The Ecophysiological Mechanisms Of Maintaining Whole-plant Water Balance

Water shortage often limits the ecological environment and agricultural productionin the arid and semi-arid area. Understanding the ecophysiological basis of highefficiency of plant water use is very important for developing water-saving agricultureand constructing ecological civilization. Plant can maintain water balance with variablehydraulic properties and aquaporins could be involved, which may help to understandthe ecophysiological mechanisms of plant water relations. This dissertation used thehydroponically grown maize （Zea mays L.）, wheat （Triticum spp.） and sweet sorghum（Sorghum bicolor （L.） Moench） seedlings as experimental materials, and used cell-androot-pressure probes, high pressure flow meter and quantitive real-time PCR tomeasure the responses of cell, single root, root and shoot and whole-plant hydraulicsand aquaporin genes transcription to short-term and long-term water stress induced byPEG6000and root excision. The main results are as follows:（1）. The leaf hydraulic conductivity （Kleaf） of Line478varied diurnally andcorrelated with whole plant hydraulic conductivity. Similar diurnal rhythms of Kleafandthe root hydraulic conductivity (K_root) could be important to maintaining whole plantwater balance. Krootsignificantly correlated with leaf transpiration rate （E）. After2h ofosmotic stress, the Krootof stressed plants significantly declined but K_leaf increased; thetranscription of four ZmPIPs was significantly up regulated in leaves, especially forZmPIP1;2, and ZmPIP2;5was down regulated in roots. The up-regulated Kleafanddown-regulated Krootmay break the water balance; and ZmPIPs genes may get involvedin the hydraulics changes during short-term water stress.（2）. The E and leaf water potential of478varied diurnally, but those of Tian4,which is more drought resistant, were more constant. Line478had advantages on leafhydraulic architecture, leaf and root morphology, K_leafand K_root, which may contribute to the higher E. The K_leafand K_root of both Tian4and478varied diurnally. The K_rootofboth Tian4and478was reduced under osmotic stress, but the K_rootof Tian4subsequently recovered. A lower and rapidly reduced leaf water loss and the recovery ofroot hydraulics during short-term osmotic stress may account for the ability ofdrought-resistant maize to maintain plant water balance.（3）. The transcription levels of ZmPIPs in mature leaves and roots of well wateredTian and478were significantly different. ZmPIP1;5was highly expressed in478but itsmRNA was not detected in Tian4. Within2h of water stress induced by PEG, thetranscription levels of ZmPIPs were up-regulated in roots but were down-regulated inleaves of Tian4, which may be help to increase water uptake and decrease water loss; inleaves and roots of478plants, the transcription levels of ZmPIPs both showed antemporary increase, which may contribute to the higher leaf water transpiration.（4）. The E, single root (Lp_root) and cell (Lp_cell) hydraulic conductivity of wheatincreased with increasing ploidy, but the V_cell was reduced. Osmotic stress significantlyreduced the E, Lp_cell, Lp_root, and the relative mRNA content of TaPIP1;2and TaPIP2;5in wheat. Under well-watered or osmotic stress conditions, Lp_rootpositively correlatedwith the E and Lp_cell; the relative mRNA content of TaPIP1;2and TaPIP2;5significantly correlated with Lp_celland Lp_root, respectively. Lp_cellwas reduced, but theLp_cell/Lp_rootincreased with increasing Vcell, suggesting that Vcellmay affect root radicalwater transport. Thus, the increased Lp_celland transcription levels of TaPIP1;2andTaPIP2;5in wheat roots provides insight into the mechanisms underlying enhanced rootwater uptake during wheat evolution.（5）. The Pnof stressed plants totally recovered three days later while gswereconsistently lower than the controls, getting an improved instantaneous water-useefficiency （WUE）. During prolonged water stress, the total water loss per plant ofstressed plants reduced significantly, while the dry mass of the whole plant did notchange, and leaf dry mass per unit area increased. The total leaf nitrogen, leaf nitrogenper unit area and leaf nitrogen concentration of stressed plants reduced significantly. Butthe photosynthetic nitrogen-use efficiency （PNUE） and dry mass based nitrogen-useefficiency （NUE） increased significantly. WUE positively correlated with PNUE. Bothimproved water-and nitrogen-use efficiencies of sweet sorghum under water stress maypartly explain its physiological acclimation to drought.Based on the experimental data obtained from maize, wheat and sweet sorghum grown under controlled conditions, some important issues on the mechanism ofwhole-plant water balance were concerned in this dissertation. The results cancontribute to understand the biological bases of plant integrative drought-resistance, toclarify the physiological mechanisms of water-nitrogen-carbon relations, and to improvethe water use efficiency of plant by gene modification.