Neck shrivel in European plum: Mechanisms and causes

缩颈是欧洲李果实的一种生理病症，其特征是花梗端部干缩，花柱端部膨大。它在收获前发育，并在收获后继续。受影响的水果被认为是劣质的，因此商业价值很低。品种(遗传)对颈枯病的感受性存在差异。人们对颈缩的机制基础知之甚少。通过茎端角质层微裂缝增加的蒸腾作用有助于颈部萎缩。然而，目前还不清楚这是否是导致颈部萎缩的唯一原因。此外，对称李果实茎端微裂纹的形成机制和花柱端微裂纹的缺失也是未知的。经验观察表明，水关系可能起到了一定作用。可以认为有两个假说可以解释果颈萎缩：假说1--在有症状的果实中，茎端而不是花柱端有负的水分平衡。在没有症状的水果中没有这样的区别。假设2-渗透势从茎端(较小的负值)到花柱端(较负的)的梯度导致水分从茎端到花柱端的重新分配。其目的是检验这些假说的正确性。我们将集中讨论以下事项：(1)为什么微裂缝在茎末端形成，这是茎末端蒸腾增加的主要原因，而不是在花柱末端形成，尽管果实基本上是对称的(假设1--由于蒸腾作用增加而导致水分负平衡)(2)木质部中不同的维管水分运输，供给感病品种的花柱末端，但不是或在较小程度上供应茎末端(假设1-由于维管流入减少而导致水分负平衡)，以及(3)水势的梯度，感病品种茎端和花柱端之间的渗透势和膨胀率(假设2-渗透势梯度引起的内部重新分布)。了解李颈枯病的发病机制是(1)培育不太感病的李新品种和(2)开发可能降低颈枯病发病率的栽培措施的先决条件。我们预计所产生的信息将适用于许多其他肉质水果作物物种，从寒冷的温带地区到热带地区。

Neck shrivel is a physiological disorder of European plum fruit characterized by a shriveled pedicel-end and a turgescent stylar-end. It develops pre-harvest and continues postharvest. Affected fruit are perceived as poor quality, so are of low commercial value. Cultivar (genetic) differences exist in susceptibility to neck shrivel. Little is known of the mechanistic basis of neck shrivel. Increased transpiration through microcracks in the cuticle of the stem-end contributes to neck shrivel. However, at present it is not known whether this is the only reason for neck shrivel. Also, the mechanism of microcrack formation in the stem end and the absence thereof in the stylar end of the symmetric plum fruit is unknown. Empirical observations suggest water relations may play a role. Two hypotheses may be thought of that account for neck shrivel: Hypothesis 1 – The stem end, but not the stylar end has a negative water balance in symptomatic fruit. There is no such difference in non-symptomatic fruit. Hypothesis 2 – A gradient in osmotic potential from the stem end (less negative) to the stylar end (more negative) causes a redistribution of water from the stem end to the stylar end. The objective is to test the validity of these hypotheses. We will focus on the following items: (1) Why do microcracks, that are the primary reason for increased transpiration of the stem end, form at the stem end, but at not the stylar end although the fruit is largely symmetric (hypothesis 1 – negative water balance due to increased transpiration)(2) Differential vascular water transport in the xylem that feeds the stylar end of susceptible cultivars, but not or to a lesser extent the stem end (hypothesis 1 – negative water balance due to reduced vascular inflow), and (3) Gradients in water potential, osmotic potential and turgor between stem end and stylar end in susceptible cultivars (hypothesis 2 – internal redistribution due to gradients in osmotic potential). Understanding the mechanism of neck shrivel is a prerequisite for (1) breeding new, less-susceptible plum cultivars and (2) developing cultural measures that might decrease the incidence of neck shrivel. We expect the information generated to be applicable to many other fleshy fruit crop species, from cool temperate regions to the tropics.