Relative Sensitivity of Photosynthetic Assimilation and Translocation of ¹⁴Carbon to Water Stress

Abstract

The relationship between photosynthesis and translocation rate changes as affected by water stress intensity and stage of plant development was evaluated in cotton and sorghum, representing a C3 and a C4 photosynthetic type, respectively. Photosynthetic rates were reduced as midday leaf water potentials declined from -14 to -27 bars in both species. Sorghum maintained higher photosynthesis and translocation rates compared to cotton at comparable leaf water potentials; however, the rate of change per bar decline in water potential was greater in sorghum than in cotton. Photosynthetic rates were reduced with increasing water stress prior to any significant change in translocation rates suggesting that photosynthesis is the more sensitive of the two processes. Severe water stress, corresponding to leaf water potentials of -27 bars, did not completely inhibit either photosynthesis or translocation.Photosynthetic rate reductions due to increasing water stress have been observed in a variety of plant species. Stomatal effects are usually considered to be the first and major limitation to CO2 fixation, although inhibitions at the chloroplast level have been proposed (5, 10). Under semiarid field conditions, stomatal closure is not evident in fully illuminated cotton leaves even at Iw2 approaching -30 bars; however, photosynthesis is severely reduced (1, 2). In this same environment, stomatal regulation is observed only prior to flowering in sorghum's response to increasing water stress (1). Photosynthetic rates of individual leaves of sorghum are significantly reduced prior to any measurable increase in stomatal resistance. After the leaf has attained maximum size no evidence of stomatal closure exists in response to increasing water stress; however, photosynthetic rates are severely affected (12, and unpublished data of Krieg).Leaf photosynthesis may be affected by accumulation of photosynthate due to effects on translocation or utilization of the assimilate (16, 18). Inhibition of assimilate translocation by water stress has been observed in a number of plant species (6,7,11,13 Disagreement exists as to the relative sensitivity of photosynthesis and translocation to water stress. Several studies have concluded that translocation is more sensitive to water deficits than is photosynthesis (6, 7). In contrast to these conclusions, others (11,13,14,19,20) have suggested that photosynthesis is more sensitive to water stress than translocation. Wardlaw (19) has indicated that the effect of water stress on the translocation process is related to the availability of photosynthate more so than by a direct effect on the translocation process mechanism per se.The objective of this study was to define the relationship between leaf water status, photosynthesis, and translocation of current assimilate as a function of photosynthetic type (C3 versus C4) and growth stage. MATERIALS AND METHODSSorghum (Sorghum bicolor L. Moench) and cotton (Gossypium hirsutum L. cv. SP 37) were chosen for study due to differences in photosynt...

The Role of Phloem Loading Reconsidered

“…High concentrations of solute in the phloem also allow the sieve elements to maintain turgor, and to continue to function, when the plant is under severe water stress (Sung and Krieg, 1979;Smith and Milburn, 1980). However, there is reason to doubt that active loading is needed to establish and maintain this pressure since many plants with the longest transport distances function without it.…”

Cotton response to mepiquat chloride and temperature

“…Hence, the best temperature regime for reproductive structure retention in cotton was day/night temperatures of 32/22 ºC. Maximum net photosynthesis in cotton leaves occurs around 32 o C (Sung and Krieg, 1979), but respiration increases almost linearly with higher temperatures resulting in less carbohydrates available for fruit growth at high temperatures, fruit shedding and decreased yields; usually a good vegetative/reproductive balance in cotton is observed when the average temperature is around 25 o C (Pettigrew, 2008). The competition between vegetative and reproductive growth for photoassimilates is decreased by mepiquat chloride use (Gwathmey and Clement, 2010).…”

Changes in the water status and osmotic solute contents in response to drought and salicylic acid treatments in four different cultivars of wheat (Triticum aestivum)

“…Such a drastic change might be induced under much severer or longer drought conditions than ours, because more than 80% of the maximum contents of the major photosynthetic pigments maintained in wheat leaves under drought (Table 2). In young leaves, expansion growth is more sensitive to water-deficit, than photosynthesis (Boyer 1970) or translocation (Sung and Krieg 1979). Therefore, we concluded that the growth rate might be determined by more watersensitive activities other than photosynthesis or pigment biosynthesis.…”