Effect of Half and Whole Root Drying on Photosynthesis, Nitrate Concentration, and Nitrate Reductase Activity in Roots and Leaves of Micropropagated Apple Plants

Half or whole root systems of micropropagated 'Gala' apple (Malus xdomestica Borkh.) plants were subjected to drought stress by regulating the osmotic potential of the nutrient solution using polyethylene glycol (20% w/v) to investigate the effect of root drying on NO3(-) content and metabolism in roots and leaves and on leaf photosynthesis. No significant difference in predawn leaf water potential was found between half root stress (HRS) and control (CK), while predawn leaf water potential from both was significantly higher than for the whole root stress (WRS) treatment. However, diurnal leaf water potential of HRS was lower than CK and higher than WRS during most of the daytime. Neither HRS nor WRS influenced foliar NO3(-) concentration, but both significantly reduced NO3(-) concentration in drought-stressed roots as early as 4 hours after stress treatment started. This reduced NO3(-) concentration was maintained in HRS and WRS roots to the end of the experiment. However, there were no significant differences in NO3(-) concentration between CK roots and unstressed roots of HRS. Similar to the effect on root NO3(-) concentration, both HRS and WRS reduced nitrate reductase activity in drought-stressed roots. Moreover, leaf net photosynthesis, stomatal conductance and transpiration rate of HRS plants were reduced significantly throughout the experiment when compared with CK plants, but the values were higher than those of WRS plants in the first 7 days of stress treatment though not at later times. Net photosynthesis, stomatal conductance and transpiration rate were correlated to root NO3(-) concentration. This correlation may simply reflect the fact that water stress affected both NO3(-) concentration in roots and leaf gas exchange in the same direction.