Effect of seed soaking with exogenous jasmonic acid on seed germination indexes of sugar beet under drought stress

Introduction Drought stress has become the major limitation factor on plant yield at global scale. It is due to the current environmental changes not only in dry areas but also in temperate ones. Deficit irrigation allows to the plants receive water lower than request. Sugar beet production requires supplementary irrigation in sugar beet producing areas such Mediterranean regions, but recently drought stress has restricted sugar beet cultivation in these areas. In arid and semi-arid regions, sugar beet production is limited by insufficient rainfall or restricted the availability of irrigation water. Breeding new cultivar for drought tolerance, developing new irrigation and soil management techniques and innovating new cultural practices may contribute to improve yield and quality of sugar beet under drought conditions. Sugar beet is one of the most important crops. It is also well recognized that drought stress is the main restrictive factor for sugar beet yield. However, the response of sugar beet to drought stress has been insufficiently studied. Jasmonic acid is a member of plant growth regulators named jasmonates which are important cellular regulators involved in several developmental processes such as seed germination, root growth, fertility, fruit ripening and senescence. Most of the plant parts contain jasmonates and the highest concentration appears to be present in reproductive tissues whereas much lower levels are found in roots and mature leaves. Till now it is considered that jasmonates particularly methyl esters of JA (Me-JA) as a chemical stress agent mimicking the effect of that appear in response to external stress factors inducing senescence.  Materials and methods To explore the germination mechanism of drought-stressed sugar beet improved by exogenous jasmonic acid, and provide a theoretical basis to sugar beet direct sowing technology for water deficit and the effects of soaking with jasmonic acid on germination status, an experiment was conducted in factorial randomized complete design with four replications in laboratory, Faculty of Agriculture, University of Shahrekord in 2016. In this experiment, treatments included five levels of drought treatments (distilled water,-0/2,-0/6,-1 and-1/4 MPa with PEG) and seed soaking with jasmonic acid (0 (control), 5 and 10 µM for 2 days. Germination percentage and rate, shoot and length, shoot weight, mean germination time, coefficient of velocity of germination, relative germination, allometry coefficient, seed length and weight vigor index were measured. Results and discussion The results showed that drought stress and jasmonic acid on germination indexes including germination percent and rate, shoot and root length, shoot weight, coefficient of velocity of germination, relative germination, allometry coefficient and seed length and weight vigor index significantly decreased under drought stress, while, mean germination time increased. Drought stress in 1/4 MPa level decreased germination percentage and rate, shoot and root length, shoot weight 90, 84, 49, 44 and 33 percent as compared with control, respectively. In other hand, soaking with exogenous jasmonic acid improved all of germination indexes, while mean germination time decreased under drought stress. Moreover, interaction effect drought and jasmonic acid on germination indexes showed 5 µM jasmonic acid had highest germination percent and rate, shoot length and weight, coefficient of velocity of germination, relative germination, and seed length and weight vigor index. The highest germination percentage and rate was detected when 5 µM jasmonic acid under non-stress that increased 21% as compared with control, while the lowest was detected when pre-soaking water under 1/4 MPa. Also, highest increase in mean germination time was recorded under 1/4 MPa with pre-soaking water, whereas, the lowest was measured under non-stress with 5 and 10 µM jasmonic acid. To conclude, soaking with 5 µM jasmonic acid improved drought tolerance in sugar beet.