Abstract:
Light fulfils two significant roles in plant growth. At first, light drives photosynthesis by
providing energy and second it is perceived by several photoreceptors, thus activating signal
pathways. Continuous light changes plant physiology by affecting both roles, thus creating
difficulties in identifying the factors that are responsible for injuries under such treatment.
As far as we know, the effect of continuous light in combination with plant’s nutritional
status or nutritional deficiency on its physiology is poorly studied. In particular, there are no
references to sulfur deficiency.
We have studied three cases of Zea may plants responses to light environment in
combination with nutrition: 1) Response under continuous light (Cc) versus normal
photoperiod under complete nutrient solution (C). 2) Response under sulfur deficiency (-S)
versus complete nutrient solution (C) under normal photoperiod. 3) Response under sulfur
deficiency and continuous light (-Sc) versus normal photoperiod in complete nutrient
solution (C). We followed the effect of the above mentioned cases on the photosystem
antennas and particularly on the extractable levels of Chla, Chlb and carotenoids from leaf
lamina or sheath, by means of dimethyl sulfoxide. The effect of growth was monitored via
fresh mass measurement. Plants were grown for seven days under normal photoperiod
condition and the following procedure included the modulation for each case, which lasted
for 3 weeks. The photon flux density was not modified during this period.
The results of this study showed that the treatments affected the time of organ appearance
as well as their presence itself. The fraction of Chla to Chlb as well as the fraction of
carotenoids to total chlorophyll proved to be useful response indicators to each treatment.
The dynamic of adjustments presented by the sheaths were different than the
corresponding ones presented by the laminas.
Under continuous light and complete nutrition, the influence focused on l
(I stands for
lamina), whilst l
7
did not occur. Sheaths’ appearance was not affected except for S
5
4
(S stands
for sheath). Injuries due to this condition focused on youngest leaf from l
onwards. The laminas’ overall average of Chla:Chlb fraction was 4.1 (an increase by 13.9%),
whilst in sheaths it was 2.7 (decreased by 6.9%). The laminas’ average Car:Chl fraction was
2.1 (decreased by 4.5%), whilst in sheaths it was 3.1 (increased by 10.7%).
A two days delay was observed in laminas l
5
, l
6
, l
7
and sheaths S
2
, S
during the treatment of
sulfur deficiency under normal photoperiod. No injuries were caused in laminas. The
laminas’ average Chla:Chlb fraction was 3.8 (increased by 5.6%), whilst the average one in
sheaths was 2.6 (decreased by 10.4%). In laminas, the average of Car:Chl fraction was 2.5
(increased by 13.6%), whilst in sheaths the corresponding average was 2.9 (increased by
3.6%).
3
With regard to the treatment with continuous light combined with sulfur deficiency, the
appearance of organs took place at the same time as in control plants, with the exception of
, l
4
5
, l
6
5
onwards and S
4
l
7
and S
. This fact indicates that the deficiency eliminated the effect of continuous light.
Aging and collapsing was observed at the oldest organs l
2
6
0
, l
1,
S
0
, S
. In laminas, the average
value of Chla:Chlb fraction was 3.8 (increased by 5.6%), whilst in sheaths the average was
2.7 (decreased by 6.9%). At the end of the experiment, in laminas the average of Car:Chl
fraction was 2.7 (increased by 22.7%), whilst in sheaths the average was 2.8 (as in control
plants).
