Rosati A, Metcalf S G, Buchner R P, Fulton A E, Lampinen B D
Istituto Sperimentale per l'Olivicoltura, Via Nursina 2, 06049 Spoleto, (PG) Italy.
Ann Bot. 2006 Jul;98(1):267-75. doi: 10.1093/aob/mcl100. Epub 2006 May 30.
Kaolin applications have been used to mitigate the negative effects of water and heat stress on plant physiology and productivity with variable results, ranging from increased to decreased yields and photosynthetic rates. The mechanisms of action of kaolin applications are not clear: although the increased albedo reduces leaf temperature and the consequent heat stress, it also reduces the light available for photosynthesis, possibly offsetting benefits of lower temperature. The objective of this study was to investigate which of these effects are prevalent and under which conditions.
A 6% kaolin suspension was applied on well-irrigated and water-stressed walnut (Juglans regia) and almond (Prunus dulcis) trees. Water status (i.e. stem water potential, psi(s)), gas exchange (i.e. light-saturated CO2 assimilation rate, Amax; stomatal conductance, g(s)), leaf temperature (T(l)) and physiological relationships in treated and control trees were then measured and compared.
In both species, kaolin did not affect the daily course of psi(s) whereas it reduced Amax by 1-4 micromol CO2 m(-2) s(-1) throughout the day in all combinations of species and irrigation treatments. Kaolin did not reduce g(s) in any situation. Consequently, intercellular CO2 concentration (C(i)) was always greater in treated trees than in controls, suggesting that the reduction of Amax with kaolin was not due to stomatal limitations. Kaolin reduced leaf temperature (T(l)) by about 1-3 degrees C and leaf-to-air vapour pressure difference (VPD(l)) by about 0.1-0.7 kPa. Amax was lower at all values of g(s), T(l) and VPD(l) in kaolin-treated trees. Kaolin affected the photosynthetic response to the photosynthetically active radiation (PAR) in almond leaves: kaolin-coated leaves had similar dark respiration rates and light-saturated photosynthesis, but a higher light compensation point and lower apparent quantum yield, while the photosynthetic light-response curve saturated at higher PAR. When these parameters were used to model the photosynthetic response curve to PAR, it was estimated that the kaolin film allowed 63% of the incident PAR to reach the leaf.
The main effect of kaolin application was the reduction, albeit minor, of photosynthesis, which appeared to be related to the shading of the leaves. The reduction in T(l) and VPD(l) with kaolin did not suffice to mitigate the adverse effects of heat and water stress on Amax.
高岭土已被用于减轻水分和热胁迫对植物生理及生产力的负面影响，但其效果各异，产量和光合速率可能增加，也可能降低。高岭土作用的机制尚不清楚：虽然反射率增加会降低叶片温度及随之而来的热胁迫，但同时也会减少可用于光合作用的光照，这可能抵消了低温带来的益处。本研究的目的是探究这些影响中哪些是普遍存在的，以及在何种条件下存在。
将6%的高岭土悬浮液施用于灌溉良好和水分胁迫的核桃（Juglans regia）树和杏仁（Prunus dulcis）树上。然后测量并比较处理树和对照树的水分状况（即茎水势，ψ(s)）、气体交换（即光饱和CO₂同化率，Amax；气孔导度，g(s)）、叶片温度（T(l)）及生理关系。
在这两个物种中，高岭土均未影响ψ(s)的日变化过程，而在所有物种与灌溉处理的组合中，它使全天的Amax降低了1 - 4 μmol CO₂ m⁻² s⁻¹。在任何情况下，高岭土均未降低g(s)。因此，处理树中的胞间CO₂浓度（C(i)）总是高于对照树，这表明高岭土导致的Amax降低并非由于气孔限制。高岭土使叶片温度（T(l)）降低了约1 - 3℃，叶 - 气蒸气压差（VPD(l)）降低了约0.1 - 0.7 kPa。在高岭土处理的树中，在所有g(s)、T(l)和VPD(l)值下，Amax都较低。高岭土影响了杏仁叶片对光合有效辐射（PAR）的光合响应：涂有高岭土的叶片具有相似的暗呼吸速率和光饱和光合作用，但光补偿点较高，表观量子产率较低，且光合光响应曲线在较高的PAR下达到饱和。当使用这些参数来模拟PAR的光合响应曲线时，估计高岭土薄膜使63%的入射PAR能够到达叶片。
施用高岭土的主要作用是使光合作用略有降低，这似乎与叶片被遮蔽有关。高岭土导致的T(l)和VPD(l)降低不足以减轻热胁迫和水分胁迫对Amax的不利影响
