title: Winter Leaf Yellowing in 'Hass' Avocado

authors: Mandemaker, Andries Jan

subjects: Persea americana

abstract: The New Zealand avocado industry is worth $39.7 million in exports of 'Hass' avocados.  Crop yields grew steadily from 1996 to 2001 to reach an average of 8.86 tonnes/ha.  Since then however, crop yields have remained steady.  To increase returns to growers, crop yields must increase.  Avocado leaves in New Zealand become yellow in winter and it is hypothesised that chilling, followed by photoinhibition, is leading to photooxidation.  Leaf yellowing leads to reduced photosynthetic capacity and early leaf abscission, at a time when carbon fixation and carbohydrate reserves are needed to support developing flowers, subsequent fruit set and vegetative flush, in addition to the existing mature crop.

The focus of this research was to determine the underlying causes of yellowing in 'Hass' avocado leaves during winter.  It is suspected that it is a result of the creation of free-radical oxygen that causes photooxidation of leaf components under excess light during low temperature conditions, such as experienced on clear winter mornings in the Bay of Plenty.

An orchard in Katikati, in the Bay of Plenty, New Zealand was selected has it had a history of leaf yellowing.  Two open flow, differential gas exchange measurement systems, The CIRAS-1 and the CMS-400 were used to monitor leaf photosynthetic performance over the course of the 2006 winter, with particular focus on the month of August.  Chlorophyll a fluorescence was measured with a Walz Mini-PAM, leaf colour with a Minolta Chroma meter CR-200b and chlorophyll content with Minolta SPAD chlorophyll meter (in addition to traditional extraction techniques).

There was conclusive evidence that the cold nights resulted in decreased net photosynthesis over the winter, with the depression starting in May and ending around the middle of August, dates that coincide closely with the period when days with mean temperatures less than 10 C occurred.  The decrease in photosynthesis appears to be due to a direct effect on the carbon reduction pathway and in unusual in that full recovery seems to occur at the same time during the day. No photodamage of significance was found and the avocado seems to be highly protected against high light when photosynthesis is inhibited.  This investigation found that leaf yellowing is not caused by photodamage following depressed photosynthesis.

A new hypothesis is proposed which suggests that leaf yellowing is produced by the re-allocation of nitrogen  from leaves during cold weather during flowering.  It is suggested that the chilled leaves are seen as unproductive, old or shaded leaves by the plant and nutrient resources are re-allocated away from these leaves.

A foliar application of 1% low biuret urea and 0.5% magnesium sulphate is currently used by avocado growers to restore leaf colour in leaves that have become yellow over winter.  An experiment was carried out on yellowed leaves on 23rd August 2006 to determine the effectiveness of the treatment.  This study concluded that the treatment was able to restore some leaf colour, but had no effect on leaf photosynthetic function.

text: Winter Leaf Yellowing in 'Hass' Avocado The New Zealand avocado industry is worth $39.7 million in exports of 'Hass' avocados.  Crop yields grew steadily from 1996 to 2001 to reach an average of 8.86 tonnes/ha.  Since then however, crop yields have remained steady.  To increase returns to growers, crop yields must increase.  Avocado leaves in New Zealand become yellow in winter and it is hypothesised that chilling, followed by photoinhibition, is leading to photooxidation.  Leaf yellowing leads to reduced photosynthetic capacity and early leaf abscission, at a time when carbon fixation and carbohydrate reserves are needed to support developing flowers, subsequent fruit set and vegetative flush, in addition to the existing mature crop.

The focus of this research was to determine the underlying causes of yellowing in 'Hass' avocado leaves during winter.  It is suspected that it is a result of the creation of free-radical oxygen that causes photooxidation of leaf components under excess light during low temperature conditions, such as experienced on clear winter mornings in the Bay of Plenty.

An orchard in Katikati, in the Bay of Plenty, New Zealand was selected has it had a history of leaf yellowing.  Two open flow, differential gas exchange measurement systems, The CIRAS-1 and the CMS-400 were used to monitor leaf photosynthetic performance over the course of the 2006 winter, with particular focus on the month of August.  Chlorophyll a fluorescence was measured with a Walz Mini-PAM, leaf colour with a Minolta Chroma meter CR-200b and chlorophyll content with Minolta SPAD chlorophyll meter (in addition to traditional extraction techniques).

There was conclusive evidence that the cold nights resulted in decreased net photosynthesis over the winter, with the depression starting in May and ending around the middle of August, dates that coincide closely with the period when days with mean temperatures less than 10 C occurred.  The decrease in photosynthesis appears to be due to a direct effect on the carbon reduction pathway and in unusual in that full recovery seems to occur at the same time during the day. No photodamage of significance was found and the avocado seems to be highly protected against high light when photosynthesis is inhibited.  This investigation found that leaf yellowing is not caused by photodamage following depressed photosynthesis.

A new hypothesis is proposed which suggests that leaf yellowing is produced by the re-allocation of nitrogen  from leaves during cold weather during flowering.  It is suggested that the chilled leaves are seen as unproductive, old or shaded leaves by the plant and nutrient resources are re-allocated away from these leaves.

A foliar application of 1% low biuret urea and 0.5% magnesium sulphate is currently used by avocado growers to restore leaf colour in leaves that have become yellow over winter.  An experiment was carried out on yellowed leaves on 23rd August 2006 to determine the effectiveness of the treatment.  This study concluded that the treatment was able to restore some leaf colour, but had no effect on leaf photosynthetic function.

year: 2007

title: An analysis of the transitions between down and up states of the cortical slow oscillation under urethane anaesthesia

authors: Wilson, Marcus T.

subjects: cortex

abstract: We study the dynamics of the transition between the low- and high-firing states of the cortical slow oscillation by using intracellular recordings of the membrane potential from cortical neurons of rats. We investigate the evidence for a bistability in assemblies of cortical neurons playing a major role in the maintenance of this oscillation. We show that the trajectory of a typical transition takes an approximately exponential form, equivalent to the response of a resistor–capacitor circuit to a step-change in input. The time constant for the transition is negatively correlated with the membrane potential of the lowfiring state, and values are broadly equivalent to neural time constants measured elsewhere. Overall, the results do not strongly support the hypothesis of a bistability in cortical neurons; rather, they suggest the cortical manifestation of the oscillation is a result of a step-change in input to the cortical neurons. Since there is evidence from previous work that a phase transition exists, we speculate that the step-change may be a result of a bistability within other brain areas, such as the thalamus, or a bistability among only a small subset of cortical neurons, or as a result of more complicated brain dynamics.

text: An analysis of the transitions between down and up states of the cortical slow oscillation under urethane anaesthesia We study the dynamics of the transition between the low- and high-firing states of the cortical slow oscillation by using intracellular recordings of the membrane potential from cortical neurons of rats. We investigate the evidence for a bistability in assemblies of cortical neurons playing a major role in the maintenance of this oscillation. We show that the trajectory of a typical transition takes an approximately exponential form, equivalent to the response of a resistor–capacitor circuit to a step-change in input. The time constant for the transition is negatively correlated with the membrane potential of the lowfiring state, and values are broadly equivalent to neural time constants measured elsewhere. Overall, the results do not strongly support the hypothesis of a bistability in cortical neurons; rather, they suggest the cortical manifestation of the oscillation is a result of a step-change in input to the cortical neurons. Since there is evidence from previous work that a phase transition exists, we speculate that the step-change may be a result of a bistability within other brain areas, such as the thalamus, or a bistability among only a small subset of cortical neurons, or as a result of more complicated brain dynamics.

year: 2010

title: Exploring the constitution of environment education as situated, critical processes of learning and change: A collaborative synthesis across diverse regional contexts

authors: Eames, Chris W.

subjects: Critical approaches

abstract: This article offers a set of unique vignettes or stories that attempt to illustrate examples of critical approaches to environmental education (EE) in diverse contexts. It draws from the experiences of five environmental educators in four different regions of the world. We detail the history of the emergence of critical consciousness in education in Brazil, and its application in a Brazilian region, then move to examples of critical educational responses to oppression in New Zealand and Zimbabwe, before closing with a critical examination of innovative teaching and research in Europe. Through this breadth of endeavour, we identify commonalities across these contexts such as the importance of participatory action and research to examine peopleenvironment relations, particularly as constituted by indigenous peoples, and to interpret realities in ways that empower through learning-led social-ecological change. We argue that this critical approach can foster emancipation through individual and collective learning in EE processes within very different contexts.

text: Exploring the constitution of environment education as situated, critical processes of learning and change: A collaborative synthesis across diverse regional contexts This article offers a set of unique vignettes or stories that attempt to illustrate examples of critical approaches to environmental education (EE) in diverse contexts. It draws from the experiences of five environmental educators in four different regions of the world. We detail the history of the emergence of critical consciousness in education in Brazil, and its application in a Brazilian region, then move to examples of critical educational responses to oppression in New Zealand and Zimbabwe, before closing with a critical examination of innovative teaching and research in Europe. Through this breadth of endeavour, we identify commonalities across these contexts such as the importance of participatory action and research to examine peopleenvironment relations, particularly as constituted by indigenous peoples, and to interpret realities in ways that empower through learning-led social-ecological change. We argue that this critical approach can foster emancipation through individual and collective learning in EE processes within very different contexts.

year: 2018