SUNFLOWER EMERGENCE AND INITIAL GROWTH IN SOIL WITH WATER EXCESS

_________________________ 2 Universidade Federal de Santa Maria/ Santa Maria/RS, Brasil. 3 SEAPA/Palmeira das Missões/RS, Brasil. 4 ESALQ-USP / Piracicaba/SP, Brasil. 5 Universidade Federal de Santa Maria/ Santa Maria/RS, Brasil. Received in: 12-16-2015 Accepted in: 2-15-2017 Eng. Agríc., Jaboticabal, v.37, n.4, p.644-655, jul./ago. 2017 SUNFLOWER EMERGENCE AND INITIAL GROWTH IN SOIL WITH WATER EXCESS


INTRODUTION
Sunflower (Helianthus annuus L.) is an oilseed crop grown and consumed worldwide.Its seeds have unsaturated fatty acids essential to the human organism (Alberio et al., 2014).However, its yield is vulnerable to abiotic stress in extreme conditions.Soil water excess and deficit are the most important factors causing plant stress (Gholinezhad & Sajedi, 2012;Grassini et al., 2007).
Water excess changes the gas phase in the soil and promotes O2 deficiency in the roots (hypoxia), which leads to anoxia (Licausi, 2011).Low ATP production and protein denaturation are some cell responses to more than 15-hour-long anoxia, which causes irreversible damage to the mitochondrial structure, metabolism and cell viability (Gorai et al., 2011;Balakhnina et al., 2012).Consequently, the leaf area, chlorophyll content, leaf proteins, photosynthesis, grain filling rate and plant yield are negatively affected by it (Shimono et al., 2012).Moreover, a significant reduction in the soil volume explored by the roots results from discontinuous growth, senescence of seminal roots and from the increased number of adventitious roots (Araki et al., 2012;Zaidi et al., 2012).Sunflower plants facing water excess present low expansion and rapid senescence of leaves, low photosynthesis rate, reduced yield and root biomass (Orchard & Jessop, 1984;Grassini et al., 2007).
Different water excess intensities have been recorded in several regions worldwide.The stress intensity depends on the duration of water excess, which is directly related to soil water storage and landform, as well as to the rainfall regimen.Wetlands with flat topography have the greatest stress risk due to water excess.Genotypic tolerance to anoxia, development stage and water excess duration together defines the stress level.Some plants are able to generate adventitious roots, adjust root growth to the available oxygen zones, regenerate the new roots and close the stomata in order to tolerate water surplus (Vartapetian & Jackson, 1997).
Sunflower emergence and initial growth in soil with water excess Eng.Agríc., Jaboticabal, v.37, n.4, p.644-655, jul./ago. 2017 645 Plant emergence of sensitive species such as wheat, soybeans, and maize is strongly affected by water excess (Lucas et al., 2015;Zaidi et al., 2012;Sung, 1995;Nakajima et al., 2015).High air temperatures have a major effect on seed germination of these species (Zaidi et al., 2012;Nakamura et al., 2012;Nakajima et al., 2015), however, few species have evolved to germinate and grow in anoxic environments (Orchard & Jessop, 1984).Rice (Oryza sativa L.) is a good example of it for being able to germinate under anoxic conditions, as amylase enzymes break down the starch under O2 absence (Yamauchi et al., 2013).Seedlings, in general, have the capacity to produce aerenchyma for root cell respiration (Vartapetian & Jackson, 1997).
The aim of the current article is to assess the effect of water-surplus on the initial growth, development, and root and shoot growth of sunflower plants under different water surplus levels and duration.

MATERIAL AND METHODS
The experiment was carried out in a polycarbonate greenhouse located in Santa Maria (latitude: 29°43'23'' S, longitude: 53°43'15'' W, and altitude 95 m), in the central region of Rio Grande do Sul State, Brazil.The maximum air temperature was set at 32°C.The experimental units (EUs) were composed by pots filled with Ultisol A horizon.The soil was homogenized, sieved, and had its fertility levels corrected.Each pot was placed inside a bucket that was suspended 3 cm from the ground surface for an effective water drainage whenever needed.The pots were distributed in a 30 m 2 area.
The present study was a factorial experiment in a complete randomized design, with three replications.Factor A consisted of a qualitative analysis of the early development of sunflower plants.The applications of water surplus for Factor A were made at the following levels: Control = no water excess, T1 = water excess right after sowing, T2 = water excess three days after sowing, T3 water excess at emergence (50% plants already emerged), T4 = water excess at V2 (two completely developed leaves), and T5 = water excess at V4 (four completely developed leaves).In turn, Factor D was a quantitative analysis of continuous periods of water surplus application, namely: 0, 48, 96, 144, 192, and 240 hours.Finally, factor E comprised three sowing dates: 08/29th/2011, 10/04th/2011, and 11/03rd/2011.
Seeds of the single-cross hybrid 'Helio 250', with 82.2% germination rate, were used in the experiment from the sowing date up to the final application of the tested treatments.At the end of the aforementioned period, plants were harvested, and the variables were assessed.The first sowing date lasted from 08/29th/2011 up to 09/29th/2011; the second, from 10/04th/2011 up to 10/31st/2011; and the third, from 11/03rd/2011 up to 11/28th/2011.Therefore, sowing dates 1, 2, and 3, lasted 31, 27, and 25 days after sowing (DAS), respectively.
Water excess treatments started after sowing and the water level was kept at 6 cm below the soil surface.Water capillary, adhesion, cohesion and the gradients of water potential in the soil allowed the rise of water level, keeping the soil surface highly moist.The water excess in the EUs was accomplished through two ways when the treatments started being applied.Water was directly supplied to the soil inside pots and buckets in order to keep the water level inside them.The water level was monitored on a daily basis, and the maximum level in the buckets was set by a hole placed 15 cm high.After applying the water excess, water was removed from the buckets, allowing drainage of the surplus through holes in the pot bottoms.Soil moisture in the EUs which received no treatment (control, and EU without water excess) remained the same due to daily irrigation.
After plant emergence, thinning was performed leaving two plants on both sides of each pot.The remaining plants were treated to the end of the experiment.The following variables were analyzed: emergence percentage (EP), leaf area (LA), plant height (PH), shoot dry matter (SDM), maximum root length (MaxRL), main root length (MRL), and root dry matter (RDM).
Plant roots were removed and washed thoroughly with fresh water, ensuring that all the adhering mud were eliminated from the roots.Next, both MaxRL (cm) and MRL (cm) were measured.Then, root material was transferred to paper bags and taken to an oven at 65°C under forced ventilation until reaching a constant weight for dry mass measurement.Dry mass variables were converted to grams per square meter (g m -2 ).Data were analyzed by analysis of variance, and the treatments were assessed by regression analysis (p<0.05).

RESULTS AND DISCUSSION
Figure 1 shows the results for plant emergence according to the duration of water excess application for three different sowing dates.A water excess application for 48 hours caused a sharp reduction in plant emergence in the third sowing date for T1 and T2 (Figures 1C and 1F), showing rates of 22.0% and 13.9% viable seeds, respectively.However, in the first and second sowing dates, these treatments showed higher rates of 95.3% and 79.3% (T1) and 71.6% and 18.9% (T2).Major reductions in plant emergence were found for treatments providing water excess for the same number of hours in the third sowing date.Furthermore, we may highlight that water excess led to severe losses in plant emergence in all sowing dates (when the germination process had already begun) right at the sowing day.Similar results were found by Sung (1995) in a study with soybeans wherein water excess for longer than 24 and 48 hours reduced emergence by 50% and 100%, respectively.Therefore, water excess in sunflower crops during germination might be harmful right after sowing, as already seen for soybean seedlings (Nakajima et al., 2015).This might be explained by an increase in respiration rate and enzyme activity after the first seed imbibition peak, causing a high demand for O2 what potentiates seed damages (Orchard & Jessop, 1984).
The air temperature during the sowing-emergence subperiod led to differences among sowing dates (Figure 2).The germination reduction of sunflower seeds due to water excess was different for each water excess duration, crop stage, and sowing dates.To understand this, one must have in mind that air temperature variations depend on the sowing date, which may enhance losses caused by water excess.Air temperature influenced the sunflower seedling emergence under water excess.Recent findings have evidenced the effects of water excess on seed germination of both soybeans (Nakamura et al., 2012) and maize (Zaidi et al., 2012).Here, we noted that emergence reduction was more harmful at 25 °C than at 15 °C and 10 ºC.According to Orchard & Jessop (1984), high air temperatures lead to increased enzymatic activity and metabolism in sunflower seeds.This must occur because the seeds under stress of excess water lack oxygen for metabolic activities (Pezeshki & Delaune, 2012).
Adverse effects of water excess on leaf area (LA) were more evident when such stress occurred right after sowing and after germination onset (Figures 3A,3B,3C,3D,3E,and 3F), showing mainly emergence failures and plant density reduction.The LA values dropped down to almost zero after 48 hours of water excess -except for the first sowing date -due to lower air temperatures.
Water excess during emergence at V2 and V4 stages considerably reduced LA in sowing the dates 2 and 3.The first sowing date showed LA reduction only at V4 stage (Figure 3M).Water excess reduced the leaf area in sowing dates 2 and 3 (Figures 3H,3K,3N,3I,3L,and 3O).For the third sowing date, LA reductions were of nearly 40% if compared to control, for 48 hours of water excess treatment; whereas the 96-hour treatment led to reductions of approximately 50%.In the second sowing date, V2-and V4-stage treatments promoted LA reductions lower than those in the third date, being of 25% after 48 hours, and 40% after 96 hours, all in comparison to the control.The experiment was carried out in a greenhouse, from August to November 2011, Santa Maria, RS, Brazil.
Such negative effect of water excess on LA was also observed on other crops such as maize (Zaidi et al., 2012), wheat (Araki et al., 2012), sorghum (Orchard & Jessop, 1984), and soybeans (Shimono et al., 2012;Lucas et al., 2015).According to Orchard & Jessop (1984), sunflower leaf expansion is strongly reduced due to water excess at V3 and V6 stages.Moreover, significant reductions in the photosynthetic rate took place after 48 hours of water excess application (Grassini et al., 2007).Yasumoto et al. (2011) described treatments using water excess in the establishment phase (V2 stage) and found sunflower growth suppression.Their results corroborated our observations, which showed a large reduction of LA due to water excess occurrence.
Leaf wilting was observed few hours after water excess treatment for all three sowing dates (Figure 4A).Furthermore, plant leaves showed photooxidative damage, mainly for V4-stage treatment in the sowing dates 2 and 3 (Figure 4B). Figure 4C highlights the differences in leaf area between plants receiving water excess or not.
According to Vartapetian & Jackson (1997), stomatal closure is an early plant response to water stress.Roots are unable to meet the water demand of plant leaves due to cell anoxia.Furthermore, photooxidative damages were visually observed in the leaves, mainly for water excess applied at V4 stage in sowing dates 2 and 3. Photooxidative damages under water excess were also observed in eggplants, tomatoes (Bansal & Srivastava, 2012), and in pigeon pea (Bansal & Srivastava, 2015).Water excess had greater adverse effect on plant height when treatment was applied before emergence since it led to major reductions (Figure 5).This effect was mostly severe during the first 2 to 4 days, especially at higher temperatures (sowing dates 2 and 3).Orchard & Jessop (1984) reported significant reductions in plant height for sunflower (V6 stage) and sorghum (V5 stage) plants caused by water excess.Likewise, Shimono et al. (2012)

FIGURE 1 .
FIGURE 1. Emergence percentage (EP) of sunflower plants in three different sowing dates (1, 2, and3) under water excess duration from 0 to 240 hours, applied immediately after sowing (A, B, and C) and 3 days after sowing (D, E, and F).The experiment was carried out in a greenhouse from August to November 2011, in Santa Maria -RS, Brazil.

FIGURE 3 .
FIGURE 3. Leaf area of sunflower plants subjected to water excess (0 to 240 hours) in three sowing dates (1, 2, and 3) right after sowing (A, B and C), 3 days after sowing (D, E and F), at plant emergence (G, H and I), at V2 stage (J, K and L) and at V4 stage (M, N and O).The experiment was carried out in a greenhouse, from August to November 2011, Santa Maria, RS, Brazil.

FIGURE 4 .
FIGURE 4. Leaf wilting (A), photooxidative damage in the leaf tissues (B), shoot and root (C) without (C, left) and with water stress (C, right).Lateral and adventitious roots on the soil surface (D) rising as a response to water excess.The experiment was carried out in a greenhouse, from August to November 2011, Santa Maria, RS, Brazil.

FIGURE 5 .
FIGURE 5. Plant height of sunflower under water excess (0 to 240 hours) in three sowing dates (1, 2 and 3) right after sowing (A, B and C), three days after sowing (D, E and, F), at plant emergence (G, H and I), at V2 stage (J, K and L) and at V4 stage (M, N and O).The experiment was carried out in a greenhouse, from August to November 2011, Santa Maria, RS, Brazil.