The impacts of high temperature and drought were studied on the seedlings of three families(superior-gangwon74, intermediate-gangwon
77 and inferior-gangwon132) of Pinus densiflora which had been selected by the based on the growth indexes of 32-year-old. The seedlings were grown in controlled-environment growth chambers with combinations of four temperatures (-3℃, 0℃, +3℃, +6℃; based on the monthly average for 30 years in Korea) and two water conditions (control, drought).
The three families under elevated temperature and drought condition, relative height, root collar diameter, and total dry weights was decreased. However, length of the needles of three families increased with the increase of temperature. The reason for the increase in the needles length with elevated temperature is hypothesized to be a strategy for maintenance of the yield of photosynthesis products.
Elevated temperature and drought stress, reduced the photosynthetic rate, stomatal conductance, and transpiration rate and caused significant damage to the photosynthetic mechanism; this resulted in decreased plant growth. In particular, drought stress inhibited photosynthesis by induction of stomatal closure, leading to reduction in the CO2 concentration in the mesophyll. In addition, elevated temperature and drought stress reduced the chlorophyll content of needles.
Reduction in the photosynthetic capacity resulted in a reduction in the total carbohydrate content in the plant body of P. densiflora. In addition, more carbohydrates were transported to the needles than to the roots under high temperatures. Elevated temperature and drought stress also increased the glucose ratio in the needles. Furthermore, since glucose is a source of nicotinamide adenine dinucleotide phosphate(NADPH), which maintains the amount of antioxidants, an increase in glucose caused by elevated temperature and drought stress appears to be a resistance response to stress conditions.
Since the content of malondialdehyde(MDA) increases under stress, it is used as an indirect factor to measure physiological damage. In the this study, MDA contents increased with elevated temperature and drought stress, and ascorbic acid, which is a major antioxidant that efficiently removes reactive oxygen species generated due to environmental stresses and prevents cell damage, and antioxidant enzyme activity increased. This indicates that P. densiflora are very sensitive to elevated temperature and drought stress.
When the growth environment is modified, the microbial population in the soil changes. Therefore, a change in the microbial population owing to elevated temperature and drought stress was analyzed. The results revealed that microbial population diversity increased with elevated temperature and decreased again when a specific temperature was attained. In addition, the total microbial activity increased with elevated temperature and decreased at extremely high temperatures. Microbial diversity and activity also decreased under drought stress.
Differences in plant growth were observed under varying environmental conditions, even in cases of cloned plants with the same genotype. These plant families exhibited different responses under various environments. In the this study, Gangwon74, which genetically has a high juvenile growth rate, showed superior growth and physiological responses as well as high stress resistance compared to that shown by the other families. These results suggest that P. densiflora can show high heritability even under unfavorable environments.
In conclusion, the family with a fast juvenile growth rate (Gangwon74) may possess a higher resistance to other environmental changes and can maintain physiological activity against environmental stresses, in order to maintain a higher growth with elevated temperature and drought stress, compared to other families. In the future, it would be desirable to consider superior families resistant to environmental stresses as well as superior families with a higher volume growth for forestation, because of the threat of global warming due to climate change. In addition, future studies are warranted to develop superior P. densiflora families resistant to environmental stresses for forestation.