Morpho-phenological Responses to Heat Stress
Temperatures 10°C above normal growth temperatures, affect a range of plant processes such as pollen meiosis, pollen germination, ovule development, ovule viability, development of the embryo (Peet et ah, 1998), seedling growth (Hong & Vierling 2001), etc. Successful flower development is critical for production of many agronomic and horticultural crops. High temperature exposures result in floral abortion in many species including Lycopersicon esculentum Mill. (Tomato; Levy et a., 1978; Abdul-Baki, 1991), Capsicum annum L. (pepper; Rylski, 1986; Erickson et ah,2002), Phaseolus vulgaris L. (bean; Konsens et ah, 1991), Vigna unguiculata (L.) Walp. (cowpea; Craufurd et al., 1998), Pisum sativum L. (pea; Guilioni et ah, 1997), Gossypium hirsutum L. (cotton; Reddy et ah, 1992), etc. In addition, a physiological disorder termed ‘blindness’ in roses is due to abortion of the flower at an early stage of development under low irradiance or high temperature conditions.
Physiological Responses of Heat Stress
The high surface area of leaves makes plants most vulnerable to heat damage. A considerable amount of energy is spent by all plants to maintain the cellular temperature. The leaf and its canopy temperature in a plant system depends on transpiration, respiration and carbon dioxide level in leaf tissues. Direct relations with several physiological processes of plants make canopy temperature one of the best parameters for heat tolerance screening.
Under field conditions, when the plants are gradually exposed to heat or drought stress, the early response is the closure of stomata, which is thought to be in response to the migration of abscisic acid (ABA) synthesized in the root. This stomatal response has been linked more closely to the soil moisture content (Tardieu et ah, 1991; Stoll et ah, 2000). Due to heat, soil moisture decreases and water loss through transpiration increases. The first response of virtually all the plants to acute water deficit under heat and drought stress is the closure of their stomata to prevent the transpirational water loss. Closure of stomata may result from direct evaporation of water from the guard cells with no metabolic involvement.
Further evidence indicated that stomatal closure is likely to be mediated by chemical signals traveling from the dehydrating root to shoots. Abscisic acid has been identified (ABA) as one of the chemical signals involved in the regulation of stomatal functioning (Davis and Zang, 1991). Stomata respond directly to the rate of water supply, through, for example, changes in xylem conductance (Salleo et al, 2000; Sperry 2000; Nardini et al, 2001). Stomatal control of water loss has been identified as an early event in plant response to water deficit under field conditions leading to limitation of carbon uptake by the leaves (Chaves, 1991; Comic and Massacci, 1996).
When heat stress is imposed slowly as case under field conditions, a reduction in the biochemical capacity for carbon assimilation and utilization may occur along with restriction in gaseous diffusion. For example, in grapevines grown in the field, CO-, assimilation gets limited to a great extent due to stomatal closure as summer drought progresses. This is accompanied with proportional reduction in the activity of various enzymes of the reductive Calvin cycle (Maroco et al, 2002 and Chaves et al., 2002).
Relative Water Content
Leaf water status is intimately related to several leaf physiological variables, such as leaf turgor, growth, stomatal conductance, transpiration, photosynthesis and respiration. Water content and water potential have been widely used to quantify the effect of heat stress in leaf tissues. Alterations in these parameters occur when plants are exposed to heat stress. Leaf water content is a usefiil indicator of plant homeostasis, since it expresses the relative amount of water present in the plant tissues. On the other hand, water potential measures the energetic status of water inside the leaf cells (Slatyer & Taylor, 1960). Measurements of water content expressed on a tissue fresh or dry weight basis have been mostly replaced by measurements based on the maximum amount of water a tissue can hold. These measurements are referred to as Relative Water Content (Bans, 1968; Boyer, 1968). Relative water content (RWC) is water content of a plant expressed as a function of its water content at foil turgidity. The relative water content (RWC) of a plant tissue is expressed as
Where, FW, DW, and TW are the fresh weight, dry weight and turgid weight of the tissues, respectively.
This index may be useful for determining the plant leaf water status. When water uptake by roots is equal to transpiration, then RWC is about 85 to 95%. Critical RWC (below which tissue death occurs) varies amongst species and tissue types, it may be -50% or less. A decline in RWC represents the severity of dehydration of plant tissues when it experiences water losses due to heat or other abiotic stresses (Morgan et al, 1997).
Using different plant species, various studies have revealed that increasing duration and severity of stress decreased the RWC of plants (Sanchez et al, 2006). Abdalla et al (2007) reported that the RWC of wheat plants decreased progressively in tolerant and susceptible varieties compared to the untr eated controls, although such decline was much pronounced in sensitive varieties (33.6%) than in the tolerant ones (28.6%) under similar level of stress treatment.