Convexity of the Photosynthetic Light-Response Curve in Relation to Intensity and Direction of Light during Growth

Abstract

Photosynthesis in the intermediate light range is most efficientwhen the convexity of the photosynthetic light-response curve is high. Factors determining the convexity were examined for intact leaves using Salix sp. and for a plant cell culture using the green microalga Coccomyra sp. I t was found that the leaf had lower convexity than diluted plant cells because the light gradient through the leaf was not fully matched by a corresponding gradient in photosynthetic capacity. l h e degree to which the leaf gradients were matched was quantified by measuring photosynthesis at both leaf surfaces using modulated fluorescence. Two principal growth conditions were identified as those causing mismatch of leaf gradients and lowering of the convexity relative to cells. l h e first was growth under low light, where leaves did not develop any noteworthy gradient in photosynthetic capacity. This led to decreased convexity, particularly in old leaves with high chlorophyll content and, hence, steep light gradients. Second and less conspicuous was growth under high light conditions when light was given bilaterally rather than unilaterally, which yielded leaves of high photosynthetic capacity at both surfaces. Two situations were also identified that caused the convexity to decrease at the chloroplast levd: (a) increased light during growth, for both leaves and cells, and (b) increased CO, concentration during measurement of high-lightgrown leaves. These changes of the intrinsic convexity were interpreted to indicate that the convexity declines with increased capacity of ribulose-1,5-bisphosphate carboxylase/oxygenase relative to the capacity of electron transport.The light-response curve of photosynthesis is accurately described by a nonrectangular hyperbola (Prioul and Chartier, 1977; Leverenz and Jarvis, 1979; Marshall and Biscoe, 1980) that has the form e p 2 -(91 + P,,,) P + +IP,,, = O (1) where P is the rate of photosynthesis ( y variable), I is the irradiance ( x variable), CP is the maximum quantum yield, 8 is the convexity, and P,,, is the light-saturated rate of photosynthesis. @ and 8 are parameters of particular importance for photosynthetic productivity under natural conditions, because it is believed that the rate of photosynthesis is limited by light most of the time (Long, 1985; Ort and Baker, 1988). As can be seen in Figure 1 from the initial line set by @ to the plateau set by P,,, (the so-called Blackman curve). This is never realized for cells and leaves that typically show 8 values within the range of 0.7 to 0.99. It should be noted that 8 does not follow a linear scale. Therefore, a curve of 8 = 0.90 is more different from a curve of 8 = 0.99 than it is from a curve of 8 = 0.75

Aclimatação de mudas de acariquara à alta irradiância

“…Isso porque, quando esse investimento é reduzido, a transição de fotossíntese limitada por A c para limitada por A j ocorre mais rápido e o formato final da curva é mais truncado. Provavelmente, por isso os valores de Θ tendem a aumentar (transição pausada de A c para A j ) à proporção que plantas se aclimatam à alta irradiância (Ogren, 1993). Independentemente do tratamento de aclimatação à alta irradiância, o fator limitante da fotossíntese foi a velocidade de carboxilação da rubisco, o que está de acordo com os resultados de Sage & Kubien (2007), que mostram que, exceto em temperaturas muito altas, a fotossíntese é mais comumente limitada pela atividade da rubisco.…”

“…A resposta da fotossíntese à irradiância é comumente descrita por uma hipérbole não retangular que inclui os parâmetros teta (Θ) e fi (Φ), em que Θ representa a convexidade da curva e Φ, o rendimento quântico máximo (Ogren, 1993). Em condições naturais, a taxa de fotossíntese é geralmente limitada pela luz, por isso Φ e Θ são parâmetros importantes.…”

Tansley Review No. 117

“…The choice of photobiont is also important because for instance the presence or absence of a CCM will influence both φ and θ (Spalding & Portis, 1985 ;Falk & Palmqvist, 1992 ;O $ gren, 1993 ;Palmqvist et al, 1994b). So far, these parameters have been extensively investigated only for Coccomyxa, which again behaved similarly to C $ plant cells (O $ gren, 1993 ;Palmqvist et al, 1994b).…”

“…However, because Rubsico is a rather large enzyme (Andrews & Lorimer, 1987) increasing its concentration will require that nitrogen can be acquired either from the environment or through reallocation within the cell or organism. Cells with higher Rubisco concentrations typically reach P max at a higher irradiance than cells with lower Rubisco concentrations (Bjo$ rkman, 1981 ;O $ gren, 1993) (Fig. 3).…”

Arctic Ecosystem Responses to Changes in Water Availability and Warming: Short and Long-Term Responses

“…The LCP and  were derived from the light response curve model described by a nonrectangular hyperbola (Leverenz and Jarvis 1979, Marshall and Biscoe 1980, Terashima and Saeki 1985, Ogren 1993. The equation presents a linear relationship between photosynthesis and light under low irradiance conditions, and an asymptotic response to light at high levels of irradiance: ) at the ecosystem level using NEE and ER as the net assimilation and respiration parameters:…”