Screening for drought tolerance in some hybrid apple rootstocks based on photosynthesis characteristics

In order to evaluate the drought tolerance of some hybrid apple rootstocks, an  experiment was conducted in a randomized complete block design with 11 genotypes from apple rootstock breeding program, AR1 to AR11, with MM111, as tolerant control, under two irrigation treatments of 40 and 80% field capacity. In this experiment, physiological parameters including photosynthesis, stomatal conductance, transpiration, sub-stomatal CO2, water use efficiency, mesophyll conductance, leaf relative water content (RWC), leaf surface temperature, and internal temperature of the crown were measured. Generally, the studied genotypes showed significant differences in physiological characteristics at 1% level. Drought stress reduced the physiological parameters and its reduction varied among different genotypes. Genotypes AR3, AR4, AR5 and AR7 had the highest stomatal conductance and photosynthesis rate under normal irrigation (control), however, AR1, AR4, AR8, AR11 showed the least and AR3, AR6, AR7, AR9 revealed the most decrease in stomatal conductance and photosynthesis rate under drought stress which may be considered as drought tolerant and drought suceptible groups of genotypes for further studies. Despite higher transpiration of drought tolerant genotypes, they maintained their leaf relative water content under drought stress which, in turn resulted in retaining their leaf turgor pressure and photosynthesis parameters. These genotypes had more ability to absorb or keep water probably due to their special root system or higher osmolites accumulation in their cells.