The Larache experimental station of INRA Tangier is located in Larache city, Morocco. It is dedicated to supporting agricultural development in the irrigated area of Loukkos, Morocco. It mainly houses research on strawberry, small red berries and horticultural crops. This research station covers 4.8 ha including 0.8 ha of greenhouses. Strawberry cultivars are grown in greenhouses and were planted with 4 complete random blocks. Each elementary parcel is made up of a ridge 60 meters long and 7 meters wide with four rows spaced 60 cm. The space between two ridges is 70 cm.

The Fortuna cv strawberry was selected in this study. Plant runners of these strawberry cultivars were used as the experimental materials. Plants runner were taken from the strawberry field in mid-October 2018 and irradiated immediately by ⁶⁰Co-gamma ray in the National Institute of Agricultural Research Center of Tangier, morocco. The dosages were 0 Gy (non-irradiation), 20 Gy, 40 Gy, 60 Gy, 80 Gy and 100 Gy. There were 80 plant runners per treatment, and all treatments were repeated four times. After the irradiation treatment, plant runners were planted immediately in the field and conventionally managed. The death rate and the change of the leaf length, leaf width, plant height of the treated runner plants were investigated in mid-April 2019. The leaf length, leaf width and plant height were measured according to description and data standard of strawberry germplasm resources [22] . The death rate of the plant runners was expressed by the number of the dead plant runners divided by the number of the treated runner plants. The experiment was conducted in a randomized complete block design (RCBD). Each box containing 20 plants was considered as a single experimental unit that was further replicated four times.

Statistical analysis of variance was carried out by software SPSS ® version 20.0. The mean difference of the study parameters among the treatments was adjusted by Duncan’s Multiple Range Test [23] .

The death rate of the runner plant is presented in Table 1. Gamma-rays in different doses affected the vegetative growth parameters in different ways. Rate of surviving plants reduced when dose rate increased. Radio-sensitivity of runners on the same cultivar was different. Results show that the death rate of the runner plant of Fortuna strawberry cultivar was higher remarkably in control treatment than in all treatment, and the death rate was between 27.85% and 74.84% (Table 1). There was significant difference in death rate of the runner plants irradiated by four different doses and there was no significant difference in the dose 40 Gy and 60 Gy. However, the death rate of the plant runners of Fortuna strawberry cultivar was higher remarkably with 20 Gy, 40 Gy and 60 Gy than in 80 Gy, and there was no significant difference between 40 Gy and 60 Gy. The death rate of runner plants of Fortuna strawberry cultivar was low with 100 Gy.

The correlation between radiation dose and cumulative percent of bud was r = −0.96 (with P value = 0.002). The equation of related regression based on radiation dose is Y = 90.21 − 0.59X (Figure 1). Based on the minimum level of 50% of runner surviving compared to control, lead us to locate the optima radiation dose between 60 and 65 Gy for the Fortuna strawberry cultivar. The yield of runner surviving of these treatments was satisfactory since the rate of survival was ranged from 27.85% to 87.5%.

As a result, GR 50 dose for the application of mutation breeding of Fortuna strawberry cultivar under Moroccan condition was determined as 62 Gy. The strawberry is an herbaceous fruit tree; the complete strawberry runner plant consists of root, stem and leaf. It has no obvious branch and bud, its bud hides in the stem. Thus, the stem of strawberry runner plants is similar to the branch of woody fruit tree [24] . [25] [26] found GR50 dose to be 20 - 25 Gy on woody cutting of grape variety.

Significant differences within the same column and means followed by the same letter do not differ at Pα ≤ 0.05 according to Duncan test.

[27] observed a higher percentage of survival at lower doses and poor survival at higher doses in all the generations of Calendula officinalis. [28] found the reduction survival of plants after exposure to gamma rays with the increase of dose in Rudbeckia laciniata. A similar trend has also been reported by [29] . [30] also reported the decrease in ability to produce shoots on increasing the radiation dose in Lonicera species. [31] reported the inhibitory effects of higher doses of gamma rays on strawberry cultures.

The application of high doses of gamma rays negatively influenced plant development, as in many studies previously done in different species [32] [33] [34] [35] .

The survival of plant runners grew further after they were planted in the field, but the plant height, leaf length, leaf width and the number of leaves of the survival of plant runners had changed compared with the control (Figure 2). However, the change was different according to irradiation dosage (Table 2). The dose 20 Gy recorded significantly maximum plant height (11.61 cm). The minimum plant height (2.03 cm) was recorded by the dose of 100 Gy. The height plant was higher significantly similar in dose 20 Gy and 40 Gy, and significantly lower similar in dose 60 Gy and 80 Gy. The dose 100 Gy shows the very less height of plant.

The increased plant vigor caused by irradiation could be explained by stimulation of biosynthesis of some amino acids like Lysine and phenylalanine [36] , modification of some enzymes activity such Polyphenol oxidase, catalases and peroxidases which are great in the leaf of treated plants [37] [38] [39] [40] for increase of primary biochemical processes, uptake of mineral nutrients [41] and photosynthesis [42] . [31] reported the inhibitory effects of higher doses of gamma rays on strawberry plant. He found that the number of roots per shoot decreased at higher gamma dosage (10 KR and 15 KR).

The same letter within the same column and means do not differ at Pα > 0.05 according to Duncan test.

Leaf number is important for photosynthesis and it differed significantly ranging from 5.78 to 20.11. The dose 20 Gy shows the highest number of leaves (20.11) and lower number for leaves was at 100 Gy doses. The doses 40 Gy and 80 Gy were not significantly different but in the number of leaves were significantly different in other doses (Table 2).

Leaf length and leaf width from each irradiated plant were studied (Table 2). In the leaf length and leaf width the dose 20 Gy show the higher value 5.03 cm and 4.26 cm respectively, the lowest value for these two parameters was observed for 100 Gy dose. In leaf width, we distinguish three groups significantly different; the dose 20 Gy and 40 Gy are not significantly different also from the dose 60 and 80 Gy they are not significantly different, only the dose 100 Gy was significantly different from all the doses. The leaf length only the dose 20 Gy and 40 Gy they are significantly not different. In this study, the gamma ray influences the leaf length and leaf width (Figure 2).

Numerous investigators [43] [44] [45] [46] have presented results which indicate that cells are more vulnerable to radiation at certain stages in mitosis. [47] obtained the number of leaves vitro plant varying from 60.52 to 49.00 in different germplasm, which was higher to the findings of the present study. But the finding about leaves plant reported by [48] was lower than the present study. They found that the number of leaves ranged from 3.33 to 6.33 plants under irrigation in semi-arid region of Punjab, which might be due to the genetic as well as the environmental effect. [49] obtained a variation in leaf number among cultivars during the later part of the season and found that cv. “Bolero” and “Everest” produced the highest and “Elsanta” the least number of leaves plant. Another study revealed a significant variation in leaves plant among strawberry germplasm and recorded 4.6 to 9.8 leaves plant at 86 days after planting in Cambridge.

Stimulatory effects of low doses of gamma rays on strawberry can be related to hypothesis that the low dose irradiation induce growth stimulation by changing the hormonal signaling network in plant cells or increase the anti-oxidative stress capacity of the cells to easily overcome the stress factors such as fluctuations of light intensity and temperatures in the growth condition [50] . In contrast, the high dose irradiation that caused growth inhibition has been ascribed to the cell cycle arrest at G2/M phase during somatic cell division and/or various damages in the entire genome [51] .

Since it is possible to alter growth rate without altering radio-sensitivity at the same time, it is no longer sufficient to say that an organism is more susceptible because it is growing rapidly or because it contains cells in mitosis. It would appear that growth, as defined, is the result of a condition which determines radio-sensitivity, rather than the cause of it. It is probably more nearly correct to say that growth activity and radio-sensitivity depend on a common physiological condition in the organism-one that remains unchanged although growth may be stopped by lowering temperature.

Stems of strawberry plants were observed (Figure 3). The strawberry plant from doses 20 Gy 40 Gy and 60 Gy show a higher number of stems in a plant like plant control. But in the plant irradiated with 80 Gy and 100 Gy, the number of stems

in a plant was lower. The character of plant robustness was influenced by the anatomical structure of the stem. This is due to the fact that strawberries are a dicotyledonous plant whose stems are subjected to secondary growth by having a lateral meristem. Additionally, the clustering life from of strawberries also supports the robustness of their stems. [52] stated that phenotypic appearances could be controlled by the genetic properties of the plant itself, which are the responses to the interaction with environmental factors.