Influence Of Water Stress Applied On Half And Whole Root On Leaf Photosynthesis In Micropropagated 'Gala' Apple (Malus Pumila Mill.) Plants And The Involvement Of Nitrate In Regulation Of Photosynthesis

Influence of water stress applied on half (HRS) and whole root (WRS) on leaf water potential, photosynthesis, NO₃^- content and the correlation between NO3" content and photosynthesis in micropropagated apple (Malus pumila Mill) plants were compared by either regulating the osmotic potential of the solution using polyethylene glycol (PEG-6000, 20% W/V), or by grafting two plants together which made it have two complete root system and then regulating soil water potential by restricting irrigation. Furthermore, artificial foliar NO₃^- accumulation was obtained by both stem feeding and foliar spray of NaNO₃ or NaNO₃+Na₂WO₄ solution, and the involvement of NO3" in regulating of photosynthesis were investigated, too. The following results were obtained:When one-year-old micropropagated apple plants were subjected to partial root drought stress, they could maintain a good water status and there was no need to accumulate osmolytes. During the water stress by PEG-6000, no significant difference of predawn leaf water potential was found between HRS and control (CK) while predawn leaf water potential from both HRS and CK was significantly higher than WRS. Though diurnal leaf water potential of HRS was lower than CK at most time of the day, foliar proline and soluble sugar didn't accumulated under HRS. However, as the result of osmoregulation, WRS made foliar proline and soluble sugar content increase.HRS had less effect on leaf photosynthesis than WRS in micropropagated apple plants. During the drought stress treatment by PEG-6000, leaf net photosynthesis, stomatal conductance and transpiration rate of HRS declined when compared with CK, but higher than WRS in the first 7 days, and no difference between HRS and WRS thereafter. Similarly during the soil drying treatment, leaf net photosynthesis, stomatal conductance and transpiration rate of HRS declined when compared with CK, but higher than WRS through out the experiment.When the plants were subjected to partial and whole root drought stress, foliar NO₃^- content had no contribution to the decreased leaf net photosynthesis. It was possible that leaf photosynthesis would be influenced indirectly by free NO₃^- in root, or by changes in root NO₃^- assimilation and other signal molecular caused by drought. Both HRS and WRS had no influence on foliar NO₃^- and NH₄⁺ content, but made NO₃^- content decreased significantly in the drought stressed roots, about half of CK, as early as the day 1, 4 hours after the plants were under drought stress. This decreased NO₃^- content was kept in roots of WRS and in the stressed roots of HRS to the end of the experiment.Low foliar NO₃^- content was the species owned property and maintaining this status was important for normal photosynthesis in micropropagated apple plants. Changes in this status had negative effect on leaf photosynthesis. Both stem feeding of 3 mM NaNO₃+0.3 mM Na₂WO₄ and foliar spray of 30 mM NaNO₃+20 mM Na₂WO₄ significantly increased foliar NO₃^- content and resulted in decreased leaf net photosynthesis, stomatal conductance and transpiration rate when the temperature and light was suitable for photosynthesis on the second day after stem feeding or foliar spray.In contrast, stem feeding 40 ml of 3 mM NaN03 alone or foliar spray of 12 mM NaNO3 alone increased foliar NR activity and had no effect on foliar NO3" content. These two treatments had no effect on leaf net photosynthesis, stomatal conductance and transpiration rate when the plants were under normal water conditions and HRS, but enhanced leaf net photosynthesis and stomatal conductance in plants subjected to WRS.The methods developed in this study, which two micropropagated apple plants were grafted together for study the effect of partial root zone drying, as well as stem feeding forien material on plant physiology, are new systems which can be adopted in other related studies of plant physiology.