday leaf water potential (LWP), but was more sensitive than LWP at mild stress levels. We conclude that the CWSI, obtained from continuous nadir-view measurements with IRTs, is a good and very sensitive indicator of water stress in pistachio. We recommend the use of canopy temperature measurements taken from 1200 to 1500 h, together with the following equation for the NWSB: (T _c − T _a) = −1.33·VPD + 2.44. Measurements of canopy temperature with VPD < 2 kPa are likely to generate significant errors in the CWSI calculation and should be avoided.     

Response of cotton water stress indicators to soil salinity

Summary A field study was conducted on cotton (Gossypium hirsutum L. c.v. Acala SJ-2) to investigate the effects of soil salinity on the responses of stress indices derived from canopy temperature, leaf diffusion resistance and leaf water potential. The four salinity treatments used in this study were obtained by mixtures of aqueduct and well water to provide mean soil water electrical conductivities of 17, 27, 32 and 38 dS/m in the upper 0.6 m of soil profile. The study was conducted on a sandy loam saline-alkali soil in the lower San Joaquin Valley of California on 30 July 1981, when the soil profile was adequately irrigated to remove any interference of soil matric potential on the stress measurements. Measurements of canopy temperature, leaf water potential and leaf diffusion resistance were made hourly throughout the day.Crop water stress index (CWSI) estimates derived from canopy temperature measurements in the least saline treatment had values similar to those found for cotton grown under minimum salinity profiles. Throughout the course of the day the treatments affected CWSI values with the maximum differences occurring in mid-afternoon. Salinity induced differences were also evident in the leaf diffusion resistance and leaf water potential measurements. Vapor pressure deficit was found to indicate the evaporative demand at which cotton could maintain potential water use for the various soil salinity levels studied. At vapor pressure deficits greater than 5 kPa, cotton would appear stressed at in situ soil water electrical conductivities exceeding 15 dS/m. The CWSI was as sensitive to osmotic stress as other, more traditional plant measures, provided a broader spatial resolution and appeared to be a practical tool for assessing osmotic stress occurring within irrigated cotton fields.     

Simulation of possible adaptive mechanisms in crops subjected to water stress

Summary The dynamics of stress development in crops involves a decrease in turgor potential of leaves which causes decreases in leaf expansion, photosynthesis, and transpiration. A study was conducted to evaluate the effectiveness of three possible adaptive mechanisms in maintaining turgor potential and growth. These mechanisms — osmotic adjustment, increased root growth, and increased stomatal resistance at full turgor — were examined by a simulation experiment using a dynamic model of the soil-crop-atmospheric system. Osmotic adjustment was found to be ineffective in maintaining turgor for crops grown in a sandy soil because of the rapid development of stress. When a ten-day drying cycle was simulated for a clay soil, cumulative transpiration, photosynthesis and growth were increased by osmotic adjustment, indicating an improved ability of the crop to maintain turgor under the simulated conditions for the clay soil. increased stomatal resistance was ineffective for the simulated conditions because of a concomitant decrease in photosynthetic rate. Increased soil volume occupied by roots was found to be the most effective adaptive mechanism for maintaining turgor, transpiration, photosynthesis and growth of crops in both soil types.Contribution from Department of Agricultural Engineering, Institute of Food and Agricultural Sciences, University of Florida. Supported in part by Grant No. FL-AGO-1911 Cooperative Research, United States Department of Agriculture. Florida Agricultural Experiment Station Journal No. 2657     

Strategies to decrease water drainage and nitrate emission from soilless cultures of greenhouse tomato

In the spring-summer season of 2005 and 2006, we explored the influence of three fertigation strategies (A-C) on the water and nitrogen use efficiency of semi-closed rockwool culture of greenhouse tomato conducted using saline water (NaCl concentration of 9.5 mol m⁻³). The strategies under comparison were the following: (A) crop water uptake was compensated by refilling the mixing tank with nutrient solution at full strength (with the concentrations of macronutrients equal or close to the corresponding mean uptake concentrations as determined in previous studies) and the recirculating nutrient solution was flushed out whenever its electrical conductivity (EC) surpassed 4.5 dS m⁻¹ due to the accumulation of NaCl; (B) the refill nutrient solution had a variable EC in order to maintain a target value of 3.0 dS m⁻¹; due to the progressive accumulation of NaCl, the EC and macronutrient concentrations of the refill nutrient solution tended to decrease with time, thus resulting in a progressive nutrient depletion in the recycling water till N-NO₃⁻ content dropped below 1.0 mol m⁻³, when the nutrient solution was replaced; (C) likewise Strategy A, but when EC reached 4.5 dS m⁻¹, crop water uptake was compensated with fresh water only in order to reduce N-NO₃⁻ concentration below 1.0 mol m⁻³ before discharge. In 2005 an open (free-drain) system (Strategy D), where the plants were irrigated with full-strength nutrient solution without drainage water recycling, was also tested in order to verify the possible influence of NaCl accumulation and/or nutrient depletion in the root zone on crop performance. In the semi-closed systems conducted following strategies A, B or C, the nutrient solution was replaced, respectively, 10, 14 and 7 times in 2005, and in 19, 24 and 14 times in 2006, when the cultivation lasted 167 days instead of 84 days in 2005. In both years, there were no important differences in fruit yield and quality among the strategies under investigation. Strategy C produced the best results in terms of water use and drainage, while Strategy B was the most efficient procedure with regard to nitrogen use. In contrast to strategies A and D, the application of strategies B and C minimized nitrogen emissions and also resulted in N-NO₃⁻ concentrations in the effluents that were invariably lower than the limit (approximately 1.42 mol m⁻³) imposed to the N-NO₃⁻ concentration of wastewater discharged into surface water by the current legislation associated to the implementation of European Nitrate Directive in Italy.     

A historical perspective on measurement of soil and plant water status and plant water status

Summary Surrounded by many sophisticated instruments, data acquisition systems and connecting computers, contemporary soil and plant scientists may not fully appreciate what really was known in earlier times and how the science has developed. A short review and a few examples of the growth of measurements and ideas are presented here.