Nutrient deficiencies under stress in legumes: An overview - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 55

transport of essential elements, disturb metabolism, and
have an impact on growth and reproduction (Xu & Shi,
2000). The heavy metals accumulated in plants include
lead (Pb), cadmium (Cd), aluminium (Al), selenium
(Se), arsenic (As), mercury (Hg), zinc (Zn) and chro-
mium (Cr).

Environmental stress
(Abiotic, biotic)

3.2.3 heat and cold stresses
Temperature is one of the major determinants of plant
growth and development. Low temperature can be
regarded as one of the most harmful stresses affecting
temperate plants. Heat stress can affect the expression of
certain genes by causing denaturation of certain vital
plant proteins. A prominent effect of high temperature-
induced protein denaturation is loss of membrane
integrity, which affects cellular structure and transport
as well.

Stress response

Resistance Susceptibility Avoidance

3.2.4 Oxidative stress
Environmental stresses lead to the production of
highly reactive substances called reactive oxygen
species (ROS), which interact with a number of cell
molecules and metabolites causing cell damage. Plants
possess both enzymatic as well as non-enzymatic anti-
oxidant defense systems, which counteract the effect
of ROS by scavenging them and protecting the photo-
synthetic and cellular functions (Nobuhiro & Mittler,
2006; Jaleel et al., 2008a, 2008b; Ahmad et al., 2010).
Overproduction of ROS results in oxidative stress
(Ahmad et al., 2010).
Whatsoever the type of stress is, plant follow similar
phases of stress and their consequences on the plant
(figure 3.3). Plants require optimal conditions for
optimal growth and development. Light absorbed by
plant pigments is used in photosynthesis or dissipated
as heat. White light or laser-induced chlorophyll fluo-
rescence of pre-darkened leaves is a very suitable tool
for early stress detection in plants. The fluorescence of
plants consists almost exclusively of the fluorescence of
leaves, which account for the largest above-ground
surface area of plants. A small part of the light energy
absorbed in photosynthesis is lost during the migration
from the antenna pigment to the reaction centres and
is dissipated by a number of non-photochemical
processes including heat and re-emission of small but
easily detectable amounts (2-5% in vivo ) of the
absorbed radiation. This re-emission occurs at longer

Acclimation Senescence Survival

Growth Death

Survival

Figure 3.3 Phases of stress and their consequences on the
plant.

wavelengths in the red and far-red spectral regions,
and is termed chlorophyll fluorescence (ChlF)
(Bradbury & Baker, 1981). The emitted light when
observed through the spectrophotometer gives the
emission spectrum (Figure 3.4).
The spectral reflectance of plants may indicate the
degree of stress imposed by various stressors. The leaf
spectral responses to environmental conditions that
inhibit plant growth generally involve an increased
amount of reflectance in the visible or the infrared
spectra. As a result of decreased absorption of light by
the pigments, reflectance at the visible wavelength is
seen to increase in stressed plants compared to
unstressed plants (Carter, 1993).