Neck shrivel in European plum: Mechanisms and causes

颈缩病是欧洲李果实的一种生理性病害，其特征是花梗末端皱缩，花柱末端肿胀。它在收获前发育，并在收获后继续发育。受影响的水果被认为质量差，因此商业价值低。品种（遗传）对颈缩病的敏感性存在差异。对于颈部萎缩的机理知之甚少。通过茎端角质层的微裂缝增加蒸腾作用导致颈部皱缩。然而，目前还不知道这是否是颈部萎缩的唯一原因。此外，微裂纹的形成机制，在茎端和对称李果实的花柱端的情况下，是未知的。经验观察表明，水关系可能发挥作用。有两种假说可以解释颈皱缩：假说1 -症状果实的茎端，而不是花柱端有负水平衡。在无症状水果中没有这种差异。假设2 -从茎端（负性较小）到花柱端（负性较大）的渗透势梯度导致水从茎端到花柱端的再分配。目的是检验这些假设的有效性。我们将重点关注以下项目：（1）为什么微裂纹是茎端蒸腾增加的主要原因，但在茎端形成，而不是在花柱端，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的负水平衡）（2）木质部中的差异维管水运输，其为敏感品种的花柱末端提供营养，但茎端不存在或在较小程度上不存在（假设1 -由于血管流入量减少而导致的负水平衡），以及（3）易感品种茎端和花柱端之间的水势、渗透势和膨压梯度（假设2 -由于渗透势梯度而导致的内部再分配）。了解颈缩病的发病机理是培育感病性较低的李新品种和制定降低颈缩病发病率的栽培措施的前提。我们希望所产生的信息适用于许多其他肉质水果作物物种，从冷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.