Abstract
Wheat leaf non-sequential senescence at the late grain-filling stage involves the early senescence of younger flag leaves compared to that observed in older second leaves. On the other hand, sequential senescence involves leaf senescence that follows an age-related pattern, in which flag leaves are the latest to undergo senescence. The characteristics of sugar metabolism in two sequential senescence cultivars and two non-sequential senescence cultivars under both natural and drought conditions were studied to elucidate the underlying mechanism of drought tolerance in two different senescence modes. The results showed that compared to sequential senescence wheat cultivars, under natural and drought conditions, non-sequential senescence wheat cultivars showed a higher leaf net photosynthetic rate, higher soluble sugar levels in leaves, leaf sheaths, and internodes, higher leaf sucrose synthase (SS) and sucrose phosphate synthase (SPS) activity, and higher grain SS activity, thereby suggesting that non-sequential senescence wheat cultivars had stronger source activity. Spike weight, grain weight per spike, and 100-grain weight of non-sequential senescence cultivars at maturity were significantly higher than those of sequential senescence cultivars under both natural and drought conditions. These findings indicate that the higher rate of accumulation and the higher mobilization of soluble sugar in the leaves, leaf sheaths and internodes of non-sequential senescence cultivars improve grain weight and drought tolerance. At the late grain-filling stage, drought conditions adversely affected leaf chlorophyll content, net photosynthetic rate, soluble sugar and sucrose content, SS and SPS activity, gain SS activity, and weight. This study showed that higher rates of soluble sugar accumulation in the source was one of the reasons of triggering leaf non-sequential senescence, and higher rates of soluble sugar mobilization during leaf non-sequential senescence promoted high and stable wheat yield and drought tolerance.
Highlights
Some crop cultivars such as wheat and rice undergo sequential leaf senescence
In Xinong 88 and NR9405 (Fig 1A and 1B), the flag leaf showed higher chlorophyll contents than the second leaf during leaf senescence, indicating that the second leaf senesced earlier than the flag leaf, which we designated as wheat leaf sequential senescence
Drought conditions significantly increased the rate of the loss of leaf chlorophyll, indicating that leaf senescence was accelerated
Summary
Some crop cultivars such as wheat and rice undergo sequential leaf senescence. During plant growth, young leaves are successively formed at the top region of the plant, and the lower older leaves gradually undergo senescence [1]. Some wheat and rice cultivars display a non-sequential mode of leaf senescence at the grain maturation stage, i.e., younger flag leaves undergo senescence earlier than older second leaves [8,9,10,11,12]. We compared the chlorophyll content, net photosynthetic rate, soluble sugar content, sucrose content, and the activity of key enzymes during sucrose metabolism in flag leaves and second leaves of sequential and non- sequential senescence cultivars during leaf senescence under natural and drought conditions. The soluble sugar content of leaf sheaths and internodes, spike weight, grain weight per spike, and 100-grain weight were examined to establish the relationship of sugar accumulation and remobilization with wheat yield, leaf senescence mode, and drought tolerance
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