Environmental and Experimental Botany 44 2000 243 – 255
Gas-exchange response and stomatal and non-stomatal limitations to carbon assimilation of sunflower under
salinity
P. Steduto
a,
, R. Albrizio
a
, P. Giorio
b
, G. Sorrentino
b
a
CIHEAM-IAMB, 6ia Ceglie
9
,
70010
Valenzano, Bari, Italy
b
C.N.R.-ISPAIM, 6ia Cupa Patacca
85
,
80056
Ercolano, Napoli, Italy Received 28 April 2000; received in revised form 27 July 2000; accepted 27 July 2000
Abstract
Sunflower Helianthus annuus was grown in both open-field and outdoor potted conditions in Southern Italy, and irrigated with water having electrical conductivity ranging between 0.9 and 15.6 dS m
− 1
obtained by different NaCl concentrations. The aim of the work was to study the leaf area and photosynthetic responses of sunflower to mild salt
stress. The response curve Ac
i
of assimilation A to leaf internal CO
2
concentration c
i
was used to determine leaf gas-exchange parameters, in order to evaluate stomatal and non-stomatal limitations to photosynthesis in relation to
salt stress. In the field, a reduction of 19 in leaf area expansion occurred, while no correlation was observed between C
l
and stomatal conductance to water vapour g
sw
ranging between 0.76 and 1.35 mol m
− 2
s
− 1
. This result was also evident at a higher salinity level reached in the pot experiment where leaf osmotic potential c
s
varied from − 1.35 to − 2.67 MPa as compared with the field experiment, where c
s
ranged from − 1.15 to − 1.42 MPa. Considering the two experiments as a unique data set, the assimilation rate, the stomatal conductance to CO
2
g
sc
and the sensitivity of A to c
i
variation g were not significantly influenced by salinity in the whole range of c
s
. As a consequence, the stomatal and non-stomatal limitations to photosynthesis were not affected by salt treatment, averaging around 20 and
80, respectively. The variation in A from 44 to 29 mmol m
− 2
s
− 1
was paralleled by the variation in g
sc
from 0.47 to 0.84 mol m
− 2
s
− 1
, with a remarkable constancy of both c
i
200 9 12.5 mmol mol
− 1
and normalized water-use efficiency 5 9 0.7 mmol mmol
− 1
kPa, showing the optimal behaviour of the plant processes. These findings indicate that, under mild salt stress, the same as observed under water deficit, sunflower controls assimilation mainly by
modulating leaf area rather than by stomatal closure, and that non-stomatal limitation of photosynthesis was not affected at all by the level of salinity reached in this study. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Leaf water potential; LAI; Photosynthesis; Helianthus annuus www.elsevier.comlocateenvexpbot
1. Introduction
Most of the studies reported in the litera- ture concerning sunflower response to saline
Corresponding author. Tel.: + 39-80-4606224; fax: + 39- 80-4606201.
E-mail address
:
stedutoiamb.it P. Steduto. S0098-847200 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 9 8 - 8 4 7 2 0 0 0 0 0 7 1 - X
conditions refer to the classical approach of Maas and Hoffman 1977, relating final yield to salinity
of the soil water extract for example, Francois, 1996. The works that explained the causes of
reduced growth and yield through investigations on leaf area expansion and gas-exchange refer to
water stress, but not to salt stress under field conditions. Rawson and Munns 1984 showed
the high sensitivity of leaf growth to salinity con- ditions for potted sunflower plants. They observed
a leaf area reduction of about 50 with 5 dS m
− 1
electrical conductivity in the nutrient solution. Giorio et al. 1996 also noticed the high sensitiv-
ity of leaf area expansion to salinity in open field experiments, in response to small reductions in
total leaf water potential. Such ‘phenotypic plas- ticity’ in modulating leaf area in response to small
reductions in leaf water potential represents one of the most relevant adaptive properties of
sunflower Connor and Sadras, 1992.
In addition to leaf area expansion the ‘sink- size’ for carbon assimilation, productivity de-
pends also on the carboxylation capacity per unit leaf area the ‘sink-intensity’ for carbon assimila-
tion, compounded by stomatal and non-stomatal contributions to photosynthesis. Most of the stud-
ies on stomatal and non-stomatal components in sunflower refer to water stress conditions but
report conflicting results Lawlor, 1995. While some authors underline stomatal limitation as the
main cause of carbon assimilation reduction Plesnicar et al., 1995, others attribute the de-
crease in photosynthetic rate to non-stomatal lim- itation Gimenez et al., 1992; Tezara and Lawlor,
1995.
Under salinity conditions, the majority of the works dealing with stomatal and non-stomatal
limitation to photosynthesis concern various crops but sunflower. In bean, a salt-sensitive spe-
cies, the reduction in assimilation was found to be mostly due to stomatal limitation Brugnoli and
Lauteri, 1991, and to both stomatal and non- stomatal limitations Seemann and Critchley,
1985. Among other salt-sensitive species, Bethke and Drew 1992 on pepper and Chartzoulakis et
al. 1995 on kiwi ascribed the observed reduction in photosynthesis to non-stomatal limitation.
These conflicting results are also found for more salt-tolerant species. For instance, Brugnoli and
Lauteri 1991 indicated that stomata played the major role in limiting photosynthesis of cotton,
whereas Dunn and Neales 1993 suggested that non-stomatal components limited the photosyn-
thesis of barley. All these works were conducted in a controlled environment with a variable range
of boundary conditions and thus resulting in a low degree of comparability. Moreover, no infer-
ences could be drawn for open-field conditions, where the degree of stress development, intensity
and duration is such that plants can activate adaptation mechanisms for adjusting to the
changing environment Lawlor, 1995.
Concerning sunflower grown in the field, no information is available on the stomatal and non-
stomatal limiting components of photosynthesis under salinity, with few studies confined to inves-
tigation on stomatal response. Katerji et al. 1994 showed a decrease in stomatal conductance with
increasing salt concentration of irrigation water of potted sunflower plants, while Giorio et al. 1996
found no response of stomatal conductance with salinity of plants grown in the field.
From that already reported, our hypothesis is that if leaf area modulation remains the most
important stress avoidance mechanism under salinity and if stomatal closure is of less signifi-
cance, non-stomatal limitations to photosynthesis should be the least or not at all induced by
salinity. This hypothesis would be consistent with the
optimization theory
of plant
processes Cowan, 1982; Givnish, 1986.
Thus, in order to verify it, the present study investigated the effect of gradually developing
salinity stress on leaf expansion, leaf gas-exchange parameters, and stomatal and non-stomatal limi-
tations to carbon assimilation.
2. Materials and methods
