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Introduction Since the leaves are the main source of production of photosynthetic substances in plants, dry matter production and crop yield potential is largely dependent on the leaf surface, and many environmental changes affect growth and yield through changes in leaf area. Hence, green leaf area per plant and leaf area index is measured in almost all studies of crop physiology to understand the mechanism of yield alteration. However, measurement of leaf area compared with the other traits such as plant height and total plant dry weight is very difficult, need to precision instruments and spend more time and cost. Therefore, according to the allometric relationships in plants, extensive studies were done to find the relationship between leaf area and the other plant traits that their measurement is easier, faster and cheaper, and does not require expensive equipment. Using these relationships will be used to estimate plant leaf area with acceptable accuracy without measuring. Plant traits that have high correlation with leaf area and usually use to estimate the plant leaf area are the number of leaves or nodes per main stem, plant height, leaf dry weight and dry weight of vegetative parts of the plant. Allometric equations was used successfully to calculate leaf area for various crops such as cotton, wheat, chickpea, faba bean, peanuts, soybean and sweet sorghum. This study was conducted to obtain the allometric relationships between green leaf area (cm2 per plant) with number of leaves or nodes per main stem, plant height, green leaf dry weight and dry weight of vegetative parts of the plant (gram per plant), and investigating the effect of plant density and planting date on these relationships in SC704 corn (Zea mays L.) hybrid. Materials and Methods This study was conducted at Gorgan University of Agricultural Sciences and Natural Resources farm located at latitude 36 o 51’ N, longitude 54 o27’ E and altitude of 13 meters above sea level in 2012. The experiment was carried out in a randomized complete block design as factorial with three replications. The experimental factors and their levels were, including plant density (4, 6, 8, 10 and 12 plants per square meter) and planting date (May 30 and June 30). Each plot was including 6 rows with 76 cm inter-row spacing and 6 m length. Corn hybrid SC704, a common hybrid in Iran and Gorgan region, was used in this study. Plant sampling was carried out once every 10 days from 15 days after planting to physiological maturity to measure leaf dry weight, plant green leaf area, the leaf (node) number per stem, dry weight of total vegetative plant parts and plant height. Green leaf area was measured using leaf area meter in laboratory. Plant samples were placed in an oven with 70 oC for 48 hours. To obtain allometric relationships various mathematical equations fitted to green leaf area against mentioned traits data. Fitting the functions to data and examining them was carried out in three steps: 1) Fitting function to each of the plant density in each planting date, 2) Fitting a function to all plant densities in each of two planting dates, separately, and 3) Fitting a function to all data. At each step, after fitting the functions and analysis of coefficients, if the differences were not significant, the next step was used. Statistical analysis was done using the software SAS and the graphs drew using Excel software. Results and Discussion The results showed that a non-linear segmented model can be used to describe the relationships between the number of leaves per stem with cumulative thermal units (R2= 0.94, RMSE =5.59%), and leaf area per plant with leaf dry weight per plant (R2= 0.98, RMSE =6.54%) and plant dry weight (R2= 0.95, RMSE =11.25%) in all plant densities and planting dates. Moreover, the results revealed that the effect of planting date and plant density on the phyllochron, time between the appearance of successive leaves on a shoot was not significant, and mean of the phyllochron was 72.33 thermal units in all treatments. In addition, the findings of this study indicated the significant effect of planting date and non-significant effect of plant densities on the relationship between leaf area per plant and number of leaves (nodes) per stem. Therefore, two individual power equations with R2= 0.99 (Y= X3.36 and Y= X0.286 for the first and second planting date, respectively) applied to describe this relationship in two planting dates. Similarly, a power equation (Y = X0.14, R2= 0.99) can be used to describe the relationship between leaf area index and plant height in all planting dates and densities. Conclusions Findings of this study showed that the obtained functions to describe the relationship between the leaf area have high accuracy to estimate green leaf area per plant. But among these relationships, relationships between plant green leaf area with plant green leaf dry weight and plant height had higher coefficient of determination, smaller root mean square, and subsequently higher estimation accuracy. Therefore, using these two traits, leaf area can be estimated more accurately. Moreover, since the relationship between green leaf area with green leaf dry weight per plant was more accurate than plant height, using this relationship in corn simulation models and for fast and easy estimating of leaf area, especially when the leaf area meter is not available, is recommended.

خشکی یکی از مهمترین تنش های محیطی است که تولید محصولات زراعی در ایران را تحت تاثیر قرار می دهد. سیب زمینی اغلب به عنوان محصولی حساس به خشکی در نظر گرفته شده که مواجهه با تنش آب حتی در دوره های کوتاه مدت از جوانه زنی تا رشد غده، می تواند باعث کاهش معنی­دار عملکرد آن گردد. به منظور بررسی اثر تنش آب در مرحله رشد رویشی بر خصوصیات ریشه و روابط آلومتریک میان آن­ها و اندام­های هوایی در ارقام دیررس و زودرس سیب زمینی، این مطالعه با استفاده از طرح کرت­های خرد شده در قالب بلوک­های کامل تصادفی در سال 1390 در گلخانه تحقیقاتی دانشکده کشاورزی دانشگاه فردوسی مشهد اجرا شد. تیمارهای آزمایشی شامل آبیاری با دو سطح تنش خشکی و عدم تنش به عنوان عامل اصلی و 20 رقم سیب زمینی در دو گروه دیررس و زودرس به عنوان عامل فرعی بودند. در تیمار تنش، آبیاری بر اساس 60 درصد آب مورد نیاز جهت رسیدن به ظرفیت زراعی انجام شد. درتیمار عدم تنش آبیاری کامل گلدان­ها به اندازه نیاز آبی بر مبنای میزان تخلیه از ظرفیت زراعی انجام شد. صفات مورد بررسی شامل عمق ریشه، وزن خشک ریشه، مجموع طول ریشه­ها، سطح ریشه، قطر ریشه، نسبت وزن ریشه به اندام­های هوایی، نسبت وزن ریشه به وزن برگ و نسبت سطح ریشه به سطح برگ بودند. نتایج نشان داد که تمامی صفات مورد بررسی تحت تاثیر تیمار تنش آب قرار گرفتند. همچنین پاسخ ارقام مختلف در گروه­های رسیدگی دیررس و زودرس به صفات فوق متفاوت بود. تنش آب سبب افزایش انتقال مواد فتوسنتزی به ریشه ها شد که خود باعث افزایش صفات ریشه نظیر وزن خشک ریشه، عمق ریشه، سطح و مجموع طول ریشه گردید.