Effects of Exposure to Short Periods of Suboptimal Temperature during Chill (5 °C) on Gas Exchange and Chlorophyll Fluorescence in Maize Seedlings (Zea Mays L.)

The effect of various periods of exposure to suboptimal temperature ('warm breaks'– WB: 14 °C for 4 h, 1 h and 0 h – control in 24 h cycles) during chilling (5 °C) of maize seedlings on the photosynthesis, the photochemical efficiency of photosystem II (Fv/Fm) and on the injuries of the cell membranes of leaves and water content in plants was compared. The measurements were conducted after 1, 3, 7 and 12 chill cycles. It was found that WB of either length distinctly diminished the chill-induced inhibition of net photosynthesis and the decrease of photochemical efficiency of PSII. The protective effect on WB on these parameters was observed shortly after completion of chilling of the plant as well as an after-effect. Daily warming up of the plants also reduced the leakage of electrolytes and diminished the water deficit of the chilled seedlings. The protective effect of WB on the measured parameters of the plants was greater in the chill-sensitive genotype than in the chill-tolerant one, especially when plants were warmed up for 4 h. The results obtained are an indication that short periods of warm weather during cold spring may diminish the injuries of the photosynthetic apparatus, as well as reduce the disturbance of water status of seedlings, contributing in this way to better condition of maize crops.     

The Effect of Short Warm Breaks during Chilling on Photosynthesis and the Activity of Antioxidant Enzymes in Plants Sensitive to Chilling

The effect of short warm breaks (from 15 min to 5 h) during chilling of three chilling-sensitive species (tomato, maize and soybean) was investigated. Injuries, intensity of net photosynthesis and antioxidant enzyme activity were measured. Throughout chilling treatment, plants were warmed by transferring them during the last few hours of the light phase from chilling temperature (5 °C for tomato and maize, 2 °C for soybean) to 20 °C. After warming, seedlings were moved back to chilling conditions. Warm breaks of 5 h almost entirely prevented the appearance of injuries, as measured by changes in leakage of electrolytes and tissue water content, during 12 days of chilling. Even a 15-min warm break ensured a significant decrease in injuries in chilled maize seedlings compared to continuously chilled seedlings. Inhibition of gas exchange and fluorescence in seedlings of two maize genotypes differing in chilling resistance was, to a small extent, prevented by 1-h warm breaks, while 4-h warm breaks reduced inhibition significantly. The length of the warm break (1 or 4 h) had no influence on changes in SOD activity compared to continuously chilled plants, but warm breaks of 4 h produced a significant increase in CAT activity. The possible influence of an alternative pathway in preventing injuries is discussed.     

Effect of Temperature and Photoperiod on the Development of Lupinus albus L. in a Controlled Environment

An experiment to determine the effect of temperature and photoperiod on Lupinus albus under controlled environmental conditions was carried out, using the three Lupinus albus genotypes 'Tifwhite', 'Esta' and 'Kiev', and three temperature (10/20, 18/28 and 20 °C continuously) and two photoperiod (8 and 16 h daylength) regimes, in all combinations. Half of the seeds were vernalized for 21 days at 4 °C to alleviate the obligate vernalization requirement of Tifwhite. Although Esta and Kiev do not have obligate vernalization requirements, they were influenced by this vernalization period. Observations included the duration of the period from planting to seedling emergence, the duration of the period from planting to the beginning of flowering and the duration of flowering. The vernalization treatment accelerated plant development in all genotypes. The period from planting to emergence was shorter under the higher temperature regime. For all genotypes, the period from planting to flowering was shorter under the longer photoperiod, the same trend as would be expected for long-day plants. Duration of flowering periods were, in contrast to pre-flowering periods, shorter for all genotypes at cooler temperatures. The results of this study confirm that photoperiod does contribute to the growth period from planting to flowering in L. albus and that this species does behave as a long-day plant.     

Chilling Tolerance of 10 Maize Genotypes as Related to Chilling-induced Changes in ACC and MACC Contents

We studied chilling-induced changes of 1-aminocydopropane-1-carboxylic acid (ACC) and of 1-(malonylamino) cyclopropane-1-carboxylic acid (MACC) contents in seedlings of ten maize genotypes with different chilling tolerance. Seedlings at the third leaf stage were chilled at 5°C and at 65% RH. Immediately before and after two and five days of chilling the contents of ACC and of MACC in the third leaf were measured. Water content and – after recovery – the degree of necrotic injuries and the percentage of seedling survival were also determined.
After 2 days of chilling, the ACC content increased in all genotypes investigated. The increase was significantly higher in the sensitive genotypes than in the tolerant ones. There was a significant correlation between ACC content and necrotic injuries of seedlings. Chilling for 5 days increased the ACC content further and the difference between the two groups of genotypes still existed.
The MACC content increased after 5 days of chilling in all genotypes investigated. The increase was greater in the tolerant genotypes than in the sensitive ones. However, the difference in MACC accumulation between the two groups of genotypes investigated was not significant, and thus no correlation between MACC accumulation and chilling susceptibility was found.
The possible causes for the increase of ACC and MACC contents under chilling conditions and the possibility of using the ACC content as an indicator of chilling tolerance in maize breeding are discussed.     

Coronatine Enhances Chilling Tolerance in Cucumber (Cucumis sativus L.) Seedlings by Improving the Antioxidative Defence System

Coronatine (COR) is a new plant growth regulator that mimics the biological activities of methyl jasmonate. We determined whether COR enhanced chilling tolerance of cucumber ( Cucumis sativus L. cv. Jinchun 4) seedlings and if such tolerance was correlated with changes in the activity of antioxidant enzymes. COR was applied to seedlings at two-leaf stage at 0 (Control), 0.1, 1, 10, and 100 n m . Seedlings were then subjected to chilling stress at 5 ± 1 °C for 4 days. Seedlings treated with COR showed significant higher tolerance to chilling stress and the optimal concentration was 1–10 n m . Compared with control, the chilling injury index (CII) of the seedlings treated with COR at 1 and 10 n m was decreased by 44.9 % and 24.5 %, respectively, while the membrane chilling stability (MCS) expressed as the change of relative conductance was increased by 37.2 % and 17.0 %, respectively. The malondialdehyde (MDA) content in leaves treated with COR at 1 n m was decreased by 39.7 %, and the O₂⁻ production rate and H₂O₂ content reduced by 28.6 % and 8.5 %, respectively. Treatment with COR at 1 n m increased the activities of superoxide dismutase (SOD) in leaves by 34.4 %, catalase (CAT) by 58.7 % and ascorbate peroxidase (APX) by 23.0 % under low temperature. The 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activities were also significantly improved by 28.9 % and 21.3 % following treatment with COR at 1 and 10 n m , respectively. The overall results suggest that COR enhanced chilling tolerance in cucumber seedlings by improving the antioxidative defence system.     

The Effect of Cold and Sucrose Concentration on its Uptake and on the Respiration Rate of Isolated Embryos of Winter Wheat

Both at 5°C and 20°C the sucrose concentrations between 8–14 % appear to be optimal from the point of view of dry mass accumulation by the seedling. The metabolic efficiency was also the highest for these concentrations of sucrose.
At both temperatures the hydration of the seedlings was strongly dependent on sucrose concentration in the nutrient, diminishing, respectively, from about 6 to about 2 and from about 10 to about 2 mg H²O × mg DW⁻¹ Such low values of the seedling hydration, however, caused only a 10–20 % decrease of their dry weight. Within the range of the applied sucrose concentration (2–20 %) its beneficial – trophic interaction prevails over the negative – osmotic interaction.
At 5°C, as compared with 20°C, a drop in the seedlings hydration was observed, which appears to be the general response of plants to low temperature of growth.
An analysis of the 24-hour respiration rate of the seedlings when grown at 5°C seems to reveal the occurrence of a process requiring additional metabolic energy and, probably, associated with the vernalization process.
A method of leveling the temperature dependent hydration of the seedling is proposed to enable the selection of high-temperature control in the investigations of the biochemical mechanism of frost hardening and vernalization.     

Electrolyte Leakage, ATP Content in Leaves and Intensity of Net Photosynthesis in Maize Seedlings at Permanent or Different Daily Exposure to Low Temperatures

Changes were determined in electrolyte leakage (EL), ATP content in leaves and intensity of net photosynthesis (FJ in maize seedlings as a result of continued exposure to low temperatures for several days or, after a cold night (5 °C, 9 h), alternating application of 5 °C and 20 °C temperatures during the lit phase of the day (h of chilling during day/night: 0/9, 5/9, 10/9, 15/9).
At continuous exposure to low temperatures, permanent reduction of ATP content occurred between days 7 and 10, while EL increased significantly between days 4 to 7 (depending on experimental conditions). Of three leaves differing in age, most injuries were found in the oldest leaf. An increased intensity of irradiation from 150 to 500 μE. m⁻².s⁻¹ caused an earlier decrease in the ATP content and a further increase in electrolyte leakage. ATP content showed a 24 h rhythm, it increased at the end of the night and decreased at the end of the day. The rhythm was particularly noticeable in the control plants (20 °C), as well as, however to a lesser extent, in plants exposed to daily fluctuating chilling temperatures. It has also been observed that higher temperatures occurring in day time may diminish electrolyte leakage induced by previous low temperatures
The intensity of F_n determined at 20 °C, 4 h after withdrawal of chilling, decreased with the prolongation of the chilling period during the day.
Thus, it may be assumed that the EL and F_n values as indicators of the plants' sensitivity are in better agreement with the induced chill doses than the ATP content in leaves, which to a greater extent depends on the interaction between chilling and other experimental factors, such as irradiation intensity or the sequence of light and darkness.     

Changes in Leaf Water Relations and Injuries in Maize Seedlings Induced by Different Chilling Conditions

The time course of injuries and electrolyte leakage as well as changes in leaf water content and relative turgidity during chilling at 5°C, 65 or 100 % relative humidity (RH) were determined for seedlings of two maize hybrids differing in chilling tolerance.
In the course of chilling of entire seedlings the statistically significant injuries and increased leakage of electrolytes occurred first, followed by significant water deficit in leaves which was independent of RH and genotype. However, during prolonged chilling high and significant correlation between the level of chilling-induced water stress and the extent of injuries was observed. Thus it has been assumed that the main factor responsible for primary chilling injuries in maize seedlings is the immediate influence of chilling temperatures on the cell membranes, and not the secondary water stress. The level of the water stress and the extent of the dehydration injuries under chilling conditions depended on the chilling sensitivity of the genotype. Differences between the examined genotypes consisted not only in delayed chilling injuries in a chilling tolerant hybrid, but also in significantly lesser effects of these injures on water relations.
However, exposure to low temperature of the aboveground part or of the roots of the seedlings only, caused first the chilling-induced marked water deficit in the leaves and next increased leakage of electrolytes. A direct effect of chilling temperature on the membranes in a situation when only a part of the seedling is being chilled, may be less harmful to the plant as a whole owing to normal functioning of the non-chilled part.     

Organic Recycling for Soil Quality Conservation in a Sub-tropical Plateau Region

The long-term effect of organic recycling on some aspects of quality in a lowland rice soil of an Indian plateau region was studied. The experiment was set up at the agricultural experimental farm of the Indian Statistical Institute, Giridih, Bihar, India. Two rice cultivars, and treatments with four organic supplements (cowdung manure, Leuceana leaves, decomposed farm residue and Sesbania ), chemical fertilizers (urea, superphosphate and muriate of potash) and no input were arranged in a factorial randomized block design. Organic supplements improved soil quality parameters such as water holding capacity, total organic C, microbial biomass C, urease and acid phosphatase activities of soils in comparison to chemical fertilizers and no input. Among the organic supplements, cowdung manure gave significantly higher organic C (1.39%), microbial biomass C (276.46 μg g⁻¹ dry soil), urease activity (32.79 and 21.22 μg urea hydrolized g⁻¹ dry soil h⁻¹ at 37 °C by the buffer and non-buffer method, respectively) and acid phosphatase activity (1.99 μmol p-nitrophenol released g⁻¹ dry soil h⁻¹ at 37 °C) than the others. The conversion of organic C into biomass C (2.46%) was highest in Leuceana -treated soil.     

Osmotically Primed Seed and Seedling Reactions to Variations in Day/Night Temperature

The effects of alternating day/night temperatures, varying in maxima and minima but all averaging 30 °C, on germination and seedling characteristics of Sorghum bicolor L. (Moench) were investigated. Seeds of sorghum CSV 15 were soaked in 2, 4 or 6 g NaCl l⁻¹ solutions for 2 days at 25 °C, soaked in water for 1 day at 25 °C, or untreated. After treatment, drying and storage, seeds were germinated at 30/30 (day/night), 35/25, 40/20 or 41/19 °C temperature regimes under a polyethylene glycol-induced drought level of −3 bar. Results revealed no advancement of germination percentage but a partial increase in germination speed by osmotic seed treatments. All three osmotic treatments also reduced the plumule/radicle ratio owing to a speculated increase in root over shoot growth. Temperature regimes significantly influenced both germination and seedling characteristics with the optimum temperature appearing to be 35/25 °C. Increasing the temperature amplitude from 30/30 to 35/25 °C increased germination speed, which was reduced by incremental increases in temperature amplitude to 40/20 or 41/19 °C.     

Effects of Low Temperature Treatment at Seedling Stage on Soybean Growth, Development and Final Yield

The after effect of continuous and variable chilling temperatures acting periodically for 7 days at seedling stage (day/night: 2/2°C, 5/5°C, 15/2°C, 15/5°C, 15/15°C and control–22/18°C) on growth, development and final yield of soybean cultivar Polan and Progres were investigated. The temperature decrease lengthened the duration of the vegetation period before flowering for both cultivars, which allowed compensation for the reduced–as an effect of chilling–rate of daily increments in the leaf area and the dry weight of the above-ground parts of the plants. The most important symptom of chilling treatment was a considerable increase in the number of axillary branches after flowering, which caused an increase in the dry weight of plants. The subsequent reaction of the growth of leaves to chilling at seedling stage was, after flowering, variety dependent. The area of leaves bigger at Polan or similar to control at Progres ensured good seed filling in an increased number of pods developed on axillary branches. Chill-induced delay in plants development allowed the avoidance of low temperature occurring during flowering in this experiment. This chilling during flowering was one of the reasons of worse-pod setting and reduction of seeds per pod number at control plants.     

Nitrogen Nutrition of Potato Cultivars Differing in Maturity

In potato, dry matter (DM) production and partitioning between plant organs and N accumulation are affected by N application; however, since cultivars differ in these processes, N fertilization must be adjusted to each cultivar. This paper studies the response of potato cultivars differing in maturity to N fertilization in the south-east of the Buenos Aires Province (37°45'S, 58°18'W) in two growing seasons. Treatments combined four N doses (0–180 kg ha⁻¹) and four cultivars: Jaerla (early), Spunta (mid-early), Mailén INTA (medium late) and Huinkul MAG (late). DM and N content were measured in leaves, stems and tubers throughout the growing season and intercepted photosynthetically active radiation was regularly assessed. There was an increase in tuber yield up to intermediate N doses (60 kg ha⁻¹ in 1990 and 120 kg ha⁻¹ in 1991). Tuber yield was similar for Spunta and Huinkul MAG. There was no interaction between cultivar and N fertilization for tuber dry matter yield. DM partitioning to leaves and tubers during the growing season differed among cultivars, but N availability affected partitioning similarly for all cultivars. Jaerla had a high and Huinkul MAG had a low radiation use efficiency between plant emergence and the beginning of tuber formation. Jaerla, Spunta and Mailén INTA reached maximum N content in foliage at ≈60 days after emergence and Huinkul MAG 20 days later. Total N content at maturity varied between 120 and 250 kg ha⁻¹ and was affected by cultivar and N dose. The results will help to improve N fertilization recommendations and management practices as related to each cultivar under the environmental conditions of this region.     

Chilling Stress Accelerates Degradation of Seed Storage Protein and Photosynthetic Protein during Cotton Seed Germination

Low temperature is one of the major abiotic stresses limiting the productivity and geographical distribution of many important crops. To gain a better understanding of chilling stress responses in cotton ( Gossypium hirsutum L . ), we carried out a comparative proteomic analysis. Cotyledon proteins of 1-week-old cotton seedlings treated with or without chilling treatment at 4 °C were extracted, separated by a two-dimensional gel electrophoresis and compared. Among 1500 protein spots reproducibly detected on each gel, 25 protein spots were down-regulated and 29 were up-regulated. Seven cotton proteins were identified by mass spectrometry analysis as beta-globulin A precursor fragments. One was identified as a Rubisco large subunit fragment, providing evidence of both seed storage protein and photosynthetic protein destruction by chilling stress. The related physiological and biochemical analysis showed that there was a significant increase in endopeptidase activity and activated oxygen generation rate, and an obvious decrease in carboxylation efficiency and maximum net photosynthetic rate of cotton seedlings under chilling stress. Based on the above results, the degradation mechanisms of beta-globulin and Rubisco under chilling stress were discussed. In conclusion, our study provides new insights into chilling-stress responses in cotton and demonstrates the advantages of proteomic analysis.     

Effects of irradiance and juvenility on the selection of chrysanthemums

Summary Chrysanthemum morifolium seedlings and cuttings from those seedlings were grown to flowering at 17°C, short days (8 h) and two levels of irradiance (10 and 30 W m^–2). The cuttings flowered more readily than the seedlings at both irradiances, Especially at 10 W m^-2 many seedlings remained vegetative. The delay in flowering of the seedlings was accompanied by a higher number of leaves produced before flowering.The number of leaves produced by the seedling in excess of the number of leaves produced by the cutting from that seedling varied considerably between genotypes. The correlations between seedlings and the corresponding cuttings for days to flowering and number of leaves were low but generally significant.It is concluded that cuttings are preferred over seedlings in breeding programmes that aim at developing chrysanthemums for low light winter production.     

Seedling Development of Maize Genotypes at Constant and at Fluctuating Temperatures

Four maize genotypes from North West Europe, tropical highlands and tropical lowlands were grown at constant temperatures (14°C; 24°C), diurnal temperature variations (18°/10°C; 28°/20°C) and at temperatures alternating every three days (18°+ 10°C; 28°+ 20°C) until the three leaves stage. Fluctuating temperatures were generally favourable for growth with most marked effects at an average of 14°C when 18°/10°C were applied. Temperature situation had pronounced effects on morphological and biochemical traits like area of second leaves and activity of phosphofructokinase but between these traits and growth rates no close relationship existed. For all traits genotype specific reaction patterns to temperature situation existed. One tropical highland variety excelled by fast growth, especially at low fluctuating temperatures. A specific chilling susceptibility of the tropical lowland variety was only marked for growth at constant 14°C.     

海水养殖与气象Ⅱ——惠东石斑鱼寒害对气候变暖的响应

冬季低温常给水产养殖造成严重损失,制约了渔业生产发展。本研究揭示了石斑鱼寒害变化特征及其对气候变暖响应规律,分析结果可以指导石斑鱼养殖确定合理的放养时段、评估寒害风险,同时为水产养殖业及政府部门制定防寒减灾决策提供科学依据。利用惠东2011年海水养殖试验点冬季逐日海温及对应气温资料,采用逐步回归分析构建了冬季海温预报方程,代入惠东气象站逐日气象资料得到对应1971—2012年冬季逐日海温资料,根据试验测得的石斑鱼寒害指标,分析了惠东石斑鱼养殖冬季寒害变化特征。结果表明：近42年来惠东地区石斑鱼冬季寒害日数略有减少,大石斑鱼寒害日数多于中石斑鱼,大石斑鱼寒害发生逐日概率有一个升降的变化过程,高发时段为1月1日—2月15日。在未来气候变暖水温升高1℃的情景下,石斑鱼冬季寒害逐日概率显著降低,大石斑鱼平均冬季寒害发生时段显著缩短。平均初寒日期由变暖前的1月4日推迟到变暖后的1月15日,平均终寒日期由变暖前的2月7日提前到变暖后的2月3日。     

Effects of soil drought during the vegetative phase of seedling growth on the uptake of 14co2 and the accumulation and translocation of 14C in cultivars of field bean (vicia faba L. Var. Minor) and field pea (pisum sativum L.) Of different drought tolerance

During the vegetative phase of growth of two field bean and two field pea cultivars of different drought tolerance, the effect of short and prolonged soil drought on gas exchange (CO₂ i H₂O), leaf water potential (ψ), stomatal diffusive resistance (r_S), uptake of CO₂, and the distribution and accumulation of ¹⁴C was studied. Differences in the response to drought conditions between resistant and susceptible cultivars were marked. After 5 days of soil drought, the decrease in net photosynthesis and transpiration rate and the increase of stomatal resistance were greater in the drought-resistant cultivars than in the drought-susceptible ones. In contrast, after 10 days of drought the decrease of leaf P_N (CO₂ assimilation rate), E (rate of transpiration) and ψ (water potential) was greater in the susceptible cultivars than in the resistant ones. Significant differences between the resistant and the susceptible cultivars were also observed in the assimilation and translocation of ¹⁴C by the green parts of the plant. The amount of carbon accumulation in roots in drought-susceptible cultivars increased less than in the drought-resistant cultivars. For treatments in which optimal soil watering was resumed after 5 or 10 days of drought there was no evidence of effects of drought on the majority of measurements, but the drought-resistant cultivars showed a general tendency for a more rapid recovery. Our results confirm the existence of genetic variability in drought tolerance among the cultivars of field bean and field pea. The recorded differences in the response to drought of experimental cultivars may indicate that, under water deficit in the soil and in plant tissues, they may use different strategies to avoid the damaging effects of temporary limitation of water supply; for example, the drought-resistant cultivars may more effectively conserve tissue hydration through effective stomatal closure. Also, the observed changes in carbon assimilation and accumulation might be the reason for their different responses to drought. The change in radioactivity losses in the control and stressed plants may result from the differences in demand for energy to maintain cell structure and function. Similarly, the less intense carbon accumulation in the roots of the sensitive cultivars could be caused by more harmful effects of drought on root growth.     

叶绿素荧光动力学O-J-I-P参数在棉花幼苗耐冷性评价上的应用

低温冷害是影响棉花生长的主要逆境因子,如何建立简捷、有效的棉花耐冷性鉴定及筛选体系是棉花耐冷研究的关键问题。在本研究中,阐述了如何用O-J-I-P参数来评价棉花幼苗的耐冷性。选用北疆棉区22个主栽棉花品种,对盆栽幼苗经过连续4d的4℃低温处理,进行了相应的冷害指数鉴定,并基于O-J-I-P参数Fv/Fm和PI所建立的冷害因子指数1及冷害因子指数2对各个棉花幼苗做了耐冷鉴定和分级评价。结果发现,冷害因子指数1和冷害因子指数2的鉴定、评价结果与冷害指数鉴定结果相近。中棉所36、新陆早25、297-5幼苗具有较强的耐冷性,而新陆早10号、新陆早12号及炮台1号的耐冷性较弱。叶绿素荧光动力学O-J-I-P可以快速、有效地鉴定棉花幼苗的耐冷性,这对棉花的耐冷性鉴定和培育耐冷型的棉花品种具有重要的意义。     

Chilling Tolerance in Hybrid Maize Induced by Seed Priming with Salicylic Acid

The optimum temperature for maize germination is between 25 and 28 °C. Poor and erratic germination at suboptimal temperature is the most important hindrance in its early sowing. This study was conducted to induce chilling tolerance in hybrid maize (Zea mays L.) by seed priming with salicylic acid (SA) and to unravel the background biochemical basis. For seed priming, maize hybrid (Hycorn 8288) seeds were soaked in 50, 100 and 150 ppm (mg l^?1) aerated solutions of SA for 24 h and were dried back. Treated and untreated seeds were sown at 27 °C (optimal temperature) and at 15 °C (chilling stress) under controlled conditions. Performance of maize seedlings was hampered under chilling stress. But seed priming with SA improved the seedling emergence, root and shoot length, seedling fresh and dry weights, and leaf and root score considerably compared with control both at optimal and chilling temperatures. However, priming in 50 mg l^?1 SA solution was more effective, followed by priming in 100 mg l^?1 SA solution. Seed priming with SA improved the chilling tolerance in hybrid maize mainly by the activation of antioxidants (including catalase, superoxide dismutase and ascorbate peroxidase). Moreover, maintenance of high tissue water contents and reduced membrane permeability also contributed towards chilling tolerance.     

Effect of Secondary Salinization on Photosynthesis in Fodder Oat (Avena sativa L.) Genotypes

The effect of secondary salinization on photosynthesis was studied in fodder oat genotypes Kent, JHO-829, JHO-881, UPO-94 and OS-6 at the flower initiation stage. With an increase in the electrical conductivity (EC) of irrigation water, the net photosynthesis rate (P_N) and the transpiration rate (E) of all the genotypes decreased. The intercellular CO₂ concentration (Ci) increased in all genotypes at 10 dS m^−1. Stomatal resistance (R_s) had a strong negative correlation with PN and E. The increase in Ci together with the increase in the R_s shows that at higher EC non-stomatal factors also start contributing to the limitation of photosynthesis. This study suggests that secondary salinization effects are strongly under stomatal control at lower saline water irrigation levels, but at higher levels non-stomatal factors may come into play.