Soil Temperature and Phosphorus Supply Interactively Affect Physiological Responses of White Birch to CO₂Elevation

Phosphorus (P) is a common limiting nutrient element to plants and its supply and uptake by plants are strongly influenced by soil temperature. However, the interactive effects of the two factors on the physiological responses of plants to global change are poorly understood. In this study, we examined how P supply and Tsoil interacted in affecting physiological responses in white birch (Betula papyrifera) to [CO2]. We exposed seedlings to 7°C, 17°C and 27°C Tsoil, 0.1479, 0.3029 and 0.5847 mM P2O5, and 360 and 720 μmol·mol-1 [CO2] for four months. We have found that both the low soil temperature and CO2 elevation resulted in photosynthetic down regulation but the specific mechanisms of the down regulation were different between the two treatments, particularly the relative contributions of biochemical and photochemical capacity, mesophyll conductance and sink strength for carbohydrate utilization to the down regulation. Furthermore, our data suggest that morphological adjustments, such as reduced leaf size and total leaf area, were the primary form of responses in white birch to low phosphorus supply and no significant physiological acclimation to P supply was detected. Our results suggest that white birch will likely enhance water use efficiency under the projected future climate conditions with doubled carbon dioxide concentration, particularly at warmer soil temperatures. Although a trade-off between water use efficiency and nutrient use efficiency is widely accepted, our results suggest that there does not have to be a trade-off between the two, for instance, CO2 elevation increased both use efficiencies and low soil temperature and reduced nitrogen efficiency without affecting water use efficiency under elevated CO2.

Foliar Application of Cytokinin Modulates Gas Exchange Features,Water Relation and Biochemical Responses to Improve Growth Performance of Maize under Drought Stress

Improvement of plant performance under drought stress is crucial to sustaining agricultural productivity.The current study investigated the ameliorative effects of foliar-applied kinetin,an adenine-type cytokinin(CK),on growth and gas exchange parameters,water relations and biochemical attributes of maize plants under drought stress.Eighteen-day-old maize plants were subjected to drought by maintaining soil moisture content at 25%field capacity for 8 days followed by foliar application of kinetin at 0,75,150 and 225 mg L^(−1)(CK0,CK75,CK150 and CK225,respectively)to the plants for two-times at the 9-day interval.Results revealed that drought stress markedly reduced stem diameter,dry weight,chlorophyll content,gas exchange parameters and water balance but increased proline,malondialdehyde and soluble sugar contents,electrolyte leakage and senescence in maize leaves.Application of exogenous CK remarkably improved maize performance by modulating growth,gas exchange-and water relation-related parameters in a dose-dependent manner under drought stress.CK225 increased chlorophyll content(by 61.54%),relative water content(by 49.14%),net photosynthesis rate(by 39.94%)and transpiration rate(by 121.36%)and also delayed leaf senescence but decreased internal CO_(2)concentration(by 7.38%),water saturation deficit(by 40.40%)and water uptake capacity(by 42.49%)in both well-watered and droughtstressed plants.Nevertheless,CK application considerably decreased electrolyte leakage,proline,malondialdehyde and soluble sugar levels in drought-stressed maize plants,as also supported by heatmap and cluster analyses.Taken together,exogenous CK at proper concentration(225 mg L^(−1))successfully improved maize performance under drought conditions,thereby suggesting CK application as a useful approach to alleviate drought-induced adverse effects in maize plants,and perhaps in other important crop plants.