Effects Of Nitrogen Form And Drought Stress On Plant Growth, Ca²⁺ Absorption And Distribution Of Malus Hupenensis （Pamp.） Rehd. Seedling

Studies were conducted over the effects of three NO3-/NH4+ratios (100/0,50/50, and0/100) and drought stress (0,0.1, and0.2kg·L-1) on Malus hupehensis (Pamp.) Rehd.seedlings during the plant growth. The relationship among the growth, cellular structure, andCa2+distribution were investigated, in order to explain the mechanism of regulating Ca2+absorption, to guide the fertilization and effective water manage of fruit trees, and to ensurethe higher Ca2+absorption and utilization.1. The relationship among the plant growth, cell structure and Ca2+distribution of theMalus hupehensis (Pamp.) Rehd. seedlings were investigated with three NO3-/NH4+ratios(100/0,50/50, and0/100) treatments. The results showed that compared to the NO3–/NH4+ratios with100/0and0/100, the seedlings at the NO3–/NH4+ratio with50/50had the higherplant biomass, chlorophyll content, leaf thickness, numbers of mesophyll cells, root area,vascular cylinder area, the numbers of xylem in the vascular cylinder, Ca2+concentration andexpression of CAX1gene, but the lower respiration rate. At the NO3–/NH4+ratio with0/100,epidermis and cortex cells in the root apical meristematic and mature region showed irregularshape with invagination of cell membrane, but the cells of vascular cylinder kept its structureintegrity. Observations the Ca2+and Ca2+-ATPase localization, the results showed that moreCa2+grains distributed in the cell walls, vacuoles, and intercellular space of mesophyll cells,meristematic zone of root tip, and maturation zone of root tip, where the Ca2+-ATPase showedthe higher activities. However, less Ca2+grains distributed in above places at the NO3–/NH4+ratio with0/100and lower Ca2+-ATPase activities were found. These results indicated that theCa2+absorption and transport as well as the Ca2+-ATPase activity were suppressed under thesole NH4+supply. In addition, numbers of Ca2+grains gathered the cell membrane ofepidermis cell of meristematic zone in root tip, the cell membrane were invaginated, and partof them were broken down, led to the imbalance of the cytosolic Ca2+homeostasis.2. The relationship among the plant growth, cell structure and Ca2+distribution of theMalus hupehensis (Pamp.) Rehd. seedlings were investigated with the different concentrations of PEG-6000(0,0.1, and0.2kg·L-1) treatments. The results showed that Malus hupehensis(Pamp.) Rehd. seedlings under the10%and20%PEG-6000treatments significantly inhibitedthe accumulation of biomass, leaf thickness, leaf area, chlorophyll content, assimilation rate,transpiration rate, stomatal conductance, and Ca2+concentration, while the intercellular CO2and respiration rate increased. In addition, the numbers of mesophyll cells were reduced anddead during the drought stress. Observations the Ca2+and Ca2+-ATPase localization, theresults showed that compared with the control, less Ca2+grains with10%and20%PEG-6000treatments were found in the vacuoles and cell walls of the cells of palisade tissue,Ca2+-ATPase activity were significantly reduced, and the structure of chloroplast wereintacted. In the cells of spongy tissue, the cells were intacted and Ca2+-ATPase activity washigher after4d of10%PEG-6000treatment, but, less Ca2+grains were found in the vacuolesand cell walls. After8d of10%PEG-6000treatment, the cell membranes of spongy tissuewere invaginated, and parts of them were broken down, deposited numbers of Ca2+grains. Inaddition, higher Ca2+-ATPase activity was observed in the plasma membrane and vacuolemembrane. After8d of20%PEG-6000treatment, Ca2+-ATPase activity and cell membraneof Ca2+grains were increased, caused the disorder of the cytosolic Ca2+homeostasis, led tothe cell death.