单粒精播密度对花生冠层结构及产量的影响

为探明密度对单粒精播花生群体冠层结构及产量的影响，大田条件下，以花育22为供试花生品种，设置 每穴单粒和双粒2种播种方式，9万穴/hm2（D1）、12万穴/hm2（D2）和15万穴/hm2（D3）3个种植密度，研究了密度对花 生冠层透光率、冠层叶面积系数、叶片干重及农艺性状的影响。结果表明，随密度的增加，冠层中下部的透光率降 低，单粒播冠层透光率大于双粒播。D1密度下，花生冠层中上部的叶面积系数、叶片干重单粒播低于双粒播，冠层 中下部高于双粒播，而D2和D3密度条件下，冠层叶面积系数、叶片干重单粒播与双粒播差异不显著。随密度的增 加，花生主茎高、侧枝长和公顷果数显著增加，而单穴果数减少，单粒精播侧枝数多于双粒播。随密度的增加，花生 荚果产量呈先升高后降低的趋势，12万穴/hm2的种植密度荚果产量最高，单粒精播花生产量略高于双粒播。总之， 合理密植是花生高产的重要措施，与双粒穴播相比，单粒精播花生冠层下部透光率和叶面积系数高，侧枝多、饱果 率高，是实现花生高产高效的重要措施。     

The control of stem numbers in potato competition experiments using either whole tubers or seed-pieces

Summary The growth and development patterns of the potato (Solanum tuberosum L.) cv. Maris Piper are compared for one- and four-eyed whole tubers, and four single-eyed seed-pieces of two different weights. In the multi-sprouted tubers some eyes, especially from the base of the tuber, failed to emerge due to the re-establishment of apical dominance after planting. Stems emerged first from eyes at the apex of the tuber, and had a larger leaf area per eye during the early harvests, than stems arising from other parts of the tuber. These differences diminished as the canopy expanded. The numbers and sizes of daughter tubers at the final harvest were similar for all four-eyed treatments. Single-eyed cores of 10 g are recommended for effective control of stem numbers with uniform canopy production.     

Tiller size/density compensation in perennial ryegrass miniature swards subject to differing defoliation heights and a proposed productivity index

The objective of this study was to investigate the effects of cutting height on tiller population density, tiller weight and herbage harvested in perennial ryegrass swards. One hundred mini-swards of Lolium perenne were established from seed on 5 May 1993 in 150 mm × 150 mm plastic pots in a greenhouse. The experimental period lasted 6 months (May to November). On 4 June all pots were cut to 50 mm and randomly allocated to five treatments (20, 40, 80, 120 and 160 mm sward surface height) with twenty replicates (four per tray). All pots were cut twice weekly to the specified surface height from 11 June, and recording began on 14 June. Every 4 weeks from 9 July to 5 November the twenty pots contained in one tray were withdrawn for destructive measurements of tiller population density and the weights of tiller components. Tiller population density increased with reduction in defoliation height, except for the 20-mm treatment where tiller density was initially restricted. The slope of the size/density compensation (SDC) line was close to ?5/2 over the range of defoliation heights 40–120 mm, but was less than ?1·0 between 120 and 160 mm. These slopes are consistent with a recent theory, which proposes that variations from a slope of ?3/2 will be linked to defoliation-induced changes in canopy leaf area and to change in tiller leaf area:volume ratio, R. At defoliation heights below 120 mm, SDC slope was increased by reduction in canopy leaf area. Above 120 mm, increase in R forced a relatively rapid tiller population decline, resulting in a reduced SDC slope and decrease in canopy leaf area. Whereas traditional measures of leafiness such as leaf:stem ratio or leaf:non-leaf ratio decreased with increasing height of defoliation, the leaf area:volume ratio, R, increased with defoliation height. Distance from an arbitrarily positioned ?3/2 SDC line was correlated with sward productivity.     

A simple non-destructive technique for healthy and diseased leaf area assessment of potato plants

Summary A method for assessing the leaf area of potato plants was based on the number of leaves coinciding with imaginary vertical lines descending from a horizontal grid of points above the plant or crop canopy. Leaves seen through each vertically aligned pair of holes in a perforated double table, covering the midpoint of the exposed area, were counted and moved aside one by one, until the ground was seen. For both green and diseased surfaces the leaf area, or its horizontal component, was calculated as the product of sum of records and the grid cell size. Tested against a destructive method, the technique worked equally well for healthy and diseased leaf area of potato plants infected byPhytophthora infestans. The theoretic estimate of the standard error for a single measurement was derived as the geometric mean of the leaf area and the grid cell size. The method is also suitable forLAI of crop stands.     

Effect of nitrogen supply on leaf appearance,leaf growth,leaf nitrogen economy and photosynthetic capacity in maize (Zea mays L.)

Leaf area growth and nitrogen concentration per unit leaf area, N_a (g m⁻² N) are two options plants can use to adapt to nitrogen limitation. Previous work indicated that potato (Solanum tuberosum L.) adapts the size of leaves to maintain N_a and photosynthetic capacity per unit leaf area. This paper reports on the effect of N limitation on leaf area production and photosynthetic capacity in maize, a C4 cereal. Maize was grown in two experiments in pots in glasshouses with three (0.84–6.0 g N pot⁻¹) and five rates (0.5–6.0 g pot⁻¹) of N. Leaf tip and ligule appearance were monitored and final individual leaf area was determined. Changes with leaf age in leaf area, leaf N content and light-saturated photosynthetic capacity, P_max, were measured on two leaves per plant in each experiment. The final area of the largest leaf and total plant leaf area differed by 16 and 29% from the lowest to highest N supply, but leaf appearance rate and the duration of leaf expansion were unaffected. The N concentration of expanding leaves (N_a or %N in dry matter) differed by at least a factor 2 from the lowest to highest N supply. A hyperbolic function described the relation between P_max and N_a. The results confirm the ‘maize strategy’: leaf N content, photosynthetic capacity, and ultimately radiation use efficiency is more sensitive to nitrogen limitation than are leaf area expansion and light interception. The generality of the findings is discussed and it is suggested that at canopy level species showing the ‘potato strategy’ can be recognized from little effect of nitrogen supply on radiation use efficiency, while the reverse is true for species showing the ‘maize strategy’ for adaptation to N limitation.     

The effects of nitrogen supply and stem density on leaf attributes and stem branching in potato (Solanum tuberosum L.)

Summary The interaction between nitrogen supply and stem density on leaf attributes and branching was studied in two field experiments. Stem densities included 8, 16, 24, 32 and 40 stems m⁻². The low-N treatments received 5 or 10 g m⁻² nitrogen, and the high-N treatments 20 or 25 g m⁻². The sizes of mature individual leaves on the main stem declined with increase in stem density and were larger for greater N supply, with additive effects of nitrogen and stem density. The specific leaf area was not affected by nitrogen supply and increased with stem density (range of values: 210 to 310 cm² g⁻¹). The leaf weight ratio was not affected by the treatments. At 8 and 16 stems m⁻² basal branches contributed substantially to the total leaf area per stem: at 24 stems m⁻² and higher densities the development of basal branches was negligible. Leaf area on apical branches was always smaller for higher plant densities and lower rates of N supply.     

Artificial Neural Network Modelling of Leaf Water Potential for Potatoes Using RGB Digital Images: A Greenhouse Study

Plant water status information of potato (Solanum tuberosum L. cv. Russet Burbank) is needed at the farm level for irrigation scheduling. This research investigated the feasibility of using a 5-megapixel digital camera to determine the leaf water potential (Ψ_L) of potato plants by capturing red, green, blue (RGB) digital images in the visible region of the electromagnetic spectrum. A greenhouse experiment was conducted in containerized cv. Russet Burbank potato plants subjected to five soil nitrate-nitrogen (N) levels and four soil water content levels. An artificial neural network (ANN) model, built with RGB images, RGB image transformations, RGB vegetation indices, and principal components analysis, found that for the validation data set, the measured Ψ_L and predicted Ψ_L results were from common populations. Other results showed: (1) a linear trend between soil nitrate-N levels and leaf reflectance in the G image band, (2) that the RG image bands were more suitable than the B image band for classifying leaf pigment from leaf shadow and leaf damage, (3) soil nitrate-N interacted with leaf greenness, affecting Ψ_L prediction, and (4) some image variables were more important than others in the ANN model. Although this greenhouse research shows promise, further field-based research is required to validate the selection of input neurons used and also validate the use of ANN modelling to determine Ψ_L at the plant canopy level with cv. Russet Burbank and other cultivars. In addition, an image acquisition method needs to be developed to obtain periodic representative sample coverage over a field.     

Alleviation of waterlogging damage in winter rape by application of uniconazole: Effects on morphological characteristics,hormones and photosynthesis

Oilseed rape seedlings (Brassica napus L.) treated with uniconazole were transplanted at the five-leaf stage into specially designed experimental containers, and then exposed to waterlogging for 3 weeks. Pre-treatment of rape seedlings with uniconazole significantly increased seedling height, shoot width, number of green leaves and leaf area per plant, and consequently the shoot, root, and total dry weight after waterlogging. Following waterlogging stress, the uniconazole treated plants had a significantly improved growth including plant height, length and width of leaves, number of green leaves, leaf area per plant, and canopy width at the stem elongation stage, and plant height, stem width, and root, shoot and total dry weight at the flowering stage. At harvest, uniconazole treatment increased the number of primary and secondary branches, seeds per pod, and number of effective pods in branches and in terminal raceme after waterlogging treatment. The uniconazole-induced increase in the number of pods per plant and number of seeds per pod were the two yield components responsible for the significantly greater seed and oil yields obtained from the uniconazole plus waterlogging treated plants, over either the control or waterlogged plants. Uniconazole also reduced waterlogging-induced rise in the erucic acid content of the seeds. The modification of GA₃, zeatin, ABA and ethylene levels due to pre-treatment of rape seedlings with uniconazole might have helped to delay the chlorosis and senescence induced by waterlogging. Uniconazole treatment also increased the leaf photosynthetic rates of waterlogged plants, in part, due to the changes in leaf conductance and hormone levels which ultimately affected various physiological processes.     

Morphology,accumulation and survival of Desmanthus under different planting densities and harvest heights

The use of legumes is an important strategy for animal feeding, especially during the dry season. The aim was to evaluate the effect of planting densities (40,000; 15,625 and 10,000 plants ha⁻¹) and harvest heights (20 and 40 cm) on the morphology, accumulation and survival of Desmanthus (Desmanthus pernambucanus [L.] Thellung), cultivated in subhumid tropical region, as well as to estimate the repeatability of the evaluated characteristics and the optimal number of measurements. The treatments were randomized in blocks, with subdivided plots and four replicates. Planting densities were evaluated in the plots and the harvest heights in the subplots. Eight harvests were carried out with an interval of 84 days. Morphological and productive characteristics and survival were evaluated. Cultivation under density of 40,000 plants ha⁻¹ produced taller plants, with higher leaf area index (LAI = 0.98), light interception (LI = 49%), individual accumulations (18.8 g DM plant⁻¹) and by area (576.5 kg DM ha⁻¹ harvest⁻¹), although resulting in reduced plant stand (66%). Harvest heights do not affect accumulation and survival. It is possible to reduce the number of measurements for stem diameter, number of leaflets per leaf (R² = 95%), plant height, canopy diameter, LAI, LI, leaf length and width, number of leaves per branch, branch diameter and LBR (R² = 90%), optimizing resources for future research. Desmanthus has potential for use in protein banks, being harvested in the rainy season, conserved and supplied in addition to animals, but plant growth is minimal during the dry season under rainfed conditions.     

Spread of potato virus S

Summary An experiment withEersteling (originally freed by meristem culture from PVX and PVS) andAlpha plants has been carried out in the field to study the spread of potato virus S. It was shown that depending on virus isolate, in a crop containing approximately 10% of plants with secondary infection, the primary infection inEersteling could rise to 56–76%, whereas that percentage inAlpha plants rose to 2–28 only. From the pattern of infected plants in was concluded that under field conditions in the Netherlands potato virus S is probably transmitted by contact.     

Effects of a short period of drought on changes in tuber volume and specific leaf weight. I. Long-term changes

Summary Single-stemmed potato plants, cv. Bintje, were grown on a nutrient solution under controlled conditions. They were exposed to a short drought stress, or remained untreated. Long-term measurements of tuber volume were carried out with a non-destructive volume-meter. Specific leaf weight was monitored with a β-gauge. Tubers from the same plant varied in rate and duration of growth. The relative growth of stressed tubers was 43–54% less than that of the control tubers. The hierarchy of tubers from one stem changed over time in plants that were exposed to the stress. Specific leaf weight declined during the stress period but there was a prompt recovery after the relief from drought. This response was similar to the response of the average rate of volume increase of the tuber.     

Plant growth strategies of Thymus vulgaris L. in response to population density

Thymus vulgaris L., known as common thyme, is a perennial subshrub native to the Mediterranean region. The use of the extracts and herb of thyme plants as antibacterial, antioxidant, natural food preservative and in cosmetic industry has increased the economic importance of this medicinal crop worldwide. Agricultural practices have an influence on growth and yield of thyme plants. Therefore, an experiment was undertaken to determine the effects of three intra-row spacing (15, 30 and 45 cm) and four harvesting times (vegetative, beginning of blooming, full blooming and fruit set) on plant growth and herbage biomass of thyme in 2007 in the research farm of Shoubak University College in the southern part of Jordan. The experiment was a split plot in randomized complete block design with four replicates. Apart from the experiment in 2007, measurements taken on the 15th of November in 2008 were treated as an independent experiment irrespective of harvesting stage. The effects of spacing were not significant for all the parameters measured in 2007. Intra-row spacing in 2008 had significant effects (P < 0.05) on plant height, canopy diameter, herbage fresh and dry weight and number of leaves on long shoots. Its effect on dry weight of shoots, fresh and dry weight of long shoots, number of long shoots, root depth, leaf area and specific leaf area was very significant (P < 0.01). Results of this study indicated that thyme plants grown using 15 cm planting space and harvested at the fruit set stage in 2007 had the highest fresh and dry weight of shoots. The highest dry weight of shoots, herbage fresh and dry weight was also obtained in 2008 in 15 cm spacing. Closer spacing resulted in significantly taller plants that exhibited higher shoot:root ratio and, therefore, plants grown using 15 cm intra-row spacing had a better use of light and accumulated more biomass compared to plants in wider spacing. The differentiation between long shoots and short shoots throughout plant development in 2007 suggests a specialization in carbon gain along different times in the growing season. Swings observed for shoot:root ratio, leaf area and specific leaf area were related to thyme growth. The decrease in specific leaf area between vegetative and full bloom stage indicated that leaves became thicker as part of an adaptive mechanism to the surrounding environment. Information obtained from growth parameters, leaf area and specific leaf area in particular, could be related to essential oil production in leaves of thyme plants.     

Reduction of leaf growth by water stress and its recovery in relation to transpiration and stomatal conductance in some potato (Solanum tuberosum L.) genotypes

Summary The effect of water stress (5 days of no watering) on leaf growth and its subsequent recovery after 5 days of watering was investigated in potted plants of eight potato (Solanum tuberosum L.) genotypes. The genotypes fell into three groups. Group A was characterized by minimum growth reduction under stress and rapid recovery on rewatering with final increase in leaf length exceeding that of the unstressed controls. In Group B, stress created moderate reduction in growth and on recovery the increase in leaf length became comparable to that of controls. Group C was characterized by large growth reduction and rewatering did not result in final leaf length increases comparable to that of controls. Statistical analysis showed a negative correlation between reduction in growth and stomatal conductance (r=−0.917;P<0.01) and between reduction in growth and transpiration (r=−0.912;P<0.01). CPRI Publication No. 812.     

Estimating potato leaf area index for specific cultivars

Summary The growth and duration of crop leaf area determines the amount of solar radiation intercepted by the canopy and therefore influences the extent of photosynthesis, evaporation, transpiration and final dry matter yield. The objective of this study was to develop cultivar specific relationships to estimate the daily leaf area index (LAI) for the potato crop (Solanum tuberosum L.) that included the effects of available soil water. The model is divided into three LAI growth stages, the durations of which are partially related to potato heat units (PHU). The LAI in the first stage is estimated from a cultivar specific leaf area-based radiation use efficiency index with a soil water reduction factor. The second stage involves the maintenance of a constant LAI with the duration related to both PHU and a soil water index that can accelerate senescence. The final stage includes a decrease in the LAI from a maximum to zero in response to a cultivar specific PHU accumulation. Model simulations compared favourably with independent LAI measurements obtained with a LI-COR plant canopy analyzer over two seasons.     

不同截杆高度对麻疯树生长的影响

把1年生的麻疯树截杆成6种不同的高度,研究其对植株生长的影响。结果表明：不同截杆高度对株高、地径、冠幅和枝长增长量以及萌芽数、成枝率、枝粗、枝下高和不同长度的萌发枝数量都有显著的影响。综合这些指标来看,在实际生产中,把麻疯树幼树截成60或80cm高更为适宜,能使植株生长更快、更健壮,促进更多的侧芽萌发和成枝,并形成更合理的树冠结构。     

油蔬两用油菜干物质积累及氮磷钾养分的吸收利用

为了解油蔬两用生产方式下油菜养分吸收利用特性以合理施肥,通过两年定点取样,以不摘薹处理为对照,比较油菜现蕾摘薹后至成熟期间甘蓝型油菜品种的干物质积累、养分吸收与分配的差异。结果发现：摘薹后油菜生育期、分枝结构、干物质积累、养分吸收利用特性等均发生了改变。与不摘臺相比,摘薹后油菜籽粒产量降低,整株干物重增多,生育期延长8d,分枝发生提前,二次分枝增多（达每株19.2枝）,籽粒产量主要来自二次分枝。氮磷钾三种元素含量随生育进程低,成熟时在角果中的含量大于根茎。摘薹后20d内,由于创伤的修复导致植株干物质积累量、氮磷钾养分吸收量增加缓慢。摘薹20d后,植株开始快速吸收养分;至成熟时,摘薹处理氮磷钾养分的积累更高。摘薹后油菜冠层结构以二次分枝为主、一次分枝为辅,二次分枝花果干物重、氮素积累量均高于一次分枝;而不摘薹处理一次分枝干物重和氮素积累量最高,主茎次之,二次分枝最低。油菜摘薹后生育期延长,干物质和养分积累持续时间变长。可见,油蔬两用生产的油菜需肥特性与常规油菜不同,需要结合特性研究具体的肥料运筹方式。     

Shoot development and flowering in potato (Solanum tuberosum L.)

Summary The shoot system of potato is a configuration of stems with terminal inflorescences. In this review, shoot development is quantified in terms of stem production, while stem development is quantified in terms of leaf and flower primordia production per stem, which are functions of the rates and the durations of primordia initiation. The effect of the position of the stem in the shoot system on number of leaves and flowers per stem is also evaluated. Flowering of individual stems is described by the ‘time to flower primordia initiation’ (expressed in number of leaves produced) and ‘flower production’ (a function of the number and the development of flower primordia). At warmer temperatures and longer daylengths the number of leaves and flower primordia per stem, and the number of stems per shoot increase by prolonging stem production and primordia development. Temperature and photoperiod also affect flower primordia survival by altering assimilate production and partitioning. The photothermal response of the number of leaves per stem is small compared to the response of stem production; at higher temperatures, flower primordia survival becomes the principal factor determining flower production. The similarity of the signals leading to flower primordia initiation and tuberization, and the relation between shoot and tuber growth are discussed.     

Effects on rice plant morphology and physiology of water and associated management practices of the system of rice intensification and their implications for crop performance

Field experiments were conducted in Bhubaneswar, Orissa, India, during the dry season (January–May) in 2008 and 2009 to investigate whether practices of the System of Rice Intensification (SRI), including alternate wetting and drying (AWD) during the vegetative stage of plant growth, could improve rice plants’ morphology and physiology and what would be their impact on resulting crop performance, compared with currently recommended scientific management practices (SMP), including continuous flooding (CF) of paddies. With SRI practices, grain yield was increased by 48% in these trials at the same time, there was an average water saving of 22% compared with inundated SMP rice. Water productivity with AWD-SRI management practices was almost doubled (0.68 g l⁻¹) compared to CF-SMP (0.36 g l⁻¹). Significant improvements were observed in the morphology of SRI plants in terms of root growth, plant/culm height, tiller number per hill, tiller perimeter, leaf size and number, leaf area index (LAI), specific leaf weight (SLW), and open canopy structure. These phenotypic improvements of the AWD-SRI crop were accompanied by physiological changes: greater xylem exudation rate, crop growth rate, mean leaf elongation rate (LER), and higher light interception by the canopy compared to rice plants grown under CF-SMP. SRI plants showed delayed leaf senescence and greater light utilization, and they maintained higher photosynthetic rates during reproductive and grain-filling stages. This was responsible for improvement in yield-contributing characteristics and higher grain yield than from flooded rice with SMP. We conclude that SRI practices with AWD improve rice plants’ morphology, and this benefits physiological processes that result in higher grain yield and water productivity.     

Versuche zur Übertragung der Kartoffelviren M und S DurchMyzus Persicae Sulz

Zusammenfassung In übertragungsversuchen mitMyzus persicae konnten die folgenden Isolate des M-Virus von Pflanzen der gegen das S-Virus immunen KartoffelsorteSaco aufSolanum demissum als Testpflanze übertragen werden: “leaf rolling mosaic”, “interveinal mosaic”, “Fortuna” und “D 1102”. Nicht übertragbar unter den gew?hlten Bedingungen (48–72 Std. Saugzeit) war das “Paracrinkle”-Isolat. Es stimmt hierin mit dem S-Virus aus demUS-S?mling 41956 überein. Die Bedeutung der übertragungsverh?ltnisse für die Klassifizierung der Kartoffelviren M und S sowie des serologisch verwandten, blattlausübertragbaren “carnation latent-Virus” wird diskutiert.

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Summary In transmission experiments withMyzus persicae the following isolates of virus M were found to be transmittable toSolanum demissum test plants from potato plants of the varietySaco, which is immune to virus S: leaf rolling mosaic, interveinal mosaic, Fortuna and D 1102. The paracrinkle isolate was not transmittable under the conditions chosen (48–72 hours sucking time). In this respect it corresponds to the virus S from theU.S. Seedling 41956. The importance of transmission conditions for the classification of potato viruses M and S as well as of the serologically related and aphid-transmittable carnation latent virus is discussed.

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Résumé Dans les essais de transmission parMyzus persicae, les isolés suivants du virus M, obtenus de la variété de pomme de terreSaco, immune au virus S, ont pu être transmis sur les plantes test deSolanum demissum: “leaf rolling mosaic”, “interveinal mosaic”, “Fortuna” et “D 1102”, L'isolé du “paracrinkle” n'était pas transmissible dans les conditions choisies (durée de pompage 48 à 72 heures). Sous ce rapport, il ressemble au virus S duUS-S?mling 41956. L'importance des conditions de transmission pour la classification des virus M et S de la pomme de terre ainsi que pour celle du “carnation latent-virus” sérologiquement apparenté et transmissible par le puceron est discutée.

     

Application of calcium and nitrogen for mitigating heat stress effects on potatoes

This study was designed to investigate the effect of calcium and nitrogen application during heat stress on leaf calcium concentration, transpiration rate, membrane thermostability, and biomass accumulation and partitioning. Micropropagated Russet Burbank potato (Solanum tuberosum L). plants were transplanted into 20 L pots containing 1:1 (v/v) soil: perlite and exposed to 30/20C (D/N) temperatures for four weeks (weeks 9–12 after transplanting) in a controlled-environment growth room. The maximum temperature was maintained for 6 hr during the middle of the 14 hr photoperiod. The nutrition treatments were N before stress (NBS), N during stress (NDS) and Ca and N during stress (Ca+NDS). Calcium was supplied as Ca(NO₃)₂. All treatments received the same total amount of nitrogen. Native soil Ca level without amendment (550 mg Ca/kg soil) was sufficient for potato plant growth under normal temperatures. Plants given Ca and N during heat stress had the highest leaf Ca concentration and transpiration rate during and 2 weeks after conclusion of the heat stress period. When measured after 4 weeks of heat stress, area and fresh and dry weight of the most recently mature leaf was significantly greater in NDS and Ca+NDS plants compared to NBS plants. Cellular membrane thermostability (measured as ion leakage from heat-treated leaf disks) was not affected by any treatment prior to heat stress. However, leaf tissue from Ca+NDS plants exhibited significantly higher membrane thermostability compared to NBS plants after 2 and 4 weeks of heat stress. At harvest, NDS and Ca+NDS plants had significantly higher leaf/stem (fresh weight ratio) values compared to NBS plants. Also, Ca+NDS plants had significantly greater total tuber and biomass values than NBS and NDS plants. Results of this study suggest that some detrimental effects of heat stress on plant growth and stomatal function may be alleviated by Ca and N application during heat stress. The data also suggest that mitigation of heat stress by Ca and N application during heat stress may maintain plant productivity when optimum growing temperatures are restored.