Larson, 1994; Kirschbaum et al., 2010a). Plants with high root TNC concentration rapidly generate feeder roots (Schupp and Hennion, 1997), providing resources for flower bud initiation (Long, 1935) and early fruit develop-ment (Nishizawa and Shishido, 1998).

In the northern hemisphere, N and TNC storage process in roots and crowns of strawberry plants starts in September with the onset of chilling temperatures, and these nutrients are remobilized for bud and new leaf development in spring, as reviewed by Kirschbaum et al. (2010b).

In previous experiments conducted in California, straw-berry runner plants cv. ‘Camarosa’ dug from HL nurseries in early October had enhanced vigor and greater early season fruit production than runner plants dug 2-3 weeks earlier in September, as a result of longer exposures to nursery chilling and decreasing photoperiods (Kirschbaum, 2012). Thus, early season fruit production in ‘Camarosa’ was also successfully increased as a result of late-summer nursery foliar-N applications (Kirschbaum et al., 2010a). Nevertheless, it is uncertain if both environmental factors will induce the same response in other strawberry cultivars since in this species the genotype x environment interaction is strong. For instance, one study showed that in order to accumulate ~40 mg.g^-1 FW of starch, in Spain, ‘Camarosa’ needed 400 chilling hours or units (CU) while ‘Pajaro’ required 700 CU (Lopez et al., 2002). ‘Ventana’, a cultivar released by the University of California, is characterized by greater early season fruit production compared to ‘Camarosa’ (Shaw and Larson, 2002), becoming an feasible cultivar for studying the role of nursery chilling and TNC and N reserves in early season fruit production.

In California, high-latitude strawberry nursery managers usually terminate N applications by the beginning of August in an effort to reduce vegetative growth and “harden off” developing runner plants. This hardening off process was thought to increase TNC accumulation, and reduce susceptibility to disease and transplant stress (Kirschbaum et al., 2010b); however, in a previous study with ‘Camarosa’, which is intermediate cultivar, we found out that late season nursery N applications were beneficial for early fruit production (Kirschbaum et al., 2010a). In light of these findings, we would like to assess if this is also valid for other cultivars with a different production cycle such as ‘Ventana’, which is an early cultivar.  

The objectives of this work were 1) to study the comparative growth and developmental responses of ‘Ventana’ and ‘Camarosa’ to progressive exposure to decreasing temperatures and photoperiods, 2) to compare the responses of the two cultivars to late summer N fertilizer applications and digging date in terms of tissue N and TNC partitioning, and 3) to relate the effects of initial tissue TNC and N contents of nursery plants to subsequent fruit production patterns in the fruiting field.

Nursery experiments

Nursery treatments were applied in a commercial runner plant propagation nurseries near Dorris (41°58'N, 121°55'W, 1292 m elevation), California. Cold-stored mother plants were used as nursery stock. In 2003, ‘Camarosa’ plants were established on 45-cm-in-row plant spacing and ‘Ventana’ on a 40-cm-in-row plant spacing on Apr 4. In 2004, ‘Camarosa’ and ‘Ventana’ planting dates were Apr 14 and 18, respectively. Water was supplied by drip irrigation. Plants were grown with standard commercial nursery management, including application of 224 kg N/ha, applied preplant (March) and through the drip irrigation system (May through the first week of August). In the 2004 experiments, N was applied foliarly in late summer. The N formulation was UAN 32 (1% concentration), and it was applied on Sep 6, Sep 13 and Sep 20, 2004 at rates of ~27 kg N/ha each. Second-daughter plants (D2, the 2^nd runner plants that develop in a series from stolons of the mother plant) were randomly harvested on Sep 20 and Oct 11, 2003, while first-daughter (D1) and D2 plants were harvested on Sept 20 and Oct 2, 2004. In 2004, plants dug on Sep-20 received two applications of foliar N of 27 kg N/ha each, and plants dug on Oct-2 received three applications of 27 kg N/ha each. Ten plants of each daughter order and nursery N treatment were cold stored at 1°C for 3-4 days, washed thoroughly, dissected into leaflets, petioles, crown and root tissues, and dried at 65°C for 4 days. For all plants, crown diameter was determined prior to drying. In 2004, after drying and dry mass (DM) determination, plants were analyzed for soluble and non-soluble total nonstructural carbohydrates, and total N. The analytical procedure for TNC consisted of enzymatic starch hydrolysis with amyloglucosidase, followed by high performance liquid chromatography (HPLC) for analysis of sugars (Smith, 1969). Total N analysis was performed with a Carlo Erba (Italy) elemental analyzer.

Growth chamber experiments

Bare-root ‘Camarosa’ and ‘Ventana’ D2 plants, consisting of leaves, crowns and roots were harvested from a HL nursery near Dorris, California on Sept 20 and Oct 11, 2003, and leaves were removed from all the plants after digging. Plants were planted in rectangular pots 13x13x15 cm (long:wide:deep) filled with "Sunshine mix" (Sun Gro Horticulture Inc., Canada) potting media, which consisted of sphagnum peat moss, perlite, dolomite limestone and gypsum. A teaspoon of controlled-release fertilizer (22-7-10 “Agriform”, Grace-Sierra Horticultural Products Co., USA) was applied to each pot at time of planting. Plants were placed in controlled environment chambers, where temperature and photoperiod were set at 25/15ºC day/night and 12 h, respectively. Plants were watered every 2-3 days. Flower emergence date and weight of harvested ripe fruit were progressively recorded for 90 days, when plants were removed from the growth chambers, dissected into component tissues (reproductive, petiols, leaflets, crown, root and fruit) and oven-dried at 65°C for 4 days. Crown diameter, leaf area, and the number of leaves, flowers and fruits (per plant) were measured prior to drying.

Fruiting field experiments

Bare-root D1 and D2 ‘Camarosa’ and ‘Ventana’ plants consisting of leaves, crowns and roots were harvested from the nursery on Sept 20 and Oct 2, 2004, cold stored at 1°C for 3-4 days and planted in experimental plots at the University of California’s South Coast Research and Extension Center, Irvine (33°39'N, 117°41'W). Preplant   soil   fumigation   was   applied   using   a (wt:wt) mixture of 2 methyl bromide : 1 chloropicrin at a rate of 392 kg.ha^-1. Plants were established in offset 4-row beds 162 cm wide x 40 cm high using a 40-cm in-row plant spacing (61,250 plants/ha). Each plot consisted of 10 plants of a unique daughter order (D1 and D2), digging date and N treatment. The experimental setup was a completely randomized design with 3 replications. All plots were maintained according to recommendations for California commercial winter plantings (Welch, 1989). Fruit were harvested weekly from Dec 21, 2004 to Apr 11, 2005, using commercial fruit maturity standards. Fruit yields were determined for each plot on a per-plant basis. Canopy diameter as well as leaf and flower numbers were determined prior to fruiting on 11 Nov 2004. Data were subjected to analysis of variance and means were separated using SAS (SAS Institute, 2003).

Compared with controls, late-season N applica-tions significantly enhanced N concentrations in leaves (Table 9), crowns (Table 10) and roots (Table 11), and, in general, decreased starch and TNC concentrations in leaves, crowns and roots. Crown tissue glucose concentration decreased as a result of N applications in seven out of eight mean comparisons; however, glucose in other tissues, as well as fructose and sucrose in general, did not show a defined trend in response to N applications.

Digging date had a significant effect on N concentration in leaves (Tables 9), generally decreasing from September to October in all tissues. However, for N-treated plants, little or no decrease in N was observed. Thus, the process of reduction of N concentrations observed in crowns and roots of control plants was reversed by late-summer N applications (Tables 10 and 11).

‘Ventana’ had greater concentrations of root glucose, fructose and sucrose, and greater concen-trations of starch and TNC than ‘Camarosa’ (Table 11), while plants of ‘Camarosa’ maintained higher concentrations of starch in leaves than plants of ‘Ventana’ (Table 9).  

In fruit evaluation plots, plants that had received late-summer N applications in the nursery had increased canopy diameters and leaf and flower numbers (Table 12), and increased early season yields (to 21 Feb) compared with control plants (Table 13). However, nursery late-summer N applications did not significantly affect total season fruit production. For ‘Ventana’ plants treated with foliar N in the nursery, early season marketable yields increased by 3 and 41% compared to controls, for plants planted on   Sept 24 and Oct 5, respectively. Plants dug in September started to produce fruit on Dec 21, 2004 and had greater early season fruit production in terms of yield and fruit number compared to October plantings, which commenced fruit production on Jan 4, 2005. ‘Ventana’ outyielded ‘Camarosa’ in early season fruit production and had greater marketable yield, mean fruit weight, marketable fruit number and better appearance.  

Currently, recommended planting dates for ‘Ventana’ in southern California are around Oct 1 (Kirk Larson, personal communication). However, prior to Feb 21, 2004, ‘Ventana’ control plants, planted on Sept 24, produced ~50% more yield than Oct-5-planted plants. As reported previously (Baum, 2005), we also observed that early season fruit production and total season fruit production were greater in ‘Ventana’ than in ‘Camarosa’. ‘Ventana’ also overcame ‘Camarosa’ in total season average fruit size, appearance and firmness.

The number of leaves and LA in the fall has been correlated with yield the following spring in biennial strawberry crops (Sproat and Darrow, 1935). We observed that plants receiving nursery N had a greater leaf number at the time of digging, resulting in larger canopy diameter and greater flower numbers after planting. By Nov 11, September-dug plants also had greater canopy diameter and leaf number than October-dug plants. Similarly, by 11 Nov, ‘Ventana’ had greater root DM, crown diameter, leaf number and LA at the time of digging than ‘Camarosa’, and also greater leaf number after planting. These correlations may partially explain the high early- season yields obtained by N-treated, September-dug, ‘Ventana’ plants.

In contrast to field experiments, growth chamber experiments allowed the evaluation of plant vigor and early season yield potential under environmental conditions that did not change from one digging date to another. Nursery runner plants harvested from a HL nursery in 2003 accumulated ~100 h of chilling when dug on Sept 20 and ~300 h when dug on Oct 11. The observed reduction in time to flowering for October-dug plants compared to September-dug plants could be due to an extended exposure to chilling temperatures, shorter photoperiods, and larger initial plant biomass. In general, after a 90-day growth period, plants dug on 11 Oct had greater fruit and flower numbers, fruit weight (fresh and dry), reproductive dry weight and whole plant dry weight than plants dug on 20 Sept and this was consistent for both cultivars.

Our observations of ‘Ventana’ plants flowering earlier and having greater fresh fruit weight and fruit DM than those of ‘Camarosa’ agree with the hypotheses that chilling stimulates the development and emergence of previously-differentiated inflorescences and that chilling sensitivity varies among cultivars (Larson, 1994). ‘Ventana’ seems to have a lower chilling requirement than ‘Camarosa’.

Since the California winter strawberry production system relies on fresh-dug runner plants without leaves (Galletta and Bringhurst, 1990), root and crown biomass play a fundamental role in plant establishment and early fruit production. Plants dug at the later dates each season had more crown and root DM than plants dug earlier. Root DM values and accumulation rates followed patterns analogous to previous studies (Kirschbaum et al., 2012; Long and Murneek, 1937; Nishizawa et al., 1998). Increased root DM during Sept-Oct could be attributed to an increased allocation of reserve nutrients to this organ (Bringhurst et al., 1960; Larson, 1994; Long and Murneek, 1937).

Previous work indicated that TNC concentration in strawberry runner plants is positively correlated with exposure to cold temperatures and short photoperiods (Kirschbaum et al., 2010b and 2012; Ruan et al., 2009) and that TNC concentration is negatively correlated with N applications (Kirschbaum et al., 2010a).

Our results support these previous observations and provide evidence for cultivar differences in regard to the effects of chilling on TNC and N accumulation and partitioning, TNC and N dynamics and TNC-N interactions.

Our results support the hypothesis that N reserves are major resources for fall-dug runner plant regrowth after transplanting. Nitrogen reserves have largely been overlooked as having a major role in strawberry transplant establishment and early fruit development in annual production systems (Kirschbaum et al., 2010b). Although details of the N cycling process in strawberries remains to be elucidated, our results indicate that late-season N applications allow plants to remain actively growing during the period of flower differentiation (Hancock, 1999; Long, 1939; Strik and Proctor, 1988), in addition to enhancing mobilization of N to crowns and roots, and with positive effects on early season flowering and fruiting. 

The authors have not declared any conflict of interest.

The authors thank Mike Fahner (Cedar Point Nursery, U.S.A.) and the UC SCREC crew for the support and experimental material, and Projects INTA PNHFA-1106073, PICT 2006-904 and PICT 2011-1170 (Argentina) for partial funding.