**1. Introduction**

Leaf senescence is an age-dependent complex process at the cellular, tissue, organ or organ‐ ism level, leading to death at the end of the life span [1]. Annual plants as grain and oil crops undergo a visual process toward the end of the reproductive stage that is accompanied by nutrient remobilization from leaf to developing seeds [2]. Senescence process is controlled by multiple genetic and environmental variables, which has strong impact on crop yield [3]. Environmental factors such as light, nutrient availability, concentration of CO<sup>2</sup> , abiotic and biotic stresses caused by disease may affect the rate of senescence. In this context, not only environmental conditions but also biotic factors influence senescence, being this an irrevers‐ ible process prematurely induced under these adverse conditions [4]. Moreover, reproductive growth is mentioned as a factor that usually impacts on leaf senescence, and particularly in sunflower, the lack of sinks delays the onset of senescence [5]. During this process, changes in gene expression result in a metabolic shift from anabolism to catabolism, which leads to decreased photosynthetic activity, progressive degradation of cellular structures and oxida‐ tive burst [6–8]. It has been documented that a delay in leaf senescence has a substantial impact on grain yield through the maintenance of the photosynthetic leaf area during the reproductive stage in different crops [3, 9, 10].

In sunflower (*Helianthus annuus* L.), the largest important oil crop worldwide, the senescence process reduces the capacity of plants to maintain their green leaf area for longer periods, especially during the grain-filling phase, affecting the yield and thus leading to economic losses [11, 12]. This production constraint has deepened since sunflower crop production has been gradually moved to marginal areas due to the rapid change of agricultural practices in crops such as soybean and maize, which have greatly increased their cultivated areas as a consequence of favorable commodity prices and because farmers found more profitable to sow transgenic crops with resistance to herbicides and insects [13, 14].

During the last years, many efforts have been achieved to build up useful functional genomics tools for cultivated sunflower involving physiological, transcriptional and metabolic profiles [15–23].

In crop species, considering the temporal gap between onset and phenotypic detection of senes‐ cence process, the availability of candidate genes and molecular markers to the early detection of senescence is indispensable to discriminate between early-senescing and late-senescing lines to be applied in the different context of breeding activities [24]. For example, the identification of functional stay‐green genotypes for breeding applications and/or for elucidating molecular mechanisms involved in this complex trait.

### **2. Senescence and crop yields: stay‐green genotypes**

Senescence is an essential process for the normal growth and development of plants, being an important mechanism for the adaptation to several environmental conditions.

The hypothesis that a delay leaf senescence increases the productivity may be valid for most crops with regard to total biomass production and tuber crops, but this assumption is more controversial with respect to seed yields [3]. However, it has been documented that a delay in leaf senescence has a high impact on grain weight and quality in different crops, including sunflower [3, 5, 25, 26].

Stay-green is a regular term given to genotypes in which the senescence phenotype is delayed in comparison with a standard reference genotype. Stay-green genotypes could be classified into five different types taking into consideration functional or cosmetic stay-green [27, 28]. Functional stay-green genotypes have a photosynthetically active leaf area showing a delay in the onset of senescence (class A), or differing in the rate of the process (class B), whereas cosmetic stay-green genotypes are those in which the senescence proceeds normally but they show problems in chlorophyll degradation (class C), or the chlorophyll content does not decline due to rapid tissue death (class D), or they have a higher chlorophyll content with no change in onset or rate of senescence development (class E) [28]. Mature leaf is a net contributor of photosynthates to the whole plant. The carbon capture phase of the leaf is fol‐ lowed by a net organic nitrogen remobilization. The transition from carbon capture to nitro‐ gen remobilization corresponds to the functional initiation of senescence [29]. In this sense, functional stay-green genotypes are present in those genotypes in which the C–N transition is delayed, or the transition occurs but the subsequent yellowing and N remobilization run slowly [29–31].

In this sense, functional stay-green genotypes could intercept more radiation, increasing pho‐ tosynthesis and yield in crops with seeds rich in carbon compounds. However, a delay in the C–N transition could negatively affect the seed quality in crops with seed rich in protein compounds [30], such as soybean [32] and cowpea [33].
