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Effect of prior heat stress on protochlorophyll and chlorophyll formation in seedlings of Colocynthis citrullus
University of lfe, lIe-lfe, Nigeria
Effect of Prior Heat Stress on Protochlorophyll and Chlorophyll Formation in Seedlings of Colocgnthis citrullus A.
O. LAWANSON
and I. C.
ONWUEME
With 2 figures Received February 19, 1973
Summary When etiolated seedlings of Colocynthis citrullus were heat-stressed at 40° C for 16 h in darkness and then illuminated, there was an initial retardation in the formation of protochlorophyll and in the rate of conversion of protochlorophyll to chlorophyll. It is suggested that these effects account for the retardation in the rate of chlorophyll accumulation previously reported in heat-stressed seedlings.
Introduction
When etiolated seedlings of Colocynthis citrullus are subjected to high temperatures in darkness and are subsequently illuminated, the rate of chlorophyll accumulation is retarded (ONWUEME and LAWANSON, 1973). Such a retardation may be due, amongst other things, to a decreased rate of protochlorophyll formation during or after heat stress; or to a decreased rate of conversion of protochlorophyll to chlorophyll during subsequent illumination. This paper reports investigations carried out to test these hypotheses. Materials and Methods Seeds of watermelon (Colocynthis citrullus cv. Bara) were germinated in darkness at 25° C on several layers of moist tissue paper in Petri dishes kept in a controlled-environment chamber. The resulting seedlings were daily supplied with tap water. On the 11 th day after sowing, one lot of seedlings was subjected to a heat stress of 40° C for 16 hours in darkness. The seedlings were then returned to 25° C and illuminated at 450 lux supplied by small fluorescent tubes. At intervals after the onset of illumination cotyledons were harvested randomly from each treatment and used for protochlorophyll analysis. Pigment extraction was done as previously described (ONWUEME and LAWANSON, 1973). Total protochlorophyll (assuming no conversion to chlorophyll), and the amount of protochlorophyll transformed to chlorophyll were estimated by the method of SMITH et al. (1954). From the values thus obtained the rate of transformation of protochlorophyll to chlorophyll was calculated.
Z. Pjlanzenphysiol. Bd. 69. S. 461-463. 1973.
462
A.
O. LAWANSON
and I. C.
ONWUEME
Results and Discussion Prior heat stress resulted in an initial decrease in the total amount of protochlorophyll formed i.e. amount present, if no conversion to chlorophyll occurred (Fig. 1). However, the amount of protochlorophyll formed in the stressed plants approached that in the unstressed plants approximately 84 hours after the cessation of stress. This indicates that the stressed plants recovered from the retarding effect of a prior heat stress on subsequent protochlorophyll formation. The retention of the capacity for recovery suggests that the stress was not lethal. A similar recovery from the effect of heat stress has been reported for nitrate reductase activity in barley (ONWUEME, 1969).
4·5 4·2 3·9
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20
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PERIOD
40
50
60
0 F ILLUMINATION (hours)
70
80
90
Fig. 1: Time-course of protochlorophyll formation (if no conversion to chlorophyll occurred) in cotyledons of heat-stressed and unstressed watermelon seedlings.
The rate of conversion of protochlorophyll to chlorophyll was initially low in the stressed plants being approximately half of that in the unstressed plants (Fig. 2). During the first 7 hours of post-stress illumination, the rate of conversion increased in the unstressed plants and thereafter began to decrease. In the stressed plants the rate of conversion remained constant for 13 hours of post-stress illumination, increased to a maximum during the following 17 hours, and thereafter decreased. However, after approximately 40 hours of illumination, the rate of conversion remained higher
Z. Pjlanzenphysiol. Bd. 69. S. 461-463. 1973.
Heat Stress and Protochlorophyll and Chlorophyll Formation
463
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PERIOD OF ILLUMINATION (hours)
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Fig. 2: Rate of conversion of protochlorophyll to chlorophyll during illumination of previously heat-stressed and unstressed seedlings of water melon.
in the stressed plants than in the unstressed. This is another indication of the ability of the plants to recover from the effects of heat stress. The maximum rates of conversion were attained in the unstressed and stressed plants at 7 hours and 30 hours of illumination, respectively. This probably explains the prolonged lag in chlorophyll accumulation following heat stress (ONWUEME and LAWANSON, 1973), especially since no lag was observed in protochlorophyll formation in the stressed plants (Fig. 1). During heat stress, there was no discernible change in the amount of protochlorophyll. It can therefore be concluded that the decreased formation of protochlorophyll and the decreased initial rate of conversion of protochlorophyll to chlorophyll following heat stress account for the retardation in chlorophyll accumulation previously reported (ONWUEME and LAWANSON, 1973). References ONWUEME, 1. c.: Ph. D. Thesis, Univ. of California, Davis, USA (1969). ONwuEME,1. C., and A. o. LAWANSON: Planta (Berl.) 110,81-84 (1973). SMITH, J. H. C., and A. BENITEZ: Plant Physiol. 29, 135-143. A. O. LAWANSON, Department of Biological Sciences University of Ife, Ile-Ife, Nigeria. 1. C. ONWUEME, Department of Plant Science University of Ife, Ile-Ife, Nigeria.
Z. Pjlanzenphysiol. Bd. 69. S. 461-463. 1973.
Effect of Prior Heat Stress on Protochlorophyll and Chlorophyll Formation in Seedlings of Colocgnthis citrullus A.
O. LAWANSON
and I. C.
ONWUEME
With 2 figures Received February 19, 1973
Summary When etiolated seedlings of Colocynthis citrullus were heat-stressed at 40° C for 16 h in darkness and then illuminated, there was an initial retardation in the formation of protochlorophyll and in the rate of conversion of protochlorophyll to chlorophyll. It is suggested that these effects account for the retardation in the rate of chlorophyll accumulation previously reported in heat-stressed seedlings.
Introduction
When etiolated seedlings of Colocynthis citrullus are subjected to high temperatures in darkness and are subsequently illuminated, the rate of chlorophyll accumulation is retarded (ONWUEME and LAWANSON, 1973). Such a retardation may be due, amongst other things, to a decreased rate of protochlorophyll formation during or after heat stress; or to a decreased rate of conversion of protochlorophyll to chlorophyll during subsequent illumination. This paper reports investigations carried out to test these hypotheses. Materials and Methods Seeds of watermelon (Colocynthis citrullus cv. Bara) were germinated in darkness at 25° C on several layers of moist tissue paper in Petri dishes kept in a controlled-environment chamber. The resulting seedlings were daily supplied with tap water. On the 11 th day after sowing, one lot of seedlings was subjected to a heat stress of 40° C for 16 hours in darkness. The seedlings were then returned to 25° C and illuminated at 450 lux supplied by small fluorescent tubes. At intervals after the onset of illumination cotyledons were harvested randomly from each treatment and used for protochlorophyll analysis. Pigment extraction was done as previously described (ONWUEME and LAWANSON, 1973). Total protochlorophyll (assuming no conversion to chlorophyll), and the amount of protochlorophyll transformed to chlorophyll were estimated by the method of SMITH et al. (1954). From the values thus obtained the rate of transformation of protochlorophyll to chlorophyll was calculated.
Z. Pjlanzenphysiol. Bd. 69. S. 461-463. 1973.
462
A.
O. LAWANSON
and I. C.
ONWUEME
Results and Discussion Prior heat stress resulted in an initial decrease in the total amount of protochlorophyll formed i.e. amount present, if no conversion to chlorophyll occurred (Fig. 1). However, the amount of protochlorophyll formed in the stressed plants approached that in the unstressed plants approximately 84 hours after the cessation of stress. This indicates that the stressed plants recovered from the retarding effect of a prior heat stress on subsequent protochlorophyll formation. The retention of the capacity for recovery suggests that the stress was not lethal. A similar recovery from the effect of heat stress has been reported for nitrate reductase activity in barley (ONWUEME, 1969).
4·5 4·2 3·9
~ 3·6 >-
'0
~3.3
"0 3 .0 .~ ] 2·7
::: 2·4
>
x
~
2·1
~
1·8
b
1·5
a:: o u o
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u.. 1·2
o
.- o·g z ::::>
~ 0·6
«
0·3
10
20
30
PERIOD
40
50
60
0 F ILLUMINATION (hours)
70
80
90
Fig. 1: Time-course of protochlorophyll formation (if no conversion to chlorophyll occurred) in cotyledons of heat-stressed and unstressed watermelon seedlings.
The rate of conversion of protochlorophyll to chlorophyll was initially low in the stressed plants being approximately half of that in the unstressed plants (Fig. 2). During the first 7 hours of post-stress illumination, the rate of conversion increased in the unstressed plants and thereafter began to decrease. In the stressed plants the rate of conversion remained constant for 13 hours of post-stress illumination, increased to a maximum during the following 17 hours, and thereafter decreased. However, after approximately 40 hours of illumination, the rate of conversion remained higher
Z. Pjlanzenphysiol. Bd. 69. S. 461-463. 1973.
Heat Stress and Protochlorophyll and Chlorophyll Formation
463
1S0-y----------------------------., 140 -::-130 t120 >-
-i; 110
~
~100 ~
-
z
90
~
80
o
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}l
>
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I
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/
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\
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/;:1
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10 10
20
30
40
SO
PERIOD OF ILLUMINATION (hours)
60
70
80
Fig. 2: Rate of conversion of protochlorophyll to chlorophyll during illumination of previously heat-stressed and unstressed seedlings of water melon.
in the stressed plants than in the unstressed. This is another indication of the ability of the plants to recover from the effects of heat stress. The maximum rates of conversion were attained in the unstressed and stressed plants at 7 hours and 30 hours of illumination, respectively. This probably explains the prolonged lag in chlorophyll accumulation following heat stress (ONWUEME and LAWANSON, 1973), especially since no lag was observed in protochlorophyll formation in the stressed plants (Fig. 1). During heat stress, there was no discernible change in the amount of protochlorophyll. It can therefore be concluded that the decreased formation of protochlorophyll and the decreased initial rate of conversion of protochlorophyll to chlorophyll following heat stress account for the retardation in chlorophyll accumulation previously reported (ONWUEME and LAWANSON, 1973). References ONWUEME, 1. c.: Ph. D. Thesis, Univ. of California, Davis, USA (1969). ONwuEME,1. C., and A. o. LAWANSON: Planta (Berl.) 110,81-84 (1973). SMITH, J. H. C., and A. BENITEZ: Plant Physiol. 29, 135-143. A. O. LAWANSON, Department of Biological Sciences University of Ife, Ile-Ife, Nigeria. 1. C. ONWUEME, Department of Plant Science University of Ife, Ile-Ife, Nigeria.
Z. Pjlanzenphysiol. Bd. 69. S. 461-463. 1973.