Consistent responses to moisture stress despite diverse growth forms within mountain fynbos communities

Understanding climate change impacts on the Cape Floristic Region requires improved knowledge of plant physiological responses to the environment. Studies examining physiological responses of mountain fynbos have consisted of campaign-based measurements, capturing snapshots of plant water relations and photosynthesis. We examine conclusions drawn from prior studies by tracking in situ physiological responses of three species, representing dominant growth forms (proteoid, ericoid, restioid), over 2 years using miniature continuous sap flow technology, long-term observations of leaf/culm water potential and gas exchange, and xylem vulnerability to embolism. We observed considerable inter-specific variation in the timing and extent of seasonal declines in productivity. Shallow-rooted Erica monsoniana exhibited steep within-season declines in sap flow and water potentials, and pronounced inter-annual variability in total daily sap flux (J_s). Protea repens showed steady reductions in J_s across both years, despite deeper roots and less negative water potentials. Cannomois congesta-a shallow-rooted restioid-was least negatively impacted. Following rehydrating rain at the end of summer, gas exchange recovery was lower in the drier year compared with the normal year, but did not differ between species. Loss of function in the drier year was partially accounted for by loss of xylem transport capacity in Erica and Cannomois, but not Protea. Hitherto unseen water use patterns, including inter-annual variability of gas exchange associated with contrasting water uptake properties, reveal that species use different mechanisms to cope with summer dry periods. Revealing physiological responses of key growth forms enhances predictions of plant function within mountain fynbos under future conditions.

Water use efficiency, grain yield, and economic benefits of common beans (Phaseolus vulgaris L.) under four soil tillage systems in Mukono District, Uganda

With the increasing climate change impacts and variabilities, water is becoming a limiting factor for rainfed crop production in Uganda. Conservation tillage practices could improve soil and water conservation in croplands. Field experiments were conducted for three consecutive seasons from April 2019 to June 2020. The experiments evaluated the effect of soil tillage treatments on soil water storage, water use efficiency, grain yield, and economic benefits of the common beans (Phaseolus vulgaris L.) in two sub-counties of Mukono District, central Uganda. The soil tillage treatments were: no-tillage, stubble-mulching, deep tillage, and conventional tillage. The no-tillage and stubble-mulching improved soil water storage by 46 and 45%, respectively, compared with the conventional tillage in the 0–100 cm soil depth over the 14 months. Soil tillage treatments significantly (p < 0.05) affected the water use efficiency, with water use efficiency values generally higher under no-tillage and stubble-mulching than under deep tillage and conventional tillage treatments. The grain yield was highest under no-tillage and stubble-mulching than deep tillage and conventional tillage treatments, with over 5, 38, and 43% higher grain yield under no-tillage than under stubble-mulching, deep tillage, and conventional tillage treatments, respectively. Although no-tillage and stubble-mulching improved soil water storage and grain yield, seasonal precipitation distribution had a greater influence on the final grain yield, soil water storage, and water use efficiency. The net profit was 3 and 5 times higher under no-tillage than under conventional tillage and deep tillage treatments, respectively. The overall results showed that no-tillage and stubble-mulching were the optimum tillage treatments for increasing soil water storage and common bean yield, enhancing water use efficiency, and improving economic returns in central Uganda.

Hydraulics and gas exchange recover more rapidly from severe drought stress in small pot-grown grapevines than in field-grown plants

Modifications of plant hydraulics and shoot resistances (R_shoot) induced by water withholding followed by rewatering, and their relationships with plant water status, leaf gas exchange and water use efficiency at the leaf level, were investigated in pot-grown and field-grown, own-rooted Syrah grapevines in an arid climate. Water stress induced anisohydric behavior, gradually reducing stomatal conductance (g_s) and leaf photosynthesis (A) in response to decreasing midday stem water potential (Ψ_s). Water stress also rapidly increased intrinsic water-use efficiency (A/g_s); this effect persisted for many days after rewatering. Whole-plant (K_plant), canopy (K_canopy), shoot (K_shoot) and leaf (K_leaf) hydraulic conductances decreased during water stress, in tune with the gradual decrease in Ψ_s, leaf gas exchange and whole plant water use. Water-stressed vines also had a lower Ψ gradient between stem and leaf (ΔΨ_l), which was correlated with lower leaf transpiration rate (E). E and ΔΨ_l increased with increasing vapour pressure deficit (VPD) in non-stressed control vines but not in stressed vines. Perfusion of xylem-mobile dye showed that water flow to petioles and leaves was substantially reduced or even stopped under moderate and severe drought stress. Leaf blade hydraulic resistance accounted for most of the total shoot resistance. However, hydraulic conductance of the whole root system (K_root) was not significantly reduced until water stress became very severe in pot-grown vines. Significant correlations between K_plant, K_canopy and Ψ_s, K_canopy and leaf gas exchange, K_leaf and Ψ_s, and K_leaf and A support a link between water supply, leaf water status and gas exchange. Upon re-watering, Ψ_s recovered faster than gas exchange and leaf-shoot hydraulics. A gradual recovery of hydraulic functionality of plant organs was also observed, the leaves being the last to recover after rewatering. In pot-grown vines, K_canopy recovered rather quickly following restoration of Ψ_s, although gas exchange recovery did not directly depend on recovery of K_canopy. In field-grown vines, recovery of water status, gas exchange and hydraulic functionality was slower than in pot-grown plants, and low g_s after rewatering was related to sustained decreased K_plant, K_canopy and K_shoot and lower water transport to leaves. These results suggest that caution should be exercised when scaling up conclusions from experiments with small pot-grown plants to field conditions.

Miniature External Sapflow Gauges and the Heat Ratio Method for Quantifying Plant Water Loss

External sapflow sensors are a useful tool in plant ecology and physiology for monitoring water movement within small stems or other small plant organs. These gauges make use of heat as a tracer of water movement through the stem and can be applied in both a laboratory and a field setting to generate data of relatively high temporal resolution. Typical outputs of these data include monitoring plant water use on a diurnal time scale or over a season (e.g., in response to increasing water deficit during drought) to gain insight into plant physiological strategies. This protocol describes how to construct the gauges, how best to install them and some expected data outputs.