12 research outputs found
Recommended from our members
Irrigation indices in almond: a comparison with an improved sap flow method
ABSTRACTIrrigation indices in almond: a comparison with an improved sap flow method
Heather K. Vice, Master of Science, 2021
Reliable strategies to assess crop water use are a persistent challenge across the agricultural sector with predicted shifts in annual precipitation patterns. Sap flow techniques provide valuable measurements of transpiration within a given plant, and thus may be useful for informing irrigation, but there are many methods from which to choose. It is also unclear how to best use sap flow data to effectively detect stress thresholds required to trigger irrigation events. The aim of the present work was to assess the potential to use continuous estimates of plant water use provided by sap flow as a complement to, or substitute for other plant-based irrigation predictors. I examined the suitability of three heat pulse methods — the double-ratio, heat-ratio and compensation heat-pulse methods (DRM, HRM and CHPM) — to estimate normalized sap velocity (NV) and normalized isothermal canopy conductance (NG, [∝ NV/VPD]) in almond [Prunus dulcis (Miller) D.A. Webb] orchards located in a semi-arid Mediterranean environment. Measurements of sap flow in twelve 20-year-old trees and three irrigation demand indices — estimated crop evapotranspiration (ETc), soil water content (θ) and stem water potential (Ψstem) — were conducted during two growing seasons, from June to September 2016 and 2017.
During well-watered periods of July, transpiration measures given by all irrigation indices demonstrated the limitations of Ψstem and θ as reliable indicators of plant water stress. Both sap flow and canopy conductance elicited clear patterns of the fluctuations in transpiration responses to changes in both θ and high evaporative demand in August, when irrigation was reduced for harvest, declining in response to low θ in comparison to relatively high unchanging rates of ETc.
Ψstem followed a similar, yet slower, pattern illustrating the relationship between VDRM and Ψstem, consistent with the concept of the “baseline” water potential varying with VPD. NG plateaued early in the day and decreased much earlier than VDRM in the afternoon, indicating stomatal closure in response to elevated VPD. These data confirm the robust performance of the DRM in almond trees and reveal that stomatal regulation of transpiration in almond is more sensitive to VPD when θ is low. Normalized isothermal canopy conductance inferred from VDRM and VPD is a promising and easily-implemented tool for interpreting short- and long-term dynamics of almond tree water status and their responses to abiotic stress
Recommended from our members
The Kok effect in Vicia faba cannot be explained solely by changes in chloroplastic CO2 concentration.
The Kok effect - an abrupt decline in quantum yield (QY) of net CO2 assimilation at low photosynthetic photon flux density (PPFD) - is widely used to estimate respiration in the light (R), which assumes the effect is caused by light suppression of R. A recent report suggested much of the Kok effect can be explained by declining chloroplastic CO2 concentration (cc ) at low PPFD. Several predictions arise from the hypothesis that the Kok effect is caused by declining cc , and we tested these predictions in Vicia faba. We measured CO2 exchange at low PPFD, in 2% and 21% oxygen, in developing and mature leaves, which differed greatly in R in darkness. Our results contradicted each of the predictions based on the cc effect: QY exceeded the theoretical maximum value for photosynthetic CO2 uptake; QY was larger in 21% than 2% oxygen; and the change in QY at the Kok effect breakpoint was unaffected by oxygen. Our results strongly suggest the Kok effect arises largely from a progressive decline in R with PPFD that includes both oxygen-sensitive and -insensitive components. We suggest an improved Kok method that accounts for high cc at low PPFD
Recommended from our members
The Kok effect in Vicia faba cannot be explained solely by changes in chloroplastic CO2 concentration.
The Kok effect - an abrupt decline in quantum yield (QY) of net CO2 assimilation at low photosynthetic photon flux density (PPFD) - is widely used to estimate respiration in the light (R), which assumes the effect is caused by light suppression of R. A recent report suggested much of the Kok effect can be explained by declining chloroplastic CO2 concentration (cc ) at low PPFD. Several predictions arise from the hypothesis that the Kok effect is caused by declining cc , and we tested these predictions in Vicia faba. We measured CO2 exchange at low PPFD, in 2% and 21% oxygen, in developing and mature leaves, which differed greatly in R in darkness. Our results contradicted each of the predictions based on the cc effect: QY exceeded the theoretical maximum value for photosynthetic CO2 uptake; QY was larger in 21% than 2% oxygen; and the change in QY at the Kok effect breakpoint was unaffected by oxygen. Our results strongly suggest the Kok effect arises largely from a progressive decline in R with PPFD that includes both oxygen-sensitive and -insensitive components. We suggest an improved Kok method that accounts for high cc at low PPFD
Accepted for the Council:
have examined the final electronic copy of this dissertation for form and content an
Tamm Review: Reforestation for resilience in dry western U.S. forests
The increasing frequency and severity of fire and drought events have negatively impacted the capacity and success of reforestation efforts in many dry, western U.S. forests. Challenges to reforestation include the cost and safety concerns of replanting large areas of standing dead trees, and high seedling and sapling mortality rates due to water stress, competing vegetation, and repeat fires that burn young plantations. Standard reforestation practices have emphasized establishing dense conifer cover with gridded planting, sometimes called \u27pines in lines\u27, followed by shrub control and pre-commercial thinning. Resources for such intensive management are increasingly limited, reducing the capacity for young plantations to develop early resilience to fire and drought. This paper summarizes recent research on the conditions under which current standard reforestation practices in the western U.S. may need adjustment, and suggests how these practices might be modified to improve their success. In particular we examine where and when plantations with regular tree spacing elevate the risk of future mortality, and how planting density, spatial arrangement, and species composition might be modified to increase seedling and sapling survival through recurring drought and fire events. Within large areas of contiguous mortality, we suggest a “three zone” approach to reforestation following a major disturbance that includes; (a) working with natural recruitment within a peripheral zone near live tree seed sources; (b) in a second zone, beyond effective seed dispersal range but in accessible areas, planting a combination of clustered and regularly spaced seedlings that varies with microsite water availability and potential fire behavior; and (c) a final zone defined by remote, steep terrain that in practice limits reforestation efforts to the establishment of founder stands. We also emphasize the early use of prescribed fire to build resilience in developing stands subject to increasingly common wildfires and drought events. Finally, we highlight limits to our current understanding of how young stands may respond and develop under these proposed planting and silvicultural practices, and identify areas where new research could help refine them
THEMES IN
Reviser of texts originally written in English, translator of texts originally written in Portugues