Study on effect of water deficit stress on growth of three Mentha species

Water is the most important factor for production of agricultural crops. Adequate water supply is necessary to obtaining maximum productivity of horticultural crops (Jones and Tardieu, 1998). According to Global Circulation Models precipitation scarcity might become worse in the near future over the world. Severe drought periods might decline yield and quality of crops (Delfine et al., 2005). Unfortunately, water deficiency is increasingly becoming a serious problem in agriculture in Iran whereas the national average annual precipitation is less than 249 mm (Baghalian et al., 2011). Therefore in our country, production of agricultural crops always accost many problems because of water scarcity (Hassani et al., 2009). Thus it is important to understand to what extent water stress impairs plant growth and yield in alternative crops (Delfine et al., 2005).
Plants respond quickly to water stress in order to prevent the photosynthesis. Stomata closure in response to water deficit stress primarily results in decline in the rate of photosynthesis.
Photosynthesis limitation causes growth and yield decreasing (Alaei et al, 2013).
Shortage of water in arid and semiarid regions of the world can reduce growth and production of medicinal and aromatic plants, especially Mentha species. Frequent irrigation is necessary during mint plant growth, as mentha species need moist soil conditions in the 100 centimeters of soil where rhizomes are located. This layer of soil has the greatest root density and always must be kept moist (Mitchell and Yang, 1998). But excessive irrigation may decrease mints yield because of limitation of oxygen for plant roots, promoting root diseases, leaching of plant nutrients especially nitrogen and losing more leaves than normal (Mitchell, 1997). According to the results of statistical analysis of Alaei et al (2013), irrigation treatments had significant effects on growth and yield of Dracocephalum moldavica. In this experiment as the amount of irrigation water declined, the plant height, leaf area, leaf number, fresh and dry weight of root and shoot, root length, branch number, and yield per pot decreased. This was while root to shoot ratio, and days to first bloom, first flower and first fruit increased. Study on effects of water deficit stress on Balm (Mellisa officinalis L.) showed that the effect of this stress on shoot yield, leaf and stem yield, stem height was significant at 1% probability level. This was while the number of lateral stem was not significant (Ardakani, 2007). Petropoulos et al., (2007) showed that parsley growth (foliage weight, root weight and leaf number) was significantly reduced by water stress, even at 30-45% water deficit levels. Water deficit has been revealed as effective on growth parameters, yield and biomass. Water stress has decreased plant height, number of secondary branches, dry and fresh weight of shoot, root mass, dry and fresh weight of root and length root (Babaee, 2010). The study on the effect of different levels of water stress on moldavian balm demonstrated that there was not a significant effect of water stress on leaf area and number of leaves, but it was significant on fresh and dry herb (Gholizadeh, 2007).
Material and In order to evaluate response of three Mentha species to water deficit stress, an experiment was carried out in a factorial-randomized design with five replications in controlled conditions and Mentha longifolia (wildmint), Mentha spicata (spearmint) and Mentha piperita (peppermint) species were subjected to four soil moisture regimes (100 (control), 80, 60, 40 of field capacity (FC)) and studied characteristics were included percent of survival, number of branches and stolon, number of leaves, length of branches and relative chlorophyll content were measured every ten days Besides all characteristics that were mentioned before, total dry weights were determined after both harvest.
The results showed that soil moisture treatments had significant effect on survival of three mint species. Trends of branch’s number and length and number of leaves indicate that adequate soil water could achieved better growth in wildmint than two other species in control and 80% of FC, but number and length of stolon in peppermint were significantly excel as compare as two other species in all treatments. However reduction of soil moisture to the 60% of FC severely decreased number and length of branches, number of leaves and number and length of stolon in all species, but spearmint had better growth in this treatment. There were low differences among SPAD of these species in control treatment, though spearmint’s SPAD was higher than two other species in 60 percent of FC in whole growing season. Total dry weight of spearmint in 80% of FC was 31 percent lower than control treatment, while in peppermint and wildmint 40 and 61 percent reduction were observed, respectively. In 60% of FC leaf dry weight of peppermint and wildmint were 92 and 96 percent lower than control, but in spearmint the mentioned parameter in this treatment was 66 percent lower than control treatment.
Water deficit stress reduced growth characteristics of all three species, but growth of spearmint in 20 and 40% deficiency of soil water (as compare with control) was better than two other species. Although spearmint was more tolerant than two other species to water deficit stress, but more study must be achieved for better understanding of mint responses to water deficit stress.
Our results regarding the effect of water stress on dry weight were in coincidence with those reported by Simon et al., (1992) in basil; Ardakani (2007) in balm; Johnson (1995) in Spanish thyme and Safikhani (2007) in moldavian balm who confirmed that water deficit could affect yield by decreasing growth.