Regulation of Photosynthetic Rate of Two Sunflower Hybrids under Water Stress

Abstract

The effect of short-term water stress on photosynthesis of two sunflower hybrids (Helianthus annuus L. cv Sungro-380 and cv , differing in productivity under field conditions, was measured. The rate of CO2 assimilation of young, mature leaves of SH-3622 under well-watered conditions was approximately 30% greater than that of Sungro-380 in bright light and elevated C02; the carboxylation efficiency was also larger. Growth at large photon flux increased assimilation rates of both hybrids. The changes in leaf composition, including cell numbers and sizes, chlorophyll content, and amounts of total soluble and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein, and in Rubisco activity and amount of ribulose-1,5-bisphosphate (RuBP) were determined to assess the factors regulating the differences in assimilation of the hybrids at high and low water potentials. Photosynthetic CO2 assimilation per unit area of leaf surface depends on the capacity of the plant's photosynthetic mechanism and on those environmental factors, such as CO2 supply and radiation, which are the substrates for the process, and on those conditions (e.g. temperature and water supply) that affect the mechanism. The rate of photosynthesis, in combination with the leaf area, determines plant productivity (2,10,11,18 CO2 fixation than C3 plants in bright light and high temperatures, and the biochemical and physiological origins of this are well established (19). However, although there are differences in assimilation rate between species within the C3 and C4 groups, differences between closely related plants are less well established. Also, the causes of differences and how they relate to the characteristics of the photosynthetic mechanism between species and varieties are poorly understood (19,21,28).Genetic variation in rates of photosynthesis per unit leaf area (Pn2) has been reported for cultivars of some crop species (2, 9, 12). Possible causes of the differences in Pn between genotypes are variations in amounts or activities of specific proteins, pigments, etc. and differences in leaf structure, cell size, or stomatal frequency. Small but consistent differences in assimilation were detected between hexaploid wheats under well-watered field conditions (9), but such differences between other wheat genotypes were attributed to ploidy (2). Substantial differences (up to 35%) have been reported between cultivars of field bean (13) and in pea (21), the latter attributed to variation in Chl content. Assimilation rates were best correlated with the ratio of cell surface area to leaf surface area (19), in analyses of 112 C3 and 6 C4 species. The correlations among cell size, ploidy, and assimilation rates (22) and also the effects of nutrition on the relationship (18) makes the analysis complex. Variation in Pn has also been attributed to the amounts and activities of Rubisco. Both the amount and the activity of Rubisco can limit photosynthesis in C3 plants (11,18,22,26,28,29). Genetic variability in Rubisco content related to ploidy ...

Partial rootzone drying: regulation of stomatal aperture and carbon assimilation in field-grown grapevines (Vitis vinifera cv. Moscatel)

“…This is in accordance with the decrease in the quantum yield of PSII photochemistry observed in NI, allowing the adjustment of electron flow to the limited carbon uptake as a result of stomatal closure. This could also be related to the high sensitivity of photophosphorylation to leaf water deficit as reported by Giménez et al (1992). However, Rubisco activity seems to be more resistant to water stress than photophosphorylation, with significant effects occurring only for severe drought.…”

Use of physiological parameters as fast tools to screen for drought tolerance in sugarcane

“…Although measurements of leaf photosynthesis rate have been shown to be reliable in distinguishing between drought tolerant and susceptible genotypes of some species such as sunflower (Gimenez et al, 1992), gas exchange techniques of assessing photosynthesis are laborious and not practical in crop improvement programs (Earl and Tollenaar, 1999). Rong-hua et al (2006), working with barley, showed that indirect and faster methods of measuring photosynthetic activity, such as chlorophyll a fluorescence technique, particularly the maximum photochemical efficiency of photosystem II -PSII (which can be assessed via the variable-to-maximum chlorophyll a fluorescence ratio, F v / F m ) and estimated chlorophyll content (SPAD index), can be as effective as the more time-consuming gas exchange techniques in revealing differences between drought tolerant and susceptible genotypes.…”

Field study of the effects of excess copper on wheat photosynthesis and productivity

“…The severe impact of Cu on wheat leaf water use efficiency, as determined by the ratio between leaf photosynthesis and transpiration (AI E) and the ratio between leaf photosynthesis and stomatal conductance (AI gs), indicates the reduced control over water loss. As observed in the are plants, lower stomatal conductances were reported in many other species in severe stress environments (Lawlor and Mitchell 1991;Gimenez et al 1992;Lidon et al 1993).…”

“…Photosynthetic CO 2 assimilation per unit area of leaf surface depends on the capacity of the plant's photosynthetic mechanism and on environmental factors, such as excess Cu, which are necessary for the process. The rate of photosynthesis, in combiriation with the leaf area, determines the plant productivity (Gimenez et al 1992). Assimilation also varies in response to environmental factors.…”