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Published online 1 September 2007
Published in Crop Sci 47:2067-2073 (2007)
© 2007 Crop Science Society of America
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CROP PHYSIOLOGY & METABOLISM

Correlation between Heat Stability of Thylakoid Membranes and Loss of Chlorophyll in Winter Wheat under Heat Stress

Zoran Ristica,*, Urska Bukovnikb and P.V. Vara Prasadb

a USDA-ARS, Plant Science and Entomology Research Unit, 4008 Throckmorton Hall, Manhattan, KS 66506
b Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506

* Corresponding author (zoran.ristic{at}gmprc.ksu.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Determining mechanisms associated with heat tolerance and identifying screening methods are vital for improvement of heat tolerance in plants. The objectives of this study were to investigate the relationship between the heat stability of thylakoids and loss of chlorophyll in winter wheat (Triticum aestivum L.) under heat stress, and to examine whether chlorophyll loss can be used as an indicator of heat tolerance in wheat. We assessed heat tolerance and measured chlorophyll content in 12 cultivars of winter wheat at flowering stage during exposure to 16-d-long heat stress. Heat tolerance was assessed using fluorescence to determine the heat stability of thylakoids, and chlorophyll content was measured with a chlorophyll meter. Experiments were conducted under controlled conditions. Heat stress caused damage to thylakoids in all cultivars as indicated by the increase in the ratio of constant fluorescence (O) and the peak of variable fluorescence (P). Heat stress also caused a decline in chlorophyll content in most cultivars. A strong negative correlation between heat-induced increases in O/P and chlorophyll content was seen. The results suggest that heat-induced damage to thylakoids and chlorophyll loss are closely associated in winter wheat. Measurements of chlorophyll content with a chlorophyll meter could be useful for high throughput screening for heat tolerance in wheat.

Abbreviations: O/P, ratio of constant fluorescence and peak of variable fluorescence • PS II, photosystem II • TTC, triphenyl tetrazolium chloride

Correlation between Heat Stability of Thylakoid Membranes and Loss of Chlorophyll in Winter Wheat under Heat Stress

Zoran Ristica,*, Urska Bukovnikb and P.V. Vara Prasadb

a USDA-ARS, Plant Science and Entomology Research Unit, 4008 Throckmorton Hall, Manhattan, KS 66506
b Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506

* Corresponding author (zoran.ristic{at}gmprc.ksu.edu).

Determining mechanisms associated with heat tolerance and identifying screening methods are vital for improvement of heat tolerance in plants. The objectives of this study were to investigate the relationship between the heat stability of thylakoids and loss of chlorophyll in winter wheat (Triticum aestivum L.) under heat stress, and to examine whether chlorophyll loss can be used as an indicator of heat tolerance in wheat. We assessed heat tolerance and measured chlorophyll content in 12 cultivars of winter wheat at flowering stage during exposure to 16-d-long heat stress. Heat tolerance was assessed using fluorescence to determine the heat stability of thylakoids, and chlorophyll content was measured with a chlorophyll meter. Experiments were conducted under controlled conditions. Heat stress caused damage to thylakoids in all cultivars as indicated by the increase in the ratio of constant fluorescence (O) and the peak of variable fluorescence (P). Heat stress also caused a decline in chlorophyll content in most cultivars. A strong negative correlation between heat-induced increases in O/P and chlorophyll content was seen. The results suggest that heat-induced damage to thylakoids and chlorophyll loss are closely associated in winter wheat. Measurements of chlorophyll content with a chlorophyll meter could be useful for high throughput screening for heat tolerance in wheat.

Abbreviations: O/P, ratio of constant fluorescence and peak of variable fluorescence • PS II, photosystem II • TTC, triphenyl tetrazolium chloride


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
ASSESSMENT OF HEAT tolerance is of primary importance in breeding programs designed to improve heat tolerance in crop plants. Several methods and approaches are available with the most common being electroconductivity, chlorophyll a fluorescence, and triphenyl tetrazolium chloride (TTC) staining. Electroconductivity measures electrolyte leakage from tissues subjected to elevated temperatures, thus, it estimates the heat stability of the plasma membrane (Sullivan, 1972; Blum and Ebercon, 1981; Ibrahim and Quick, 2001). Chlorophyll a fluorescence assesses damage to photosystem II (PS II) and thylakoid membranes caused by heat (Krause and Weis, 1984; Ristic and Cass, 1993; Maxwell and Johnson, 2000; Sayed, 2003), and TTC evaluates the mitochondrial electron transport chain (Chen et al., 1982; Krishnan et al., 1989; Fokar et al., 1998).

Despite their reliability and common use, electroconductivity, TTC staining, and chlorophyll a fluorescence have some limitations. Electroconductivity and TTC have limited applications because of the amount of labor involved in variable field conditions. Similarly, measurements of chlorophyll a fluorescence require use of expensive instrumentation and in some cases necessitates dark adaptation of the leaf tissue, which limits the number of plants that can be screened in a given day. Therefore, there is a need to develop more efficient, less expensive alternatives for high throughput screening for heat tolerance.

Thylakoid membranes and PS II are considered the most heat-labile cell structures (Santarius, 1974; Schreiber and Berry, 1977). In wheat (Triticum aestivum L.) and related species, for example, thylakoids are more affected than the chloroplast envelope, stromal enzymes, or the integrity of cell compartments (Thebud and Santarius, 1982; Monson et al., 1982; Kobza and Edwards, 1987; Sayed et al., 1989; Al-Khatib and Paulsen, 1989). Thylakoids harbor chlorophyll, a portion of which is associated with the proteins of PS II (Schreiber and Berry, 1977; Vacha et al., 2007). Damage to thylakoids caused by heat could be, therefore, expected to lead to chlorophyll loss. Indeed, heat-induced damage to thylakoid membranes and chlorophyll loss have been observed in many crop plants including wheat (Reynolds et al., 1994; Fokar et al., 1998; Al-Khatib and Paulsen, 1984). However, to our knowledge, the relationship between chlorophyll loss and damage to thylakoids has not been clearly established.

The objectives of this study were to (i) investigate the relationship between the heat stability of thylakoid membranes and the loss of chlorophyll in winter wheat under heat stress conditions, and (ii) to test the possibility of using chlorophyll loss, as determined by a chlorophyll meter (Fanizza et al., 1991; Reynolds et al., 1998), as an indicator of heat tolerance in wheat. We assessed thylakoid stability using fluorescence and measured chlorophyll content in 12 cultivars of winter wheat under heat stress conditions.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Plant Material and Experimental Conditions
Seeds of 12 cultivars of winter wheat were obtained from the Institute of Field and Vegetable Crops, Novi Sad, Serbia (for cultivar names see Fig. 1 ). Two independent experiments were conducted under controlled environment conditions in the spring of 2006. For each experiment, seeds of each cultivar were germinated in 4-cm-deep trays containing commercial Metro Mix 200 potting soil (Hummert Int., Topeka, KS) in a greenhouse. Ten-day-old seedlings were vernalized at 4°C for 8 wk. Following vernalization, seedlings of each cultivar were transplanted into 10 pots (three seedlings per pot; pot diameter at the top and the bottom was 21 and 16 cm, respectively; pot depth 20 cm) containing Metro Mix 200 potting soil. Plants were grown in a greenhouse and watered daily. Miracle Gro fertilizer (24–8–16; Stern's Miracle-Gro Products, Inc., Port Washington, NY) was added (according to manufacturer instructions) once every 7 d during the entire duration of the experiment. At the beginning of flowering stage (50% of the plants at growth stage Feekes 10.5.1 [Large, 1954]), plants of each cultivar were divided into control (five pots) and high-temperature treatment (five pots) groups. In each group, five plants were randomly selected (one plant per pot) and one flag leaf per selected plant was randomly chosen and tagged (total of five flag leaves per group were tagged). The tagged flag leaves were later used for assessment of damage to thylakoid membranes and measurement of chlorophyll content. The control group was maintained under growth conditions in a greenhouse, and the treatment group was exposed to heat stress for 16 d (day/night temperature, 36/30°C; relative humidity, 90–100%; and photoperiod, 16/8 h; photosynthetic photon flux, 280 µmol m–2 s–1 [Sylvania cool white fluorescent lamps, Radiant Lamp Co., Jacksonville, FL]) in a growth chamber (Conviron, Model CMP4030, Winnipeg, MB). For each cultivar, heat treatment started when 50% of the plants reached growth stage Feekes 10.5.1 (Large, 1954). Air temperatures, relative humidity, and light levels were continuously monitored at 10-min intervals during the entire period of experimentation in the growth chamber. In the greenhouse data on air temperatures were measured at hourly intervals (the average daily temperature in the greenhouse was 22.7 ± 2.8°C). To minimize or avoid possible dehydration of the leaf tissue during heat treatment, all the pots including controls were kept in trays containing approximately 1-cm-deep water and irrigation was provided every day as necessary. Plants were assessed for damage to thylakoid membranes and PS II and chlorophyll loss after 0, 8, 10, 12, 14, and 16 d of heat stress treatment.


Figure 1
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  		Figure 1. (A) The ratio of constant fluorescence and the peak of variable fluorescence (O/P) and (B) chlorophyll content in flag leaves from 12 cultivars of winter wheat under heat stress conditions. Chlorophyll a fluorescence and chlorophyll contents were measured on the same flag leaves after 0, 8, 10, 12, 14, and 16 d of exposure to heat stress. Increases in O/P indicate damage to thylakoid membranes, the greater the damage the lower the tolerance to heat stress (Ristic and Cass, 1993). Plotted data are from Experiment 1. Bars indicate ±standard errors; n = 5. Similar results were observed in a duplicate experiment.

 
Assessment of Damage to Thylakoid Membranes and Measurement of Chlorophyll Content
Damage to thylakoid membranes was assessed by measuring chlorophyll a fluorescence on intact flag leaves after 1 h of dark adaptation (Ristic and Cass, 1993). Fluorescence was measured halfway between the base and the tip of the blade of the flag leaf. The ratio of constant fluorescence to the peak of variable fluorescence, O/P, was used to assess damage to thylakoid membranes (Krause and Weis, 1984; Ristic and Cass, 1993). Fluorescence measurements were conducted at room temperature (25°C) using a pulse modular fluorometer (Model OS5-FL, Opti-Sciences, Hudson, NH). Data were analyzed using two approaches: (i) data from five replicate plants were averaged and used for statistical analysis, and (ii) individual data were used for statistical evaluation.

Chlorophyll content was measured in the same flag leaves, in the same blade area that was used for fluorescence measurements using a self-calibrating SPAD chlorophyll meter (Model 502, Spectrum Technologies, Plainfield, IL). Five flag leaves per treatment (control and heat stress) were used for measurements of each cultivar. Data from five replicates were used for statistical analysis using the approach described for fluorescence data.

Statistical Analysis
Correlation analysis was used to test the relationship between heat-induced damage to thylakoid membranes and loss of chlorophyll. Data from two independent experiments were analyzed in two different ways: (i) data from each experiment were analyzed separately, and (ii) data from two experiments were averaged and used for correlation analysis. PROC CORR procedures in Statistical Analysis System (SAS Institute, 2003) were used to quantify the relationship between the variables. Similarly, the effects of heat stress and cultivars on chlorophyll a fluorescence and chlorophyll content were analyzed using PROC ANOVA in SAS with five replications.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
We assessed heat tolerance in 12 cultivars of winter wheat by estimating damage to thylakoid membranes using chlorophyll a fluorescence. Heat stress caused damage to thylakoids in all wheat cultivars as indicated by increases in O/P (Fig. 1A). However, cultivars differed in the extent of the damage. The greatest damage, indicating lowest tolerance to heat stress, was seen in cultivars Zlatka, Stepa, and Rana Niska (O/P > 520% after 16 d of heat stress). Relatively little increase in O/P (O/P < 175% after 16 d of heat stress) was seen in cultivars Proteinka, Ljiljana, Partizanka, Stamena, and Jefimija (Fig. 1A). We speculate that the observed differences in heat tolerance of cultivars may be partly due to possible differential expression of a highly conserved (Bhadula et al., 2001) chloroplast protein, elongation factor EF-Tu. This protein has been shown to play a role in heat tolerance by acting as a molecular chaperone (Rao et al., 2004), and our recent study showed that wheat cultivars that display greater tolerance to heat stress express higher levels of EF-Tu under heat stress conditions (Ristic et al., 2006).

Heat stress also affected chlorophyll content in these wheat cultivars. Under control conditions no significant changes in chlorophyll content were observed (data not shown). However, under heat stress conditions all cultivars, except Ljiljana, showed progressive loss of chlorophyll over time (Fig. 1B). Cultivars that showed chlorophyll loss differed in their ability to retain chlorophyll under heat stress (Fig. 1B). As indicated by chlorophyll content after 16 d of heat stress, the greatest loss of chlorophyll (>75%) was observed in cultivars Zlatka, Stepa, and Rana Niska, and the least amount of loss (<20%) in cultivars Proteinka, Partizanka, Stamena, and Jefimija. The cultivar differences in chlorophyll loss seen in our study are consistent with Wardlaw et al. (1980) and Blum (1986) who demonstrated the presence of genetic variability in chlorophyll content in wheat cultivars when exposed to heat stress.

Chlorophyll loss naturally occurs in plants undergoing senescence (Thimann, 1987), and it can also prematurely occur in plants experiencing heat stress (Reynolds et al., 1994; Fokar et al., 1998; Al-Khatib and Paulsen, 1984). In our experiments, control plants of all wheat cultivars did not show any significant changes in chlorophyll content or senescence. Therefore, it is likely that the chlorophyll loss in our heat-stressed plants was primarily due to the effects of high temperature rather than natural senescence. The mechanism by which high temperature may have caused chlorophyll loss is, however, unclear. Al-Khatib and Paulsen (1984) and Harding et al. (1990) have suggested that a major effect of high temperature on wheat is acceleration of senescence, which is manifested by an increase in the activity of proteolytic enzymes leading to protein degradation and chlorophyll loss. We speculate that this may be the case in our study. Alternatively, chlorophyll loss in these wheat cultivars may be a consequence of heat-induced damage to thylakoid membranes and PS II. Further studies are needed to elucidate the mechanism(s) of chlorophyll loss in wheat under heat stress conditions.

We analyzed the relationship between chlorophyll content and damage to thylakoid membranes. This analysis was done by expressing chlorophyll content in heat-stressed plants in two different ways and plotting it against O/P. First, we expressed chlorophyll content in heat-stressed plants as a percentage of that in control plants (no heat stress). Chlorophyll content in heat-stressed plants was also expressed as a percentage of the chlorophyll content in the same plants before the beginning of heat stress treatment (Day 0 of heat stress). The chlorophyll content in heat-stressed plants was expressed in two different ways to test the possibility of using chlorophyll content at the beginning of heat stress treatment as a control. This would be useful for measurements of chlorophyll content under field conditions where environmental factors including temperature are highly variable, making it difficult to have control plants that do not experience heat stress. In both cases, a highly significant negative linear correlation (P < 0.0001) between chlorophyll content and O/P was observed (Fig. 2 and 3 , Tables 1 and 2 ). This correlation was evident in two independent experiments when data were plotted and analyzed for each individual day of heat stress (Fig. 2 and Table 1) as well as when data for all days of stress treatment (Days 8–16) were plotted collectively (Fig. 3 and Table 2). In addition, this correlation was also evident when both averages from five replicate plants (Fig. 2 and 3, and Table 1) and individual data (Table 2) were used for correlation analysis.


Figure 2
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  		Figure 2. Correlation between the ratio of constant fluorescence and the peak of variable fluorescence (O/P) and chlorophyll content expressed as percentage of control (plants not exposed to heat) in the flag leaf from 12 cultivars of winter wheat. Data from five replicate plants of each cultivar were averaged and used for correlation analysis. Plotted data are from Experiment 1; n = 12. Similar results were obtained in a duplicate experiment. HS, heat stress.

 

Figure 3
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  		Figure 3. Correlation between the ratio of constant fluorescence and the peak of variable fluorescence (O/P) and chlorophyll content in the flag leaves of 12 cultivars of winter wheat. Data represent an average of two experiments in which in each experiment data from five replicate plants of each cultivar were averaged. (A) The chlorophyll content in heat-stressed plants, measured on Days 8, 10, 12, 14, and 16 of heat treatment, was expressed as percentage of that in control (plants not exposed to heat stress). (B) The chlorophyll content in heat-stressed plants, measured on Days 8, 10, 12, 14, and 16 of heat treatment, was expressed as percentage of chlorophyll content measured in the same plants at the beginning of heat stress (Day 0 of heat stress). Data for all days of heat stress treatment when fluorescence and chlorophyll content were measured are plotted on the same graph (n = 60).

 

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  		Table 1. Correlation coefficients of the relationship between the ratio of constant fluorescence and the peak of variable fluorescence (O/P) and chlorophyll content in wheat under heat stress in two separate experiments when chlorophyll content is expressed as percentage of chlorophyll content measured in the same plants at the beginning of heat stress (Day 0 of heat stress). Data from five replicate plants of each cultivar were averaged and used for correlation analysis.

 

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  		Table 2. Correlation coefficients of the relationship between the ratio of constant fluorescence and the peak of variable fluorescence (O/P) and chlorophyll content in 12 cultivars of winter wheat under heat stress. O/P is expressed as percentage of that in control (plants not exposed to heat stress).{dagger}

 
The observed correlation between chlorophyll content and O/P suggests that under heat stress conditions loss of chlorophyll and damage to thylakoid membranes are closely associated. Moreover, this correlation also suggests that chlorophyll loss under heat stress can be used to indicate heat tolerance and that measurements of chlorophyll content using a chlorophyll meter will be useful as a method for high throughput screening for heat tolerance in wheat. This is particularly useful due to the relatively low cost of the chlorophyll meter compared with fluorometers. In addition, there is no need for dark adaptation of plants before measurement.

This study revealed a quantitative relationship between the unitless SPAD chlorophyll meter readings and the physiological state of thylakoid membranes, as determined by chlorophyll a fluorescence. Such a result is critical for using SPAD meter readings to indicate thermotolerance. Wheat cultivars that lose less chlorophyll under heat stress, as determined by SPAD, can thus be expected to be more heat tolerant than cultivars that lose more chlorophyll. The ability of a plant to retain chlorophyll under stress is generally known as the "stay-green trait" (Reynolds et al., 1997; Thomas and Howarth, 2000), and the identification of plants displaying this trait could aid in producing new wheat cultivars with improved tolerance to heat stress.

Our study also revealed that chlorophyll content at the beginning of heat stress could be used as a "control" for determination of chlorophyll loss and assessment of heat tolerance. As stated earlier, this may be of particular importance under field conditions where it is difficult to have plants that do not experience heat stress. The exact timing (beginning and duration) of chlorophyll content measurements is difficult to predict; however, it is reasonable to suggest that initial measurements should be taken when wheat begins to experience temperatures that are generally considered as heat stress temperatures for wheat (≥28–32°C) (Mullarkey and Jones, 2000). Wheat may experience heat stress and suffer injury during vegetative or reproductive phases depending on the location and season (Kolderup, 1979), but most commonly it encounters stress in the later part of the growing season (Wardlaw et al., 1989), during flowering. Hence, in most cases measurements of chlorophyll content under heat stress conditions could begin at the beginning of flowering and continue thereafter for 7 to 21 d. Heat-induced chlorophyll loss will probably depend on environmental conditions or field location and wheat heat tolerance. If field air temperature is not sufficiently high or the high temperatures do not last for a prolonged period of time, loss of chlorophyll may not be observed. Therefore, it is important that measurements of chlorophyll content and assessment of heat tolerance are conducted in hot environments.

In summary, our study revealed a highly significant negative linear correlation between chlorophyll content and damage to thylakoid membranes and PS II in winter wheat under heat stress. Cultivars of wheat that suffered more damage to thylakoid membranes (displayed lower tolerance to heat stress) under heat stress lost more chlorophyll than cultivars that suffered less damage (displayed higher tolerance to heat stress). The results suggest that loss of chlorophyll under heat stress, as determined with a chlorophyll meter, could be used as a reliable and high throughput approach or method for screening for heat tolerance in wheat.


   ACKNOWLEDGMENTS
 
The authors are grateful to Dr. Novica Mladenov and Dr. Radivoje Jevtic, Institute of Field and Vegetable Crops, Novi Sad, Serbia, for generously providing seeds of cultivars of winter wheat. The authors are also grateful to Dr. Thomas Elthon, University of Nebraska, Lincoln, NE; Dr. David Horvath, USDA-ARS, Fargo, ND; Dr. Jeffrey Pedersen, USDA-ARS, Lincoln, NE; and Dr. Kassim Al-Khatib, Kansas State University, Manhattan, KS, for suggestions on the manuscript. This publication is approved as Kansas Agriculture Experiment Station No. 07-90-J. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture, and does not imply its approval to the exclusion of other products that may also be suitable.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication October 20, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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Z. Ristic, U. Bukovnik, P. V. V. Prasad, and M. West
A Model for Prediction of Heat Stability of Photosynthetic Membranes
Crop Sci., July 1, 2008; 48(4): 1513 - 1522.
[Abstract] [Full Text] [PDF]

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