8th INTERNATIONAL WHEAT CONFERENCE

effeCTS of PRogReSSIVe WATeR STReSS oN
PhoToSyNTheSIS IN WheAT (TRITICum AeSTIVum L.)
Ricardo Ferraz de Oliveira, Mariam Sulaiman
and Saulo de Tarso Aidar
Escola Superior de Agricultura “Luiz de Queiroz” – USP. Departamento de Ciências Biológicas.
1348-900, Piracicaba, SP, Brazil.
E-mail Address of presenting author: rfo@esalq.usp.br
For more than half of the world population, wheat (Triticum aestivum L.) is a primary
food source. Of all the cultivated food crops, wheat is among the heaviest ‘water-users’;
30% of the freshwater used in crop cultivation is allocated solely to wheat. Consequently,
water availability is often the limiting factor in wheat production. Water scarcity and
drought are only two consequences that pose as potential threats to our food security.
A rapid, non-invasive technique such as fluorescence measurement can be employed to
monitor growth and performance in response to water deficit. This would allow the determination
of optimal water input for maximum crop yield. Fluorescence studies can also
be applied in crop improvement program, enabling the selection of stress-resistant plants.
The literature is abundant with studies of resurrection plants and their responses to water
stress. Very few investigated the similar effects on irrigated crops such as wheat, especially
up to extreme water stress. The aim of this experiment was to investigate the effects of
progressive water deficit on wheat, accompanying changes in classic parameters (such as
assimilation rate, etc.) and fluorescence kinetics. In the study of plant physiology, photosynthetic
activity is often related to CO 2 assimilation rates (A max ) i.e. the amount of carbon
dioxide being fixed per unit time. A positive rate indicates a net assimilation of CO 2
by plants through photosynthesis. A negative value, on the other hand, could either mean
that the plant stops photosynthesizing or that the amount of CO 2 released through respiration
offset those being fixed. However, it would be imprudent to judge photosynthetic
activity based on solely A max . This is because photosynthesis consists of 2 components: (1)
Light reaction-Conversion of light energy into chemical energy (in the form of reducing
powers, NADPH and ATP) by PSI and PSII. (2) ‘Dark’ reaction-Fixation or reduction of
CO 2 by the enzymes of Calvin Cycle, into carbohydrates. A max only gives a measure of the
latter. To analyze the former, chlorophyll fluorescence ratios and coefficients such as F V /
F M, should be used. F V /F M quantifies the efficiency of Photosystem II in converting light
energy into chemical energy. It depends on the ability of leaves to transfer electrons away
from quinone acceptors of PSII. Our findings indicate that carbon assimilation is more
susceptible to water deficit than photochemical conversion. A 10% decline in RWC leaf was
sufficient to reduce A max to 0. Yet F V /F M was still maintained at 0.7. A possible reason for
this could be the existence of mechanisms that maintain the integrity of the photosynthetic
apparatus. Albeit being the more frequently used Chl fluorescence ratio, precaution
must be taken when using F V /F M to analyze efficiency of photochemical conversion. It
gives the maximum possible efficiency of PSII photochemistry, in dark-adapted leaves.
Whether plants are as effective in the light cannot be ascertained only from this parameter.
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