[Show abstract][Hide abstract] ABSTRACT: Circularly polarized chlorophyll luminescence (CPL) may serve as a measure of chiral macroaggregates of the light-harvesting chlorophyll-protein complexes (LHC II) in both isolated chloroplasts and intact leaves (Gussakovsky et al (2000) Photosynth Res 65: 83-92). In the present work, we applied the CPL approach to study the effect of fast (1-2 min) thermal impacts on LHC II macroaggregates. The results revealed unexpected temperature-response kinetics, composed of initial bell-shaped changes in the CPL signal, followed by degradation down to a steady state (equilibrium). The bell-shape effect was dependent upon illumination, and vanished in the dark. A mathematical analysis of the temperature-response kinetics uniquely indicated that LHC II chiral macroaggregates may persist in both left- and right-handed forms. These forms differ in their response to high temperatures. Both forms are more thermostable in leaves than in isolated chloroplasts. The cooperative degradation of LHC II macroaggregates, which is induced by the thermal impact, is irreversible. It is therefore suggested that the native LHC II macroaggregates are stable, stationary, non-equilibrium, spatially heterogeneous (dissipative) structures. The dissipative properties probably allow the interconversion between left- and right-handed forms under perturbation by certain factors. Illumination probably serves as one such perturbation factor, initiating the interconversion of dark-adapted, left-handed to light-dependent, right-handed LHC II macroaggregates. The chiral heterogeneity of the LHC II macroaggregates is a newly revealed aspect which needs to be taken into consideration in future circular dichroism or CPL studies.
Photosynthesis Research 04/2006; 87(3):253-65. DOI:10.1007/s11120-005-9003-2 · 3.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The effect of a cold night on photosynthesis in herbaceous chilling-sensitive crops, like tomato, has been extensively studied and is well characterized. This investigation examined the behaviour of the sub-tropical fruit tree, mango, to enable comparison with these well-studied systems. Unlike tomato, chilling between 5 degrees C and 7 degrees C overnight produced no significant inhibition of light-saturated CO(2) assimilation (A:) during the first hours following rewarming, measured either under controlled environment conditions or in the field. By midday, however, there was a substantial decline in A:, which could not be attributed to photoinhibition of PSII, but rather was associated with an increase in stomatal limitation of A: and lower Rubisco activity. Overnight chilling of tomato can cause severe disruption in the circadian regulation of key photosynthetic enzymes and is considered to be a major factor underlying the dysfunction of photosynthesis in chilling-sensitive herbaceous plants. Examination of the gas exchange of mango leaves maintained under constant conditions for 2 d, demonstrated that large depressions in A: during the subjective night were primarily the result of stomatal closure. Chilling did not disrupt the ability of mango leaves to produce a circadian rhythm in stomatal conductance. Rather, the midday increase in stomatal limitation of A: appeared to be the result of altered guard cell sensitivity to CO(2) following the dark chill.
[Show abstract][Hide abstract] ABSTRACT: Sugars are key regulatory molecules that affect diverse processes in higher plants. Hexokinase is the first enzyme in hexose metabolism and may be a sugar sensor that mediates sugar regulation. We present evidence that hexokinase is involved in sensing endogenous levels of sugars in photosynthetic tissues and that it participates in the regulation of senescence, photosynthesis, and growth in seedlings as well as in mature plants. Transgenic tomato plants overexpressing the Arabidopsis hexokinase-encoding gene AtHXK1 were produced. Independent transgenic plants carrying single copies of AtHXK1 were characterized by growth inhibition, the degree of which was found to correlate directly to the expression and activity of AtHXK1. Reciprocal grafting experiments suggested that the inhibitory effect occurred when AtHXK1 was expressed in photosynthetic tissues. Accordingly, plants with increased AtHXK1 activity had reduced chlorophyll content in their leaves, reduced photosynthesis rates, and reduced photochemical quantum efficiency of photosystem II reaction centers compared with plants without increased AtHXK1 activity. In addition, the transgenic plants underwent rapid senescence, suggesting that hexokinase is also involved in senescence regulation. Fruit weight, starch content in young fruits, and total soluble solids in mature fruits were also reduced in the transgenic plants. The results indicate that endogenous hexokinase activity is not rate limiting for growth; rather, they support the role of hexokinase as a regulatory enzyme in photosynthetic tissues, in which it regulates photosynthesis, growth, and senescence.
The Plant Cell 08/1999; 11(7):1253-66. DOI:10.2307/3870747 · 9.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Summary Many green decorative branches ("greens"), which form an important part of ornamental bouquets, originate in shade plants, and are commercially grown in Israel under black shade nets. We are studying the possibility to manipulate the nature of the vegetative growth of greens in order to improve the yield and quality desirable for various ornamental crops. The present paper summarizes the results obtained for Pittosporum variegatum grown under shade nets of various optical properties: green, red, blue, grey, black, and reflective. The knitting density and design of the nets were adjusted to give the same transmittance of sunlight in the PAR (photosynthetically active radiation, 400-700 nm) region. The experiments were carried out in a commercial plot. A single layer of 50% shade net covered the plot during the winter season, and two layers in the summer, according to the common practice. Data were collected mostly during the second growth year. The main results obtained included: (i) pronounced stimulation of branch elongation under the Red net; (ii) dwarfing under the Blue net; (iii) the Grey net markedly enhanced branching, resulting in "bushy", dense plants with short side shoots and small leaves; (iv) the reflective, thermal net (Aluminet®) enhanced long branching. The results were reproducible over two successive harvesting years. We suggest that the coloured net
[Show abstract][Hide abstract] ABSTRACT: Nets are frequently used to protect agricultural crops from excessive solar radiation, environmental hazards or pests. We have developed a new concept, by which the nets are designed to specifically filter sunlight, concomitant with providing the desired physical protection. A series of photoselective nets (ColorNets) was developed for outdoor use, each one absorbing different spectral bands, and at the same time increasing the relative proportion of scattered/diffused light. The spectral manipulation is aimed at specifically promoting physiological responses, while light scattering improves light penetration into the inner canopy. The relative enriching of the intercepted light with "good" parts of the spectrum, while reducing "bad" parts, may allow better utilization of the solar energy. Earlier studies of ornamental crops, traditionally grown under black shade nets, revealed differential responses to the photoselective shading. Ongoing studies of low-shading ColorNets in numerous fruit crops and climatic regions showed that nets of the same shading factor but different chromatic properties can differentially affect various attributes of tree performance, including production, fruit size and quality, and advancement or delay in the timing of harvest. The netting further ameliorates extreme climatic fluctuations, reduces heat/chill stresses, and improves the resulting canopy activity and water use. The results demonstrate the potency of photoselective netting for improving the agro-economical performance of horticultural crops, especially (but not only) in harsh climates and arid zones. INTRODUCTION Agriculture is being gradually pushed towards less amenable environments because of the combination of urbanization processes and global climate changes. One of the ways to cope with this trend is by protecting the crops. Netting is an effective solution, because of (i) protection from environmental hazards (e.g., excessive solar radiation, wind, hail, flying pests); (ii) improved plant microclimate (thus reducing heat/chill, drought stresses); (iii) moderation of rapid climatic stresses; and (iv) being cheaper and less energy-consuming than greenhouses. We have taken the use of netting one step further, and developed the concept of photoselective netting. The concept of photoselective netting is to apply differential filtration of the solar radiation, together with physical protection. In collaboration with Polysack Plastics Industries, Israel, we have developed a series of photoselective nets (ColorNets) for out-door use, each type differentially absorbing the UV, Blue, Green, Yellow, Red, FR or IR spectral regions, and at the same time increasing the relative content of scattered/diffused light (Shahak et al., 2004a, b). The spectral manipulation is aimed at specifically promoting photomorphogenetic/physiological responses, while light scattering improves light penetration into the inner canopy. The relative enriching of the intercepted light with productive components of the spectrum, while reducing the less productive ones, may allow better utilization of the solar energy. The target responses are those determining the commercial value of each crop, including yield, product quality and timing of maturation.