Effect of Soil Moisture and Determination of Critical Soil Moisture Contents of Cassava

The aims of this research were to investigate the effects of soil moisture on cassava growth and physiological processes and to determine the critical soil moisture contents. To fulifll the research objectives, cassava was grown under five levels of soil moisture, including 50, 40, 30 and 20% of soil available water holding capacity (AWHC) along with control (no irrigation). Physiological traits and plant growth parameters were measured. The results showed that cassava grown under 50% of AWHC exhibited the highest photosynthesis rate, stomatal conductance, predawn leaf water potential and chlorophyll activity. All physiological traits decreased significantly when the moisture content was less than 40 and 20% of AWHC in sandy clay loam and loamy sand soil, respectively. Predawn leaf water potential was used to determine the critical point of soil moisture. It was found that the critical soil moisture contents were 39.0 and 15.7% of AWHC in sandy clay loam and loamy sand soil, respectively.

Predictors and mechanisms of the drought-influenced mortality of tree species along the isohydric to anisohydic continuum in a decade-long study of a central US temperate forest

Using decade-long continuous observations of tree mortality and predawn leaf water potential (ψpd) at the Missouri Ozark AmeriFlux (MOFLUX) site, we studied how the mortality of important tree species varied along the isohydric to anisohydric continuum and how such variations may be predicted. Water stress determined inter-annual variations in tree mortality with a time delay of one year or more, which was predicted by predawn leaf water potential integral (PLWPI), mean effective precipitation interval (a time period with no daily precipitation rates exceeding a threshold) with a daily threshold precipitation at 5 mm day−1 (MEPI5), and precipitation variability index (PVI). Positive temperature anomaly integral (PTAI) and vapor pressure deficit integral (VPDI) also worked reasonably well, particularly for moderate droughts. The extreme drought of the year 2012 drastically increased the mortality of all species in the subsequent year. Regardless of the degree of isohydry and drought intensity, the ψpd of all species recovered rapidly after sufficiently intense rain events. This, together with a lack of immediate leaf and branch desiccation, suggests that hydraulic disconnection in the xylem was absent even during extreme drought and tree death was caused by significant but indirect effects of drought. We also found that species occupying middle positions along the isohydric to anisohydric continuum suffered less mortality than those at either extremes (i.e. extremely isohydric or extremely anisohydric). Finally, our study suggested that species differences in mortality mechanisms can be overwhelmed and masked in extreme droughts and should be examined in a broad range of drought intensity.

Rubber Trees Affected by Necrotic Tapping Panel Dryness Exhibit Poor Transpiration Regulation under Atmospheric Drought

The objective was to study the responses in water status and transpiration regulation of rubber trees affected by the necrotic Tapping Panel Dryness (N) by comparison with healthy trees (H). The experiment was done with 5 N trees and 5 H trees of clone RRIM600 during well soil watered periods differing in evaporative demand intensity, May and August 2007. The study compared predawn leaf water potential (ψpd), midday leaf water potential (ψmid), whole tree hydraulic conductance (K), midday sapflow density (Js) and tree transpiration (ET) with the average girth size 51.54 cm of H tree and 52.66 cm of N tree. These variables, investigated in the high evaporative demand day (ETO = 3.71 mm day-1) on 23 May 2007 and low evaporative demand day (ETO = 1.75 mm day-1) on 22 Aug 2007, did not significantly differ between tree types. However, over a long period, in high evaporative demand, ET tended to be higher in N trees. Expression of ET versus ETO confirmed different relationships between the two tree types with a higher plateau of maximum transpiration for N trees. Our results suggested that individuals with relatively poor transpiration regulation could be more sensitive to necrotic Tapping Panel Dryness syndrome.

Can within-season grapevine predawn leaf water potentials be predicted from meteorological data in non-irrigated Mediterranean vineyards?

<p style="text-align: justify;"><strong>Aims</strong>: Climate-based indices exist for viticulture, particularly for modelling phenological events, but not for vine water status. In this work, climatic variables are linked to the temporal evolution of the water status of the vine</p><p style="text-align: justify;"><strong>Methods and Results</strong>: Within-season time-series of predawn leaf water potential from previous studies across 8 seasons in the south of France were collated. Meteorological data were also collected at each site. A stepwise linear regression model was developed to predict the mean predawn leaf water potential of a vineyard block at a given date from climatic variables and time. Variables selected were growing degree days, short-term humidity and temperature effects. Application of the stepwise model to an independent data set, collected at the other two research sites, produced a linear response but required a local calibration at each site.</p><p style="text-align: justify;"><strong>Conclusion</strong>: The analysis demonstrated that climatic variables can be used for predicting the temporal evolution of vine water stress in non-irrigated vineyards.</p><p style="text-align: justify;"><strong>Significance and impact of study</strong>: The modelling can assist with within-season crop management and future vineyard planning/development in non-irrigated regions. In irrigated vineyards the application is more restricted but it may be used to identify when to start irrigation.</p>